Erratic Hatching in Aedes Eggs: A New Interpretation

J. D. Gillett, E. A. Roman, V. Phillips


Erratic hatching of eggs in Aedes aegypti (and other aedine mosquitoes) has long been something of an enigma, although the temporal scatter it provides, when coupled with drought resistance, is advantageous to species utilizing small, temporary breeding places. It is well known that eggs of Aedes hatch when the amount of dissolved oxygen (p$_{\text{O}_{2}}$) in the surrounding water is lowered, as when the medium becomes contaminated with bacteria. But erratic hatching in small instalments may occur in the absence of any obvious stimulus. We have re-examined this problem, postulating that the eggs themselves harbour microbial colonies and that these, by lowering the p$_{\text{O}_{2}}$ in the microenvironment of each egg, quantitatively influence hatching. When laid the eggs are sterile, but each acquires surface colonies of bacteria, which they pick up from the environment. By rearing each egg in isolation we have shown that the number of bacteria acquired in a given time differs markedly from egg to egg and, as predicted, those with the highest counts hatch first. Under natural conditions, with eggs of different ages massed together, the first larvae to hatch browse over the surfaces of the remaining eggs, presumably consuming the bacteria and thus lowering the likelihood of further hatching. Thus, in this simple fashion, and not, it seems, by means of some hypothetical pheromone (as previously suggested), the first eggs to hatch influence the prospects of the other eggs, ensuring that a proportion of them remain unhatched, thus serving as a reserve for the future.