Abstract

Triatomine behaviour as a basis for functional genetics - Kissing-bugs have become an important model organism for neuroethological studies in the last two decades ( Guerenstein & Lazzari 2009Guerenstein PG, Lazzari CR 2009. Host-seeking: how triatomines acquire and make use of information to find blood. Acta Trop 110 : 148-158. , Manrique & Lorenzo 2012Manrique G, Lorenzo MG 2012. The sexual behaviour of Chagas disease vectors: chemical signals mediating communication between male and female Triatomine Bugs. Psyche (Camb Mass) 2012 : 8 pp. , Lazzari et al. current issue). The range of topics covered includes host search mechanisms ( Guerenstein & Lazzari 2009Guerenstein PG, Lazzari CR 2009. Host-seeking: how triatomines acquire and make use of information to find blood. Acta Trop 110 : 148-158. ), pheromone communication in contexts like mating ( Manrique & Lorenzo 2012Manrique G, Lorenzo MG 2012. The sexual behaviour of Chagas disease vectors: chemical signals mediating communication between male and female Triatomine Bugs. Psyche (Camb Mass) 2012 : 8 pp. ), shelter recognition ( Lorenzo & Lazzari 1996Lorenzo MG, Lazzari CR 1996. The spatial pattern of defaecation in Triatoma infestans and the role of faeces as a chemical mark of the refuge. J Insect Physiol 42 : 903-907. ) and predation risk ( Ward 1981Ward J 1981. A comparison of the behavioural responses of the haematophagous bug, Triatoma infestans , to synthetic homologues of two naturally occurring chemicals (n-and iso-butyric acid). Physiol Entomol 6 : 325-329. , Manrique et al. 2006Manrique G, Vitta AC, Ferreira RA, Zani CL, Unelius CR, Lazzari CR, Diotaiuti L, Lorenzo MG 2006. Chemical communication in Chagas disease vectors. Source, identity and potential function of volatiles released by the metasternal and Brindley’s glands of Triatoma infestans adults. J Chem Ecol 32 : 2035-2052. ), circadian rhythms and microclimatic preferences ( Lazzari 1991Lazzari CR 1991. Temperature preference in Triatotna infestans (Hemiptera: Reduviidae). Bull Ent Res 81 : 273-276. , Roca & Lazzari 1994Roca MJ, Lazzari CR 1994. Effects of relative humidity on the haematophagous bug Triatoma infestans : hygropreference and eclosion success. J Insect Physiol 40 : 901-907. , Lorenzo & Lazzari 1999Lorenzo MG, Lazzari CR 1999. Temperature and relative humidity affect the selection of shelters by Triatoma infestans , vector of Chagas disease. Acta Trop 72 : 241-249. , Guarneri et al. 2002Guarneri AA, Lazzari CR, Diotaiuti L, Lorenzo MG 2002. The effect of relative humidity on the behaviour and development of Triatoma brasiliensis . Physiol Entomol 27 : 142-147. , 2003Guarneri AA, Lazzari CR, Xavier AAP, Diotaiuti L, Lorenzo MG 2003. The effect of temperature on the behaviour and development of Triatoma brasiliensis . Physiol Entomol 28 : 185-191. ), state dependency ( Bodin et al. 2009aBodin A, Vinauger C, Lazzari CR 2009a. Behavioural and physiological state dependency of host seeking in the blood-sucking insect Rhodnius prolixus . J Exp Biol 212 : 2386-2393. , bBodin A, Vinauger C, Lazzari CR 2009b. State-dependency of host-seeking in Rhodnius prolixus : the post-ecdysis time. J Insect Physiol 55 : 574-579. ) and diverse forms of learning ( Vinauger et al. 2011aVinauger C, Buratti L, Lazzari CR 2011a. Learning the way to blood: first evidence of dual olfactory conditioning in a blood-sucking insect, Rhodnius prolixus . I. Appetitive learning. J Exp Biol 214 : 3032-3038. , bVinauger C, Buratti L, Lazzari CR 2011b. Learning the way to blood: first evidence of dual olfactory conditioning in a blood-sucking insect, Rhodnius prolixus . II. Aversive learning. J Exp Biol 214 : 3039-3045. , 2012Vinauger C, Pereira MH, Lazzari CR 2012. Learned host preference in a Chagas disease vector, Rhodnius prolixus . Acta Trop 122 : 24-28. , 2013Vinauger C, Lallement H, Lazzari CR 2013. Learning and memory in Rhodnius prolixus : habituation and aversive operant conditioning of the proboscis extension response. J Exp Biol 216 : 892-900. ).

Triatomine host searching mechanisms include orientation to airstreams laden with CO₂ and other host odours ( Núñez 1982Núñez JA 1982. Food source orientation and activity in Rhodnius prolixus Stål (Hemiptera: Reduviidae). Bull Ent Res 72 : 253-262. , Taneja & Guerin 1995Taneja J, Guerin PM 1995. Oriented responses of the triatomine bugs Rhodnius prolixus and Triatoma infestans to vertebrate odours on a servosphere. J Comp Physiol A 176 : 455-464. , Barrozo & Lazzari 2004Barrozo RB, Manrique G, Lazzari CR 2003. The role of water vapour in the orientation behaviour of the blood-sucking bug Triatoma infestans (Hemiptera, Reduviidae). J Insect Physiol 49 : 315-321. ), exploitation of bird and mammal emission of infrared radiation ( Lazzari & Núñez 1989Lazzari CR, Núñez JA 1989. The response to radiant heat and the estimantion of the temperature of distant sources in Triatoma infestans . J Insect Physiol 35 : 525-529. , Flores & Lazzari 1996Flores GB, Lazzari CR 1996. The role of the antennae in Triatoma infestans : orientation towards thermal sources. J Insect Physiol 42 : 433-440. , Ferreira et al. 2007Ferreira RA, Lazzari CR, Lorenzo MG, Pereira MH 2007. Do haematophagous bugs assess skin surface temperature to detect blood vessels? PLoS ONE 2 : e932. ) and orientation to sources of water vapour ( Barrozo et al. 2003Barrozo RB, Manrique G, Lazzari CR 2003. The role of water vapour in the orientation behaviour of the blood-sucking bug Triatoma infestans (Hemiptera, Reduviidae). J Insect Physiol 49 : 315-321. ). Furthermore, the intensity of these responses depends on modulatory factors such as the phase of the daily cycle ( Barrozo et al. 2004Barrozo RB, Minoli SA, Lazzari CR 2004. Circadian rhythm of behavioural responsiveness to carbon dioxide in the blood-sucking bug Triatoma infestans (Heteroptera: Reduviidae). J Insect Physiol 50 : 249-254. , Bodin et al. 2008Bodin A, Barrozo RB, Couton L, Lazzari CR 2008. Temporal modulation and adaptive control of the behavioural response to odours in Rhodnius prolixus . J Insect Physiol 54 : 1343-1348. ), bug nutritional status ( Bodin et al. 2009aBodin A, Vinauger C, Lazzari CR 2009a. Behavioural and physiological state dependency of host seeking in the blood-sucking insect Rhodnius prolixus . J Exp Biol 212 : 2386-2393. ) and experience ( Vinauger et al. 2011aVinauger C, Buratti L, Lazzari CR 2011a. Learning the way to blood: first evidence of dual olfactory conditioning in a blood-sucking insect, Rhodnius prolixus . I. Appetitive learning. J Exp Biol 214 : 3032-3038. , bVinauger C, Buratti L, Lazzari CR 2011b. Learning the way to blood: first evidence of dual olfactory conditioning in a blood-sucking insect, Rhodnius prolixus . II. Aversive learning. J Exp Biol 214 : 3039-3045. ).

