Cryptotendipes

Saether, Ole A., 2010, Cryptotendipes Lenz from Manitoba, Canada, with keys to known immatures of the genus (Diptera: Chironomidae), Zootaxa 2412, pp. 1-20 : 18-19

Cryptotendipes species in Lake Winnipeg

Amongst the five Cryptotendipes species occurring in Lake Winnipeg C. darbyi and C. casuarius both are very common and distributed all over the lake ( Fig. 1 ). The larvae of C. darbyi are most common in the South Basin, while C. casuarius is more common in the North Basin. The remaining three species of the genus are represented by a few specimens with only a pupa of C. pseudotener present in the bottom samples ( Fig. 1 ). Imagines of C. casuarius and C. darbyi were present during the full sampling period with from mid June to mid September, while C. emorsus and C. pilicuspis were caught in June–July, and C. pseudotener in October. C. tuberosus from Southern Indian Lake was collected in June.

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) question 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. The inverse of 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 %.

In a recent paper on Cryptochironomus from Lake Winnipeg ( Saether 2009) found that in populations of Cryptochironomus stylifera (Johannsen) the median of the head capsules of third instar were 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; in C. digitatus (Malloch) the same ratios were 62% and 62.5 %; in C. ramus Mason 61 % and 60 %. In Lake Winnipeg populations of Cryptotendipes casuarius the median of the head capsules of third instar are 61 % as long as the median of the head capsules of fourth instar and the head capsules of second instar 66 % as long as in third instar ( Fig. 8 ); in C. darbyi the same ratios are 56% and 70 %. However, too few second instar larvae were collected to make the relation between head capsule lengths of second and third instars reliable.

Figures

FIGURE 1. Map of Lake Winnipeg showing sampling stations and the larval distribution of Cryptotendipes darbyi (Sublette) and Cryptotendipes casuarius (Townes) in the lake.

FIGURE 8. Head capsule lengths of different instars of Cryptotendipes darbyi (Sublette) and Cryptotendipes casuarius (Townes) in Lake Winnipeg.