Sexual differences in development and behaviour of larval Ischnura verticalis (Odonata: Coenagrionidae)

The emergence period of Ischnura verticalis from a pond in southern Ontario lasted at least 97 days; the sex ratio of emerging larvae was not significantly different from 1:1. A 17-month study on larvae from the same pond indicated that the life cycle was univoltine and that male larvae tended to develop faster than female larvae. Analysis of instar distributions of larval I. verticalis collected from a series of ponds also indicated that males were in more advanced instars than females. In the laboratory, male I. verticalis larvae in the final instar developed faster than female larvae in the final instar but male and female larvae in the penultimate instar developed at approximately the same rate. Male larvae in the antepenultimate instar consistently spent more time moving and crawled farther than female larvae in the antepenultimate instar. Sexual differences in larval development and behaviour could help explain sexual differences in instar distributions and altered sex ratios at emergence.