Kissing bugs also communicate through pheromones in diverse behavioural contexts (Lazzari et al. current issue). Disturbed adult triatomines emit alarm pheromones to trigger avoidance of the emission spot by conspecifics ( Ward 1981Ward J 1981. A comparison of the behavioural responses of the haematophagous bug, Triatoma infestans , to synthetic homologues of two naturally occurring chemicals (n-and iso-butyric acid). Physiol Entomol 6 : 325-329. , Manrique et al. 2006Manrique G, Vitta AC, Ferreira RA, Zani CL, Unelius CR, Lazzari CR, Diotaiuti L, Lorenzo MG 2006. Chemical communication in Chagas disease vectors. Source, identity and potential function of volatiles released by the metasternal and Brindley’s glands of Triatoma infestans adults. J Chem Ecol 32 : 2035-2052. ) and this has been suggested to mediate the avoidance of predators ( Manrique et al. 2006Manrique G, Vitta AC, Ferreira RA, Zani CL, Unelius CR, Lazzari CR, Diotaiuti L, Lorenzo MG 2006. Chemical communication in Chagas disease vectors. Source, identity and potential function of volatiles released by the metasternal and Brindley’s glands of Triatoma infestans adults. J Chem Ecol 32 : 2035-2052. ). A pheromone also mediates triatomine aggregation inside shelters ( Schofield & Patterson 1977Schofield G, Patterson J 1977. Assembly pheromone of Triatoma infestans and Rhodnius prolixus nymphs (Hemiptera: Reduviidae). J Med Entomol 13 : 727-734. , Figueiras et al. 1994Figueiras ANL, Kenigsten A, Lazzari CR 1994. Aggregation in the haematophagous bug Triatoma infestans : chemical signals and temporal pattern. J Insect Physiol 40 : 311-316. , Lorenzo & Lazzari 1996Lorenzo MG, Lazzari CR 1996. The spatial pattern of defaecation in Triatoma infestans and the role of faeces as a chemical mark of the refuge. J Insect Physiol 42 : 903-907. , Pires et al. 2002bPires HHR, Lorenzo MG, Diotaiuti L, Lazzari CR, Figueiras AL 2002b. Aggregation behaviour in Panstrongylus megistus and Triatoma infestans : inter and intraspecific responses. Acta Trop 81 : 47-52. ). The use of shelters during daylight hours is fundamentally driven by their strong negative phototaxis ( Reisenman et al. 1998Reisenman CE, Lazzari CR, Giurfa M 1998. Circadian control of photonegative sensitivity in the haematophagous bug Triatoma infestans . J Comp Physiol A 183 : 533-541. ) and at a later phase by an intense thigmotactic behaviour. In addition, these insects locate bug aggregations searching for shelters marked with triatomine faeces. Once inside refuges, a contact chemical signal present in their cuticule induces their arrestment ( Figueiras et al. 2009Figueiras AL, Girotti JR, Mijailovsky SJ, Juárez MP 2009. Epicuticular lipids induce aggregation in Chagas disease vectors. Parasit Vectors 2 : 8. ).Finally, a sex pheromone is emitted by female adults to attract males ( Manrique et al. 2006Manrique G, Vitta AC, Ferreira RA, Zani CL, Unelius CR, Lazzari CR, Diotaiuti L, Lorenzo MG 2006. Chemical communication in Chagas disease vectors. Source, identity and potential function of volatiles released by the metasternal and Brindley’s glands of Triatoma infestans adults. J Chem Ecol 32 : 2035-2052. , Pontes et al. 2008Pontes G, Bohman B, Unelius C, Lorenzo MG 2008. Metasternal gland volatiles and sexual communication in the triatomine bug, Rhodnius prolixus . J Chem Ecol 34 : 450-457. , Vitta et al. 2009Vitta AC, Bohman B, Unelius CR, Lorenzo MG 2009. Behavioral and electrophysiological responses of Triatoma brasiliensis males to volatiles produced in the metasternal glands of females. J Chem Ecol 35 : 1212-1221. , May-Concha et al. 2013May-Concha I, Rojas JC, Cruz-López L, Millar JG, Ramsey JM 2013. Volatile compounds emitted by Triatoma dimidiata , a vector of Chagas disease: chemical analysis and behavioural evaluation. Med Vet Entomol 27 : 165-174. ). Sexual signals also mediate male aggregation around mating pairs, apparently promoting polyandric reproduction in some bug species ( Crespo & Manrique 2007Crespo J, Manrique G 2007. Mating behavior of the hematophagous bug Triatoma infestans : role of Brindley’s and metasternal glands. J Insect Physiol 53 : 708-714. , Pontes & Lorenzo 2012Pontes G, Lorenzo MG 2012. Female metasternal gland odours mediate male aggregation in Rhodnius prolixus , a triatomid bug. Med Vet Entomol 26 : 33-36. ). Nevertheless, the latter seems not to be generalised in the subfamily Triatominae ( Pires et al. 2004Pires HHR, Lorenzo MG, Lazzari CR, Diotaiuti L, Manrique G 2004. The sexual behaviour of Panstrongylus megistus (Hemiptera: Reduviidae): an experimental study. Mem Inst Oswaldo Cruz 99 : 295-300. ).

These insects seem to evaluate the adequacy of potential refuges by their physical properties ( Lorenzo & Lazzari 1999Lorenzo MG, Lazzari CR 1999. Temperature and relative humidity affect the selection of shelters by Triatoma infestans , vector of Chagas disease. Acta Trop 72 : 241-249. ). Temperature and relative humidity range preferences apparently vary from species to species ( Lazzari 1991Lazzari CR 1991. Temperature preference in Triatotna infestans (Hemiptera: Reduviidae). Bull Ent Res 81 : 273-276. , Roca & Lazzari 1994Roca MJ, Lazzari CR 1994. Effects of relative humidity on the haematophagous bug Triatoma infestans : hygropreference and eclosion success. J Insect Physiol 40 : 901-907. , Guarneri et al. 2002Guarneri AA, Lazzari CR, Diotaiuti L, Lorenzo MG 2002. The effect of relative humidity on the behaviour and development of Triatoma brasiliensis . Physiol Entomol 27 : 142-147. , 2003Guarneri AA, Lazzari CR, Xavier AAP, Diotaiuti L, Lorenzo MG 2003. The effect of temperature on the behaviour and development of Triatoma brasiliensis . Physiol Entomol 28 : 185-191. , Pires et al. 2002aPires HHR, Lazzari CR, Schilman PE, Diotaiuti L, Lorenzo MG 2002a. Dynamics of thermopreference in the Chagas disease vector Panstrongylus megistus (Hemiptera: Reduviidae). J Med Entomol 39 : 716-719. , Schilman & Lazzari 2004Schilman PE, Lazzari CR 2004. Temperature preference in Rhodnius prolixus , effects and possible consequences. Acta Trop 90 : 115-122. ). The level of illumination and light spectral qualities of their environments clearly affect their spatial choices and the intensity of these responses seems to depend on the phase of the daily cycle ( Reisenman et al. 1998Reisenman CE, Lazzari CR, Giurfa M 1998. Circadian control of photonegative sensitivity in the haematophagous bug Triatoma infestans . J Comp Physiol A 183 : 533-541. , 2002Reisenman CE, Insausti TC, Lazzari CR 2002. Light-induced and circadian changes in the compound eye of the haematophagous bug Triatoma infestans (Hemiptera: Reduviidae). J Exp Biol 205 : 201-210. ). Vision also plays an important role in locomotion, flight orientation and spatial recognition by triatomines ( Lazzari & Varjú 1990Lazzari CR, Varjú D 1990. Visual lateral fixation and tracking in the haematophagous bug Triatoma infestans . J Comp Physiol A 167 : 527-531. , Reisenman et al. 1998Reisenman CE, Lazzari CR, Giurfa M 1998. Circadian control of photonegative sensitivity in the haematophagous bug Triatoma infestans . J Comp Physiol A 183 : 533-541. , 2000Reisenman CE, Figueiras ANL, Giurfa M, Lazzari CR 2000. Interaction of visual and olfactory cues in the aggregation behaviour of the haematophagous bug Triatoma infestans . J Comp Physiol A 186 : 961-968. , Minoli & Lazzari 2006Minoli SA, Lazzari CR 2006. Take-off activity and orientation of triatomines (Heteroptera: Reduviidae) in relation to the presence of artificial lights. Acta Trop 97 : 324-330. , Reisenman & Lazzari 2006Reisenman CE, Lazzari CR 2006. Spectral sensitivity of the photonegative reaction of the blood-sucking bug Triatoma infestans (Heteroptera: Reduviidae). J Comp Physiol A Neuroethol Sens Neural Behav Physiol 192 : 39-44. , Insausti et al. 2013Insausti TC, Le Gall M, Lazzari CR 2013. Oxidative stress, photodamage and the role of screening pigments in insect eyes. J Exp Biol 216 : 3200-3207. ).

