Some Causes of Variation in Estimates of Mean Weights for Hydropsyche Larvae (Trichoptera: Hydropsychidae)

Abstract

I present monthly values of mean dry weight per larval instar for 1 yr for Hydropsyche betteni Ross, H. bronta Ross, H. morosa Hagen, H. slossonae Banks, and H. sparna Ross. Larvae of each species were collected from two or four sites in the Credit and Number rivers of southern Ontario. Month-to-month changes in mean weight were more irregular in spring, summer, and autumn than in winter; summer values tended to have the highest standard errors. When growth was fast in summer, any one instar was constantly receiving low-weight individuals from the previous instar while losing heavier individuals to the next instar; successive monthly collections were sampling instars at various stages in the recruitment–loss continuum. Sexual differences were additional causes of wide ranges in weight in instar V; larvae destined to produce female pupae could be twice as heavy as those developing into male pupae. Because metamorphosis into males tended to precede that into females, heavier potentially female larvae were sometimes the only individuals collected during certain prepupal periods in summer. A mixture of cohorts and generations in summer also contributed to high variance. Winter weights, in contrast, were less variable from month to month and tended to have a smaller range per instar. When a particular instar was present in winter (December–March), its mean winter weight closely approximated the overall annual mean.