Cryptochironomus

Cryptochironomus species in Lake Winnipeg

Amongst the seven Cryptochironomus species occurring in Lake Winnipeg three are very common and distributed all over the lake (Fig. 1). The larvae of C. ramus are most common outside Saskatchewan River, an area with moderate oligotrophy but with rich sediments due to previous pollution (Saether 1979). C. digitatus is evenly distributed all over the lake, while C. stylifera is scarce in the Narrows. The remaining four species of the genus are represented by few specimens with only C. blarina with more than one larva present in the bottom samples (Fig. 1). Imagines of all the common species were present during the full sampling period with most specimens caught between mid June to mid July (Chang et al. 1994).

Dyar (1890) studied 28 species of Lepidoptera larvae and reported that width of the head capsule consistently increased by a factor of 1.4 at each molt. This ratio has become known as Dyar's Rule. Soponis & Russell (1982) questions the usefulness of applying Dyar’s rule to chironomid larvae since the existence of four larval instars is well established in chironomids and the groupings of instars obvious. However, the rule is most useful when several similar species are present in the same locality and especially the early instars difficult to separate, as is the case for Cryptochironomus in Lake Winnipeg. The inverse of as Dyar's Rule or 0.71 is more useful in order to identify earlier instars. McCauley (1974) did a study of the instar differentiation of larval chironomids and gave the head capsule length and width of about 40 species. The head capsule length of third instar as a percentage of the head capsule length of the fourth instar calculated from his tables varies between 56 and 71 %, with a mean of 63 %. The head capsule length of second instar as a percentage of the head capsule length of the third instar varies between 52 and 79 %, with a mean of 64 %. The variation probably would be less if the median instead of the mean were used. However, some species such as Pagastiella sp. A from Marion Lake, British Columbia (McCauley 1974 fig. 5C), and Diamesa valkanovi Saether (Saether 1968) from Finse, Norway, clearly have a higher than normal growth ratio. Both species are without a clear division between any two consecutive instars, and both have a mixture of one and two year generations. In Lake Winnipeg populations of C. stylifera the median of the head capsules of third instar are 57 % as long as the median of the head capsules of fourth instar and the head capsules of second instar 57 % as long as in third instar (Fig. 9); in C. digitatus the same ratios are 62% and 62.5 %; in C. ramus 61 % and 60 %. This facilitates the recognition of early instars as shown in Fig. 9.