Triatomine behaviour is finely controlled by circadian clocks ( Lazzari 1992Lazzari CR 1992. Circadian organization of locomotion activity in the haematophagous bug Triatoma infestans . J Insect Physiol 38 : 895-903. , Barrozo et al. 2004Barrozo RB, Minoli SA, Lazzari CR 2004. Circadian rhythm of behavioural responsiveness to carbon dioxide in the blood-sucking bug Triatoma infestans (Heteroptera: Reduviidae). J Insect Physiol 50 : 249-254. , Guerenstein & Lazzari 2009Guerenstein PG, Lazzari CR 2009. Host-seeking: how triatomines acquire and make use of information to find blood. Acta Trop 110 : 148-158. ). In fact, activities are distributed within two precise temporal windows: one after dusk devoted to host search and another at dawn, dedicated to shelter location, egg-hatching and ecdysis ( Lazzari et al. 2013Lazzari CR, Pereira MH, Lorenzo MG 2013. Behavioural biology of Chagas disease vectors. Mem Inst Oswaldo Cruz 108 : 34-47. ). Each activity peak is under the control of a specific endogenous oscillator that establishes the proper timing ( Lazzari 1992Lazzari CR 1992. Circadian organization of locomotion activity in the haematophagous bug Triatoma infestans . J Insect Physiol 38 : 895-903. ).

There is a clear state dependency in the motivation of these insects to search for hosts, shelters and mates. Host search is modulated by age, nutritional status and reproductive condition ( Bodin et al. 2008Bodin A, Barrozo RB, Couton L, Lazzari CR 2008. Temporal modulation and adaptive control of the behavioural response to odours in Rhodnius prolixus . J Insect Physiol 54 : 1343-1348. , 2009aBodin A, Vinauger C, Lazzari CR 2009a. Behavioural and physiological state dependency of host seeking in the blood-sucking insect Rhodnius prolixus . J Exp Biol 212 : 2386-2393. ). Recently-fed insects and gravid females avoid host associated odours ( Bodin et al. 2009aBodin A, Vinauger C, Lazzari CR 2009a. Behavioural and physiological state dependency of host seeking in the blood-sucking insect Rhodnius prolixus . J Exp Biol 212 : 2386-2393. ). The search for refuges also depends on the nutritional state and the phase of the diel cycle ( Lorenzo & Lazzari 1998Lorenzo MG, Lazzari CR 1998. Activity pattern in relation to refuge exploitation and feeding in Triatoma infestans (Hemiptera: Reduviidae). Acta Trop 70 : 163-170. ). The search for reproductive mates and mating receptivity depend upon the age since the adult ecdysis and the nutritional state of the adult insects ( Manrique & Lazzari 1994Manrique G, Lazzari CR 1994. Sexual behaviour and stridulation during mating in Triatoma infestans (Hemiptera: Reduviidae). Mem Inst Oswaldo Cruz 89 : 629-633. , Vitta & Lorenzo 2009Vitta AC, Lorenzo MG 2009. Copulation and mate guarding behavior in Triatoma brasiliensis (Hemiptera: Reduviidae). J Med Entomol 46 : 789-795. ).

The advent of next-generation sequencing (NGS), gene expression/regulation techniques and heterologous expression systems in the post-genomic era - Many insect genomes have been sequenced to date, forming a rich source of appropriate orthologues of behaviour controlling genes to initiate searches in the Rhodnius prolixus genome. Particularly, the genome of the pea aphid Acyrthosiphon pisum represents one of the best candidates for guiding BLAST searches due to their closer phylogenetic relation ( The International Aphid Genomics Consortium 2010The International Aphid Genomics Consortium 2010. Genome Sequence of the pea aphid Acyrthosiphon pisum . PLoS Biol 8 : e1000313. ). This will be more relevant whenever a greater functional characterisation of this genome is made available. An assortment of gene sequencing, silencing, deletion and heterologous expression techniques have enabled more elaborate studies on the genetic bases of biological processes ( Figure ). Insect physiology benefited from this wealth of novel techniques and has shown impressive progress concomitant with the amazing potential of one particular insect model, Drosophila melanogaster ( Table ). In the last decades, it has been adopted as a main model for the study of the genetic and molecular bases of behaviour, being central to current neuroscience. The molecular mechanisms underlying circadian rhythms, plasticity and the formation of memories, sensory function and even, sexual behaviour have been studied in Drosophila ( Table ). Nevertheless, it presents limitations for neuroscience studies due its small size that restricts manipulation. For example, studies like those developed by VB Wigglesworth using R. prolixus as an insect model for the study of metamorphosis and neurosecretory function could only be performed thanks to an extremely practical model that allowed surgical procedures with minimal deleterious consequences. The present paper intends to propose R. prolixus as a new tool for the study of insect neuroscience due to these three characteristics: manipulation-friendly size, background as a classical insect physiology model and deep knowledge of diverse aspects of its behaviour. These facts, together with the recent characterisation of its genome sequence, will allow the resurgence of an excellent insect physiology model, as in Wigglesworth’s time, but in the omics era. Next follows a series of aspects of triatomine behaviour, as well as related candidate genes uncovered for other insects, whose characterisation and study would be invaluable in R. prolixus and other relevant Chagas disease vectors.

The molecular basis of insect behaviour as a framework for studies on triatomines - Sensory ecology of host searching - Host location in triatomines is dependent on environmental and physiological conditions evaluated by their brain in order to regulate a proper expression of this behaviour. When properly motivated to feed, these insects need to locate warm-blooded animals for a blood meal. As mentioned above, this task is performed detecting host signals through an array of sensory channels that include the detection of CO₂, diverse odours, heat and water vapour. Once host detection is achieved, these insects approach the animal and finally need to find an appropriate biting site to pierce a blood vessel. All these behavioural steps depend on the detection of stimuli at sensory neurons in the periphery of their nervous system.

