Embryonic Development

Abstract

Embryonic development begins with the first mitotic division of the zygote nucleus and terminates at hatching. Not surprisingly, in view of their diversity of form, function, and life history, insects exhibit a variety of embryonic developmental patterns, though, as more knowledge is obtained, certain evolutionary trends are becoming apparent. Eggs of most species contain a considerable amount of yolk. In exopterygote eggs there is such a preponderance of yolk that the egg cytoplasm is readily obvious only when it forms a small island surrounding the nucleus. In eggs of endopterygotes, the yolk/cytoplasm ratio is much lower than that of exopterygotes and, as a result, the cytoplasm can be seen as a conspicuous network connecting the central island with a layer of periplasm lying beneath the vitelline membrane. This trend toward reduction in the relative amount of yolk in the egg, carried to an extreme in certain parasitic Hymenoptera and viviparous Diptera (Cecidomyiidae), whose eggs are yolkless and receive nutrients from their surroundings, has some important consequences. Broadly speaking, the eggs of endopterygotes are smaller (size measured in relation to the body size of the laying insect) and develop more rapidly than those of exopterygotes. The increased quantity of cytoplasm leads to the more rapid formation of more and larger cells at the yolk surface which, in turn, facilitates the formation of a larger embryonic area from which development can take place. Compared with that of exopterygotes, development of endopterygotes is streamlined and simplified. There has been, as Anderson (1972b, p. 229) puts it, “reduction or elimination of ancestral irrelevancies,” which when taken to an extreme, seen in the apocritan Hymenoptera and cyclorrhaph Diptera, results in the formation of a structurally simple larva which hatches within a short time of egg laying. However, superimposed on this process of short-circuiting may be developmental specializations associated with an increasing dissimilarity of juvenile and adult habits.