Lake outlet blackflies - the dynamics of filter feeders at very high population densities

Larvae of the blackfly Simulium noelleri aggregated at very high population densities (up to 1.2 × 10⁶ individuals m^?2) at a lake outlet in Kent, United Kingdom. During 1983 and 1984 their first appearance in these large numbers was in late-June and they completed three summer generations before the overwintering larval generation appeared in October. It is not known where the larvae overwinter but they recolonized the concrete steps of this outlet in May, together with larvae of the S. ornatum group which, however, were not found after completing one generation at this location.
Female flies from the overwintering generation oviposited en masse during late-June and the result was a well-synchronized growth of larvae in the first summer generation. Within this, and other generations, there was A wide range of emergence times for adults; they could emerge early and were then relatively small, or could emerge later and were then relatively large. Females were always larger than males and the emergence of flies was protandrous. A very similar pattern of growth and emergence times was found at a site in Finland.
In all generations, sex ratio was biased to males and the sex ratio in each generation was inversely correlated with population density. This ensured that there were sufficient males emerging, and surviving adult mortality, to guarantee fertilisation of the females which were more expensive to produce.