Chemoreceptor mediated host searching - CO₂ is considered a long distance host cue promoting both the activation of resting triatomines and their orientation through odour modulated anaemotaxis ( Barrozo & Lazzari 2004Barrozo RB, Manrique G, Lazzari CR 2003. The role of water vapour in the orientation behaviour of the blood-sucking bug Triatoma infestans (Hemiptera, Reduviidae). J Insect Physiol 49 : 315-321. ). The physiological and molecular bases of detection of this host cue are unknown for triatomines, but relevant progress has been made to uncover a similar mechanism in two dipterans ( Jones et al. 2007Jones W, Cayirlioglu P, Kadow I, Vosshall LB 2007. Two chemosensory receptors together mediate carbon dioxide detection in Drosophila . Nature 445 : 86-90. , Lu et al. 2007Lu T, Qiu Y, Wang G, Kwon J, Rutzler M, Kwon H-W, Pitts R, van Loon J, Takken W, Carlson JR, Zwiebel LJ 2007. Odor coding in the maxillary palp of the malaria vector mosquito Anopheles gambiae . Curr Biol 17 : 1533-1544. ). According to these reports, both Drosophila and Anopheles gambiae detect CO₂ with heterodimers composed of gustatory receptors (GRs) that are co-expressed in specific olfactory receptor neurons (ORNs). These receptors ( DmGr21a and DmGr63a ) are expressed in the antennae of D. melanogaster and the maxillary palps of An. gambiae ( AgGr22, AgGr23 and AgGr24 ). These GRs belong to three ancient lineages of GRs also found in moths and beetles ( Robertson & Kent 2009Robertson HM, Kent L 2009. Evolution of the gene lineage encoding the carbon dioxide receptor in insects. J Insect Sci 9 : 19. ). Surprisingly, these gene lineages have not been found in other arthropods like aphids, lice, honey bees, water fleas and black-legged ticks. Nevertheless, some of these arthropods are known to detect CO₂, suggesting that they evolved a different molecular mechanism to detect this cue. The study of the molecular bases of CO₂ detection by triatomines is therefore relevant and will rely on diverse bioassays already developed.

Host odours other than CO₂ play a fundamental role in the orientation of triatomine bugs and their detection is probably mediated by proteins belonging to two different families: the odourant receptors (ORs) and the ionotropic receptors (IRs). In insects, ORs present seven transmembrane domains characteristic of all members of this protein family ( Vosshall et al. 1999Vosshall LB, Amrein H, Morozov PS, Rzhetsky A, Axel R 1999. A spatial map of olfactory receptor expression in the Drosophila antenna. Cell 96 : 725-736. ). In the ORNs expressing them, these receptors need to be co-expressed together with a phylogenetically conserved chaperon protein currently named “odourant receptor coreceptor-OrCo” for all insects ( Larsson et al. 2004Larsson MC, Domingos AI, Jones WD, Chiappe ME, Amrein H, Vosshall LB 2004. Or83b encodes a broadly expressed odorant receptor essential for Drosophila olfaction. Neuron 43 : 703-714. , Vosshall & Hansson 2011Vosshall LB, Hansson BS 2011. A unified nomenclature system for the insect olfactory coreceptor. Chemical Senses 36 : 497-498. ). ORs are divergent proteins with low sequence identity between insect species and their roles cannot be predicted by sequence homology alone. Olfactory sensory neurons express different receptor genes, such that individual neurons are functionally distinct ( Table ). Numbers of ORs are extremely variable in insects, ranging from only 10 in the human louse to 265 in the flour beetle ( TGSC et al. 2008TGSC - Tribolium Genome Sequencing Consortium, Richards S, Gibbs R, Weinstock G, Brown S, Denell R, Beeman R, Bucher G, Friedrich M, Grimmelikhuijzen C, Klingler M, Lorenzen M, Roth S, Schröder R, Tautz D, Zdobnov E, Muzny D, Attaway T, Bell S, Buhay C, Chandrabose M, Chavez D, Clerk-Blankenburg K, Cree A, Dao M, Davis C, Chacko J, Dinh H, Dugan-Rocha S, Fowler G, Garner T, Garnes J, Gnirke A, Hawes A, Hernandez J, Hines S, Holder M, Hume J, Jhangiani S, Joshi V, Khan Z, Jackson L, Kovar C, Kowis A, Lee S, Lewis L, Margolis J, Morgan M, Nazareth L, Nguyen N, Okwuonu G, Parker D, Ruiz S-J, Santibanez J, Savard J, Scherer S, Schneider B, Sodergren E, Vattahil S, Villasana D, White C, Wright R, Park Y, Lord J, Oppert B, Wang L, Liu Y, Worley K, Elsik C, Reese J, Elhaik E, Landan G, Graur D, Arensburger P, Atkinson P, Beidler J, Demuth J, Drury D, Du Y-Z, Fujiwara H 2008. The genome of the model beetle and pest Tribolium castaneum . Nature 452 : 949-955. , Kirkness et al. 2010Kirkness E, Haas B, Sun W, Braig H, Perotti M, Clark J, Lee S, Robertson HM, Kennedy R, Elhaik E, Gerlach D, Kriventseva E, Elsik C, Graur D, Hill C, Veenstra J, Walenz B, Tubío J, Ribeiro J, Rozas J, Johnston J, Reese J, Popadic A, Tojo M, Raoult D, Reed D, Tomoyasu Y, Kraus E, Krause E, Mittapalli O, Margam V, Li H-M, Meyer J, Johnson R, Romero-Severson J, Vanzee J, Alvarez-Ponce D, Vieira F, Aguadé M, Guirao-Rico S, Anzola J, Yoon K, Strycharz J, Unger M, Christley S, Lobo N, Seufferheld M, Wang N, Dasch G, Struchiner C, Madey G, Hannick L, Bidwell S, Joardar V, Caler E, Shao R, Barker S, Cameron S, Bruggner R, Regier A, Johnson J, Viswanathan L, Utterback T, Sutton G, Lawson D, Waterhouse R, Venter J, Strausberg R, Berenbaum M, Collins F, Zdobnov E, Pittendrigh B 2010. Genome sequences of the human body louse and its primary endosymbiont provide insights into the permanent parasitic lifestyle. Proc Natl Acad Sci USA 107 : 12168-12173. ).

A second family of insect ORs includes IRs, recently described in Drosophila , which present three transmembrane domains and constitute cation channels ( Benton et al. 2009Benton R, Vannice K, Gomez-Diaz C, Vosshall LB 2009. Variant ionotropic glutamate receptors as chemosensory receptors in Drosophila . Cell 136 : 149-162. ). These proteins have been suggested to mediate olfaction and taste in protostomes, an apparently ancient function for detecting chemical signals from the environment ( Croset et al. 2010Croset V, Rytz R, Cummins S, Budd A, Brawand D, Kaessmann H, Gibson T, Benton R 2010. Ancient protostome origin of chemosensory ionotropic glutamate receptors and the evolution of insect taste and olfaction. PLoS Genet 6 : e1001064. ). IRs have been proposed to act as dimers or trimers of subunits co-expressed in the same neuron, which comprise individual odour-specific receptors and one or two broadly expressed coreceptors ( Abuin et al. 2011Abuin L, Bargeton B, Ulbrich M, Isacoff E, Kellenberger S, Benton R 2011. Functional architecture of olfactory ionotropic glutamate receptors. Neuron 69 : 44-60. ). The suggested coreceptor units for IRs have thus been named IR25a, IR8a and IR76b. The ORNs expressing IRs are restricted to chemoreceptor hairs characterised by being double-walled, wall pore sensilla (e.g., grooved-pegs) and appear to have a relatively conserved role for detection of a restricted set of odourants in different insects ( Pophof 1997Pophof B 1997. Olfactory responses recorded from sensilla coeloconica of the silkmoth Bombyx mori . Physiol Entomol 22 : 239-248. , Diehl et al. 2003Diehl P, Vlimant M, Guerenstein PG, Guerin PM 2003. Ultrastructure and receptor cell responses of the antennal grooved peg sensilla of Triatoma infestans (Hemiptera: Reduviidae). Arthropod Struct Dev 31 : 271-285. , Yao et al. 2005Yao CA, Ignell R, Carlson JR 2005. Chemosensory coding by neurons in the coeloconic sensilla of the Drosophila antenna. J Neurosci 25 : 8359-8367. , Qiu et al. 2006Qiu YT, van Loon JJ, Takken W, Meijerink J, Smid HM 2006. Olfactory coding in antennal neurons of the malaria mosquito, Anopheles gambiae . Chem Senses 31 : 845-863. ). According to Silbering et al. (2011), odourant detection by IRs represents an insect olfactory subsystem that evolved in parallel to OR based olfaction. Numbers of IRs also show a relevant degree of variation ranging from 12 in the human body louse to 95 in Aedes aegypti ( Croset et al. 2010Croset V, Rytz R, Cummins S, Budd A, Brawand D, Kaessmann H, Gibson T, Benton R 2010. Ancient protostome origin of chemosensory ionotropic glutamate receptors and the evolution of insect taste and olfaction. PLoS Genet 6 : e1001064. ).

