Observations on the biology and taxonomy of the eggs of Anopheles mosquitos

Abstract

A study of the egg shells of 11 species of Anopheles was made with the stereoscan electron microscope. A number of structures have been found that are useful in distinguishing between eggs of different species; all of them can also be seen with the light microscope. A list is given of these structures, together with others already known to be of taxonomic importance.The functional significance of the network of the outer layer of the chorion of Anopheles eggs is discussed. It is suggested that the film of gas held by the network is of selective advantage in that it contributes to the buoyancy of the egg, provides it with a part of its protective coloration, and functions as a plastron when the egg is submerged. The plastron of A. labranchiae atroparvus Van Thiel resisted wetting at an excess pressure of more than half an atmosphere.The relation of Anopheles eggs to the surface film is discussed. A highly stable line of contact with the water is formed, such that the water surface is depressed along the sides of the egg and raised at each end. Because like menisci are attracted and unlike ones repelled, eggs make contact with each other end to end or side to side, never end to side. Water spreads on most natural objects, forming menisci like those at the ends of the egg, which is thus provided with anautomatic trapping device that tends to attach the egg endwise to an object projecting from the water. The selective advantage of endwise over sideways attachment is not known, but it may facilitate hatching.The structure of the egg of A. pseudopunctipennis Theobald has been disputed, and it is therefore described in some detail.