Larval Identification and Instar Association in Some Species of Hydropsyche and Cheumatopsyche (Trichoptera: Hydropsychidae)

Abstract

Features of hydropsychid larvae are described which allowed coexisting species in southern Ontario to be distinguished while in the 1st 3 instars; these young stages cannot usually be identified by existing taxonomic keys. Cheumatopsyche differed from Hydropsyche in having a long tapered seta on each antero-lateral corner of the pronotum. Species of Hydropsyche were distinguished according to the length and degree of tapering of the pronotal seta, by background color of head and thoracic sclerites, by round spots and oval patches on the dorsum of the head capsule, and by abdominal setae and scale hairs. After species had been differentiated, measurements of head widths showed that successive instars increased in size in a regular geometric progression (Dyar's Rule). The factor of increase was ca. 1.5 in all species. The use of Dyar's Rule is suggested as a taxonomic tool to associate young instars of coexisting hydropsychids with the more easily identified older instars.