The identification of receptor genes mediating host odour detection in the R. prolixus genome along with an analysis of olfactory-driven behaviour in these insects may enable researchers to understand the mechanisms linking host recognition and triatomine behaviour. These receptors and their ligands need to be characterised using a multidisciplinary approach including the study of gene expression, its physiological modulation and the electrophysiological properties of the ORNs expressing them ( Figure ). A deeper comprehension of the olfactory physiology underlying triatomine host detection would permit the design of antagonists to block these functions, both for OR and IR proteins involved.

Thermo and hygroreceptor mediated host searching - Triatomines make use of their highly developed thermal sense to detect potential hosts. These bugs detect the heat emitted by warm-blooded animals in the form of infrared radiation ( Lazzari & Núñez 1989Lazzari CR, Núñez JA 1989. The response to radiant heat and the estimantion of the temperature of distant sources in Triatoma infestans . J Insect Physiol 35 : 525-529. , Schmitz et al. 2000Schmitz H, Trenner S, Hofmann MH, Bleckmann H 2000. The ability of Rhodnius prolixus (Hemiptera, Reduviidae) to approach a thermal source solely by its infrared radiation. J Insect Physiol 46 : 745-751. ). The antennae of these insects house infrared detection organs ( Lazzari & Wicklein 1994Lazzari CR, Wicklein M 1994. The cave-like sense organ in the antennae of Triatominae bugs. Mem Inst Oswaldo Cruz 89: 643-648. , Flores & Lazzari 1996Flores GB, Lazzari CR 1996. The role of the antennae in Triatoma infestans : orientation towards thermal sources. J Insect Physiol 42 : 433-440. ) and are also critical for the bilateral integration of thermal information necessary to approach hosts ( Flores & Lazzari 1996Flores GB, Lazzari CR 1996. The role of the antennae in Triatoma infestans : orientation towards thermal sources. J Insect Physiol 42 : 433-440. ). Triatomine thermoreceptor neurons have been poorly studied and information on their electrophysiological properties is very limited ( Bernard 1974Bernard J 1974. Étude électrophysiologique de récepteurs impliqués dans l’orientation vers l’hôte et dans l’acte hématophage chez un Hémiptère, Triatoma infestans, PhD Thesis, Université de Rennes, Rennes, 286 pp. ). The physiology of thermoreceptor neurons and their receptor proteins should be studied in these insects, which represent one of the most practical models to determine the molecular basis of heat perception in animals. Several genes belonging to the transient receptor potential A (TRPA) subfamily are known to mediate thermoreception in D. melanogaster, An. gambiae and Apis mellifera . The TRPA proteins belong to the TRP superfamily, which is composed of seven subfamilies of transmembrane protein channels with a relevant role in diverse sensory modalities including vision, taste, smell, thermo and mechanosensation ( Table ). The members of the TRPA subfamily are characterised by presence of six transmembrane domains and large numbers of ankyrin motifs in the N-terminal domain ( Montell 2005Montell C 2005. The TRP superfamily of cation channels. Sci STKE 2005 : re3. ). Orthologous sequences in the genome of R. prolixus need to be identified. Thermoreceptor genes with functional roles such as TRPA1, pyrexia and painless have been described in Drosophila ( Tracey Jr et al. 2003Tracey Jr WD, Wilson RI, Laurent G, Benzer S 2003. painless , a Drosophila gene essential for nociception. Cell 113 : 261-273. , Lee et al. 2005Lee Y, Lee Y, Lee J, Bang S, Hyun S, Kang J, Hong S-T, Bae E, Kaang B-K, Kim J 2005. Pyrexia is a new thermal transient receptor potential channel endowing tolerance to high temperatures in Drosophila melanogaster . Nat Genet 37 : 305-310. , Wang et al. 2009Wang G, Qiu Y, Lu T, Kwon H-W, Pitts R, van Loon J, Takken W, Zwiebel LJ 2009. Anopheles gambiae TRPA1 is a heat-activated channel expressed in thermosensitive sensilla of female antennae. Eur J Neurosci 30 : 967-974. ).

Water vapour has been implicated as a host signal used by triatomines during their approach to blood-meal sources ( Barrozo et al. 2003Barrozo RB, Manrique G, Lazzari CR 2003. The role of water vapour in the orientation behaviour of the blood-sucking bug Triatoma infestans (Hemiptera, Reduviidae). J Insect Physiol 49 : 315-321. ) and the molecular basis of its detection have been well studied in D. melanogaster ( Liu et al. 2007Liu L, Li Y, Wang R, Yin C, Dong Q, Hing H, Kim C, Welsh M 2007. Drosophila hygrosensation requires the TRP channels water witch and nanchung . Nature 450 : 294-298. ). Briefly, vinegar flies detect air humidity levels by means of two different TRP proteins belonging to the TRPV subfamily, nanchung (involved in detecting dry air) and water witch (required to detect moist air). An evaluation of possible orthologues of these genes in the genome of R. prolixus would allow functional studies on their role in host location.

Host recognition and biting - After their final approach, triatomines need to recognise surface properties on the potential host in order to trigger biting responses properly. These can be reinforced by stimuli perceived through diverse sensory channels, but fundamentally through thermoreception and contact chemoreception. The first is mediated by antennal thermodetection structures recognising warm surfaces and even warmer blood vessels ( Flores & Lazzari 1996Flores GB, Lazzari CR 1996. The role of the antennae in Triatoma infestans : orientation towards thermal sources. J Insect Physiol 42 : 433-440. , Ferreira et al. 2007Ferreira RA, Lazzari CR, Lorenzo MG, Pereira MH 2007. Do haematophagous bugs assess skin surface temperature to detect blood vessels? PLoS ONE 2 : e932. ). The second can be mediated by proteins belonging to at least two different families, the GRs and pickpocket receptors (ppks). Insect GRs are membrane proteins generally dedicated to detect non-volatile substances ( Table ). These are G protein-coupled receptors presenting seven transmembrane domains that mediate the recognition of substances present on substrates and are expressed in the cilia of neurons housed inside contact chemoreceptor sensilla ( Clyne et al. 2000Clyne P, Warr C, Carlson JR 2000. Candidate taste receptors in Drosophila . Science 287 : 1830-1834. ). Pickpocket receptors belong to the Degenerin/epithelial sodium channel gene family and present two transmembrane helixes, two short intracellular domains and a large cysteine-rich extracellular loop ( Ben-Shahar 2011Ben-Shahar Y 2011. Sensory functions for degenerin/epithelial sodium channels (DEG/ENaC). Adv Genet 76 : 1-26. ). In Drosophila , ppk receptors have been related with the detection of water, e.g. ppk28 ( Cameron et al. 2010Cameron P, Hiroi M, Ngai J, Scott K 2010. The molecular basis for water taste in Drosophila . Nature 465 : 91-95. , Chen et al. 2010Chen Z, Wang Q, Wang Z 2010. The amiloride-sensitive epithelial Na+channel ppk28 is essential for Drosophila gustatory water reception. J Neurosci 30 : 6247-6252. ) and salt, e.g. ppk11 and ppk19 ( Liu et al. 2003Liu L, Leonard A, Motto D, Feller M, Price M, Johnson W, Welsh M 2003. Contribution of Drosophila DEG/ENaC genes to salt taste. Neuron 39 : 133-146. ). Their presence in the genome of R. prolixus should be assessed and their potential role in host recognition in triatomines evaluated.

