Life-History Tactics of Rain-Pool Dwellers

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Abstract
Larvae of the midge Chironomus imicola are a principal inhabitant of rain pools in tropical Africa. Paradoxically they lack the ability to survive desiccation of this ephemeral habitat. A reduction in the duration of aquatic stages increases the chances of an individual emerging as a fly before the pool it inhabits dries, and periods from egg to adult may be as short as 12 days. Not all individuals reach adulthood at the same time, nor are emerging flies all of the same size. In the laboratory, emergence occurs over a 40-day period and females vary from about 100-500 .mu.g in weight. Adult female size relates positively to number of ovules, and to flight duration in the laboratory, i.e., to fecundity and dispersal ability. Since pools are ephemeral, dispersal is necessary to provide offspring with a chance of survival; hence, larger females are more fit than smaller ones. As with many insects, size distribution over the whole emergence period is sensitive to the density at which individuals are reared. Because pools evaporate, population density changes conspicuously during development. Laboratory experiments suggest that patterns of emergence are associated with high and low densities, adaptively appropriate to the corresponding stage of evaporation. At initial low densities, safe, early emergence produces small females; increased risk for larvae emerging late is compensated for by achieving larger and therefore fitter adults. At high density, when habitat extinction is imminent, females emerge earlier but are also larger. Females both large and early are unexpected but have a high risk, only about 2% of larvae reaching adulthood. The adaptive value of the life style is compared with a species able to survive desiccation, Polypedilum vanderplanki. The latter has a long larval life, significantly less size variation, an inability to survive high larval population densities and a poor dispersal ability.