Utility of larval instar, size and development data for recognition of cohorts in a merovoltine Chironomus population
Frequency distributions of larval instMs or body lengths are inadequate to separate cohorts of a profundal Chironomus population. A developmental index, in conjunction with body length or weight, permits segregation of 2 discrete groups of larvae that coexist in instM IV both before and after the single, annual emergence period. A 3-year life-cycle is proposed for this population, with inStM IV reached after 1 year, and the remaining 2 years spent-in this final instM. Among 2,yearold larvae, females are typically longer than males. Larvae,appear to gain little length over the third year, although females may gain weight. Considerable’maturation toward pupation occurs in the final year, permitting use of a developmental-phase index for separation of emerging and nonemerging cohorts.