Pheromones: the molecular basis of chemical communication processes - As already mentioned, pheromones are used by triatomines to exchange information in diverse behavioural contexts. The substances composing them are most probably detected by triatomine ORs, IRs, GRs and ppks which still need to be characterised. Potential agents blocking their functions may become alternatives for triatomine control, which suggests that the identification of genes coding for these receptors would be extremely relevant to allow their manipulation. In the case of sex pheromone detection, any receptors showing exclusive expression in adults, males or female triatomines should be main research targets and may be identified through NGS techniques such as RNAseq ( Grosse-Wilde et al. 2011Grosse-Wilde E, Kuebler LS, Bucks S, Vogel H, Wicher D, Hansson BS 2011. Antennal transcriptome of Manduca sexta . Proc Natl Acad Sci USA 108 : 7449-7454. , Pitts et al. 2011Pitts RJ, Rinker DC, Jones PL, Rokas A, Zwiebel LJ 2011. Transcriptome profiling of chemosensory appendages in the malaria vector Anopheles gambiae reveals tissue and sex-specific signatures of odor coding. BMC Genomics 12 : 271. , Bengtsson et al. 2012Bengtsson JM, Trona F, Montagné N, Anfora G, Ignell R, Witzgall P, Jacquin-Joly E 2012. Putative chemosensory receptors of the codling moth, Cydia pomonella , identified by antennal transcriptome analysis. PLoS ONE 7 : e31620. ).

Spatial and temporal orientation - Negative phototaxis is a common behavioural trait of most triatomine bug species ( Lazzari et al. 1998Lazzari CR, Reiseman CE, Insausti TC 1998. The role of the ocelli in the phototactic behaviour of the haematophagous bug Triatoma infestans . J Insect Physiol 44 : 1159-1162. , Reisenman et al. 1998Reisenman CE, Lazzari CR, Giurfa M 1998. Circadian control of photonegative sensitivity in the haematophagous bug Triatoma infestans . J Comp Physiol A 183 : 533-541. , 2000Reisenman CE, Figueiras ANL, Giurfa M, Lazzari CR 2000. Interaction of visual and olfactory cues in the aggregation behaviour of the haematophagous bug Triatoma infestans . J Comp Physiol A 186 : 961-968. , Reisenman & Lazzari 2006Reisenman CE, Lazzari CR 2006. Spectral sensitivity of the photonegative reaction of the blood-sucking bug Triatoma infestans (Heteroptera: Reduviidae). J Comp Physiol A Neuroethol Sens Neural Behav Physiol 192 : 39-44. ). This behavioral feature induces avoidance of open areas during daylight hours ( Lorenzo & Lazzari 1996Lorenzo MG, Lazzari CR 1996. The spatial pattern of defaecation in Triatoma infestans and the role of faeces as a chemical mark of the refuge. J Insect Physiol 42 : 903-907. , Mota & Lorenzo 2012Mota T, Lorenzo MG 2012. Lack of segregation between two species of Chagas disease vectors. Am J Trop Med Hyg 87 : 109-116. ), which is mediated both by their compound eyes and ocelli ( Lazzari et al. 1998Lazzari CR, Reiseman CE, Insausti TC 1998. The role of the ocelli in the phototactic behaviour of the haematophagous bug Triatoma infestans . J Insect Physiol 44 : 1159-1162. , Reisenman et al. 1998Reisenman CE, Lazzari CR, Giurfa M 1998. Circadian control of photonegative sensitivity in the haematophagous bug Triatoma infestans . J Comp Physiol A 183 : 533-541. ). The sensitivity of behavioural responses mediating negative phototaxis in these insects varies in a circadian manner ( Reisenman et al. 1998Reisenman CE, Lazzari CR, Giurfa M 1998. Circadian control of photonegative sensitivity in the haematophagous bug Triatoma infestans . J Comp Physiol A 183 : 533-541. ) and the migration of screening pigments in both visual organs is also under the control of a circadian clock ( Reisenman et al. 2002Reisenman CE, Insausti TC, Lazzari CR 2002. Light-induced and circadian changes in the compound eye of the haematophagous bug Triatoma infestans (Hemiptera: Reduviidae). J Exp Biol 205 : 201-210. ). In contrast to their robust negative phototaxis, light sources become attractive to triatomines when adults initiate flight in search for new habitats, food and mates ( Noireau & Dujardin 2001Noireau F, Dujardin J-P 2001. Flight and nutritional status of sylvatic Triatoma sordida and Triatoma guasayana . Mem Inst Oswaldo Cruz 96 : 385-389. , Vazquez-Prokopec et al. 2004Vazquez-Prokopec GM, Ceballos LA, Kitron U, Gürtler RE 2004. Active dispersal of natural populations of Triatoma infestans (Hemiptera: Reduviidae) in rural northwestern Argentina. J Med Entomol 41 : 614. , Minoli & Lazzari 2006Minoli SA, Lazzari CR 2006. Take-off activity and orientation of triatomines (Heteroptera: Reduviidae) in relation to the presence of artificial lights. Acta Trop 97 : 324-330. ).

Insect visual pigments, as those of vertebrates, are photoreceptors composed of an opsin protein and a light-sensitive chromophore derived from retinal ( Terakita 2005Terakita A 2005. The opsins . Genome Biol 6 : 213. ). Since most organisms synthesise a single type of chromophore, the diversity of visual pigment absorption spectra essentially depends on the opsin structure. Thus, the spectral sensitivity of a given species is controlled by the type of opsins existing in its genome and expressed in the retina cells of its visual organs ( Briscoe & Chittka 2001Briscoe AD, Chittka L 2001. The evolution of color vision in insects. Annu Rev Entomol 46 : 471-510. ). R. prolixus offers an excellent model to study the molecular basis of vision in nocturnal arthropods. Characterisation of its visual receptor genes would enable understanding their relation to behaviour.

Little is known about the mechanisms of photoreception and visual processing in triatomines. Some behavioural studies suggest the ability of these bugs to discriminate between distinct spectral properties of visual stimuli ( Reisenman et al. 1998Reisenman CE, Lazzari CR, Giurfa M 1998. Circadian control of photonegative sensitivity in the haematophagous bug Triatoma infestans . J Comp Physiol A 183 : 533-541. , 2000Reisenman CE, Figueiras ANL, Giurfa M, Lazzari CR 2000. Interaction of visual and olfactory cues in the aggregation behaviour of the haematophagous bug Triatoma infestans . J Comp Physiol A 186 : 961-968. , Minoli & Lazzari 2006Minoli SA, Lazzari CR 2006. Take-off activity and orientation of triatomines (Heteroptera: Reduviidae) in relation to the presence of artificial lights. Acta Trop 97 : 324-330. , Reisenman & Lazzari 2006Reisenman CE, Lazzari CR 2006. Spectral sensitivity of the photonegative reaction of the blood-sucking bug Triatoma infestans (Heteroptera: Reduviidae). J Comp Physiol A Neuroethol Sens Neural Behav Physiol 192 : 39-44. ), but it remains unclear whether triatomine vision relies on chromatic and/or achromatic mechanisms ( Briscoe & Chittka 2001Briscoe AD, Chittka L 2001. The evolution of color vision in insects. Annu Rev Entomol 46 : 471-510. ). True colour vision depends on subtractive interactions between at least two photoreceptor types ( Menzel & Backhaus 1991Menzel R, Backhaus W 1991. Colour vision in insects. In P Gouras (ed.), Vision and visual dysfunction , The perception of color , Vol. 6 , Macmillan, London, p. 262-293. ), but so far the number of photoreceptor types in triatomine visual organs has not been reported. The study of opsin gene expression in triatomines could be combined with behavioural and neurobiological approaches to provide insights into the visual processing mechanisms displayed by these insects ( Figure ).

The expression of the triatomine behaviours listed so far is under a strict control of circadian clocks ( Lazzari et al. 2004Lazzari CR, Minoli SA, Barrozo RB 2004. Chemical ecology of insect vectors: the neglected temporal dimension. Trends Parasitol 20 : 506-507. ). This affects host search activity, egg hatching, moulting, the expression of thermopreference and the use of shelters ( Lazzari et al. 2004Lazzari CR, Minoli SA, Barrozo RB 2004. Chemical ecology of insect vectors: the neglected temporal dimension. Trends Parasitol 20 : 506-507. ). The study of the molecular bases of circadian rhythms has been one of the most elaborate areas in insect physiology in the last decades ( Sandrelli et al. 2008Sandrelli F, Costa R, Kyriacou CP, Rosato E 2008. Comparative analysis of circadian clock genes in insects. Insect Mol Biol 17 : 447-463. ). A complex network of genes controlling the circadian expression of insect behaviour, including main roles like those of clock , cycle , timeless and period ( Table ), has been characterised in Drosophila ( Sandrelli et al. 2008Sandrelli F, Costa R, Kyriacou CP, Rosato E 2008. Comparative analysis of circadian clock genes in insects. Insect Mol Biol 17 : 447-463. ). It is probable that orthologues of these and other clock genes exist in R. prolixus and their identity should be determined through bioinformatic searches. Likewise, their expression cycling profiles should be clarified to allow experimental studies on their control of relevant biological features of triatomine biology such as oscillations in olfactory sensitivity.

Motivation: modulation and plasticity - The levels of expression of locomotor activity in triatomines seem extremely plastic, varying from almost null in immature insects to intense in starved individuals. This particular aspect of triatomine behaviour makes their physiology a cyclic process of long starvation intervals interspersed with short gaps of blood repletion that trigger a cascade of neuroendocrine events promoting moulting. This discrete separation allows clearly associating experimental manipulation to gene expression alterations, establishing cause-effect relations not easily attributable for Drosophila or other classic models that feed continuously. The underlying physiological processes regulating locomotor activity have not been studied in detail and these may involve mechanisms regulating gene expression. In the last decade, the study of a gene named foraging ( for ), encoding a cGMP-dependent protein kinase, has allowed a better comprehension of the genetic basis of locomotion in several insects ( Reaume & Sokolowski 2011Reaume CJ, Sokolowski MB 2011. Conservation of gene function in behaviour. Phil Trans R Soc B 366 : 2100-2110. ). For example, individuals showing two distinct profiles of locomotor activity controlled by different alleles of the for gene, called rovers and sitters , exist in D. melanogaster ( Osborne et al. 1997Osborne KA, Robichon A, Burgess E, Butland S, Shaw RA, Coulthard A, Pereira HS, Greenspan RJ, Sokolowski MB 1997. Natural behavior polymorphism due to a cGMP-dependent protein kinase of Drosophila . Science 277 : 834-836. ). For locusts, changes in the expression levels of this gene are associated with behavioural shifts characteristically triggered at high population densities ( Lucas et al. 2010Lucas C, Kornfein R, Chakaborty-Chatterjee M, Schonfeld J, Geva N, Sokolowski MB, Ayali A 2010. The locust foraging gene. Arch Insect Biochem Physiol 74 : 52-66. ). In addition, variations in the expression of the for gene have been indicated to promote nurse honey bees to become foragers ( Ben-Shahar 2005Ben-Shahar Y 2005. The foraging gene, behavioral plasticity and honeybee division of labor. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 191 : 987-994. ). The existence, function and regulation of this gene in the R. prolixus genome and its potential relation to the ample regulation of locomotor activity in triatomines deserve to be explored.

Brain production of diverse neuropeptides has been described in R. prolixus and their roles have been related to the control of bug physiology ( Ons et al. 2009Ons S, Richter F, Urlaub H, Pomar RR 2009. The neuropeptidome of Rhodnius prolixus brain. Proteomics 9 : 788-792. ). Nevertheless, their impact on triatomine behaviour has not been analysed and their potential on the modulation of behaviour needs to be addressed. Many aspects of triatomine behaviour have been shown to be extremely plastic. Therefore, the study of the genetic and molecular bases of behaviour modulation in triatomines is necessary.

In recent years, several reports have shown that R. prolixus is capable of diverse forms of learning ( Vinauger et al. 2011aVinauger C, Buratti L, Lazzari CR 2011a. Learning the way to blood: first evidence of dual olfactory conditioning in a blood-sucking insect, Rhodnius prolixus . I. Appetitive learning. J Exp Biol 214 : 3032-3038. , bVinauger C, Buratti L, Lazzari CR 2011b. Learning the way to blood: first evidence of dual olfactory conditioning in a blood-sucking insect, Rhodnius prolixus . II. Aversive learning. J Exp Biol 214 : 3039-3045. , 2012Vinauger C, Pereira MH, Lazzari CR 2012. Learned host preference in a Chagas disease vector, Rhodnius prolixus . Acta Trop 122 : 24-28. , 2013Vinauger C, Lallement H, Lazzari CR 2013. Learning and memory in Rhodnius prolixus : habituation and aversive operant conditioning of the proboscis extension response. J Exp Biol 216 : 892-900. , Minoli et al. 2013Minoli SA, Palottini F, Manrique G 2013. The main component of an alarm pheromone of kissing bugs plays multiple roles in the cognitive modulation of the escape response. Front Behav Neurosci 7 : 77. ). These include simple habituation ( Vinauger et al. 2013Vinauger C, Lallement H, Lazzari CR 2013. Learning and memory in Rhodnius prolixus : habituation and aversive operant conditioning of the proboscis extension response. J Exp Biol 216 : 892-900. ), associative learning with positive or aversive rewards ( Vinauger et al. 2011aVinauger C, Buratti L, Lazzari CR 2011a. Learning the way to blood: first evidence of dual olfactory conditioning in a blood-sucking insect, Rhodnius prolixus . I. Appetitive learning. J Exp Biol 214 : 3032-3038. , bVinauger C, Buratti L, Lazzari CR 2011b. Learning the way to blood: first evidence of dual olfactory conditioning in a blood-sucking insect, Rhodnius prolixus . II. Aversive learning. J Exp Biol 214 : 3039-3045. ) and operant conditioning ( Vinauger et al. 2013Vinauger C, Lallement H, Lazzari CR 2013. Learning and memory in Rhodnius prolixus : habituation and aversive operant conditioning of the proboscis extension response. J Exp Biol 216 : 892-900. ). All these facts indicate that this species might represent a friendly model to study molecular aspects of learning. Memory related genes, such as CrebB ( Yin et al. 1994Yin J, Wallach J, Del Vecchio M, Wilder E, Zhou H, Quinn W, Tully T 1994. Induction of a dominant negative CREB transgene specifically blocks long-term memory in Drosophila . Cell 79 : 49-58. ) or stripe ( Lutz & Robinson 2013Lutz CC, Robinson GE 2013. Activity-dependent gene expression in honey bee mushroom bodies in response to orientation flight. J Exp Biol 216 : 2031-2038. ) have already been described in other insects and their characterisation in triatomines would allow a deeper understanding of behavioural plasticity.

Behaviour modulation due to parasite infection is another relevant issue not properly evaluated for triatomines. Parasite-host associations are unapparent elements in an ecological community and affect competition relations intra and inter species, the distribution and abundance of species and even community composition ( Horwitz & Wilcox 2005Horwitz P, Wilcox BA 2005. Parasites, ecosystems and sustainability: an ecological and complex systems perspective. Int J Parasitol 35 : 725-732. ). In this way hosts cannot be considered alone, as their relation with the environment and conspecifics necessarily includes their natural parasites. The ability of parasites to manipulate host phenotypes facilitating transmission is an important and well known paradigm in the study of the evolutionary biology of host-pathogen interactions ( Thomas et al. 2005Thomas F, Adamo S, Moore J 2005. Parasitic manipulation: where are we and where should we go? Behav Processes 68 : 185-199. , Moore 2013Moore J 2013. An overview of parasite-induced behavioral alterations and some lessons from bats. J Exp Biol 216: 11-17. ). Changes in vector behaviour have been reported in different systems ( Molyneux & Jefferies 1986Molyneux D, Jefferies D 1986. Feeding behaviour of pathogen-infected vectors. Parasitology 92 : 721-736. , Schaub 1989Schaub G 1989. Trypanosoma cruzi : quantitative studies of development of two strains in small intestine and rectum of the vector Triatoma infestans . Exp Parasitol 68 : 260-273. , 1992Schaub GA 1992. The effects of trypanosomatids on insects. Adv Parasitol 31 : 255-319. , Killick-Kendrick & Molyneux 1990Killick-Kendrick R, Molyneux DH 1990. Interrupted feeding of vectors. Parasitol Today 6 : 188-189. , Alekseev 1991Alekseev AN 1991. Group and individual behaviour of infected and noninfected arthropods - vectors of diseases 2, Zoological Institute, St Petersburg, 15 pp. , Hurd 2003Hurd H 2003. Manipulation of medically important insect vectors by their parasites. Annu Rev Entomol 48 : 141-161. , Lefevre & Thomas 2008Lefevre T, Thomas F 2008. Behind the scene, something else is pulling the strings: Emphasizing parasitic manipulation in vector-borne diseases. Infect Genet Evol 8 : 504-519. ). Generally, these alterations affect feeding behaviour, fundamentally when parasites are transmitted by vector bites, as in African trypanosome infected tsetse flies ( Jenni et al. 1980Jenni L, Molyneux D, Livesey J, Galun R 1980. Feeding behaviour of tsetse flies infected with salivarian trypanosomes. Nature 283 : 383-385. ), Leishmania -infected sandflies ( Beach et al. 1985Beach R, Kiilu G, Leeuwenburg J 1985. Modification of sand fly biting behavior by Leishmania leads to increased parasite transmission. Am J Trop Med Hyg 34 : 278-282. ) and Plasmodium -infected mosquitoes ( Ribeiro et al. 1985Ribeiro J, Rossignol P, Spielman A 1985. Aedes aegypti : model for blood finding strategy and prediction of parasite manipulation. Exp Parasitol 60 : 118-132. , Rossignol et al. 1986Rossignol P, Ribeiro J, Spielman A 1986. Increased biting rate and reduced fertility in sporozoite-infected mosquitoes. Am J Trop Med Hyg 35 : 277-279. , Koella et al. 1998Koella JC, Sörensen FL, Anderson R 1998. The malaria parasite, Plasmodium falciparum , increases the frequency of multiple feeding of its mosquito vector, Anopheles gambiae . Proc Biol Sci 265 : 763-768. ). R. prolixus has its feeding behaviour affected when infected by Trypanosoma rangeli ( Garcia et al. 1994Garcia E, Mello C, Azambuja P, Ribeiro J 1994. Rhodnius prolixus : salivary antihemostatic components decrease with Trypanosoma rangeli infection. Exp Parasitol 78 : 287-293. ). Authors suggest that this parasite interferes the synthesis of antihemostatic molecules during salivary gland infection, but Paim et al. (2013)Paim RM, Pereira MH, Araújo RN, Gontijo NF, Guarneri AA 2013. The interaction between Trypanosoma rangeli and the nitrophorins in the salivary glands of the triatomine Rhodnius prolixus (Hemiptera; Reduviidae). Insect Biochem Mol Biol 43 : 229-236. showed an unspecific reduction of stored proteins in the salivary glands in the presence of T. rangeli . Other behavioural effects on vectors, such as alterations in locomotory activity in Ae. aegypti infected by dengue virus ( Lima-Camara et al. 2011Lima-Camara TN, Bruno RV, Luz PM, Castro MG, Lourenço-de-Oliveira R, Sorgine MH, Peixoto AA 2011. Dengue infection increases the locomotor activity of Aedes aegypti females. PLoS ONE 6 : e17690. ) and changes in foraging and defecation in Mepraia spinolai infected by Trypanosoma cruzi ( Botto-Mahan et al. 2006Botto-Mahan C, Cattan PE, Medel R 2006. Chagas disease parasite induces behavioural changes in the kissing bug Mepraia spinolai . Acta Trop 98 : 219-223. ) have been eventually reported, but many aspects of infected vector behaviour are still unstudied.

In spite of the significant advances described here for triatomine behaviour studies, little is known about whether trypanosome infection affects bug behaviour. The few studies concerning behavioural modifications in triatomines infected by trypanosomes suggest that such alterations may occur ( Garcia et al. 1994Garcia E, Mello C, Azambuja P, Ribeiro J 1994. Rhodnius prolixus : salivary antihemostatic components decrease with Trypanosoma rangeli infection. Exp Parasitol 78 : 287-293. , Botto-Mahan et al. 2006Botto-Mahan C, Cattan PE, Medel R 2006. Chagas disease parasite induces behavioural changes in the kissing bug Mepraia spinolai . Acta Trop 98 : 219-223. ). Triatomines host several parasite species, such as T. cruzi , the causative agent of Chagas disease and T. rangeli. Alterations in triatomine behaviour caused by these parasites could affect their distribution or even increase parasite transmission rates. In case trypanosomes are capable of inducing changes in triatomine behaviour, the molecular mechanisms triggering this would need to be characterised.

The study of the molecular basis of behaviour in R. prolixus would allow a better understanding of fundamental features of vector biology. Besides, it would enable accessing relevant aspects of insect physiology mostly unknown for models other than classic holometabolous insects, like dipterans and moths. This would encourage comparative approaches to study insect sensory function, as well as cognition, at the molecular, cellular and organism level.

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