Fig. 501.—Micropyles: a, of Nepa cinerea; b, of Locusta viridissima; c, of a bug (Pyrrhocoris apterus).—From Gerstäcker.
The micropylar region is generally, at least in Orthoptera and Odonata, covered by a gelatinous cap (Figs. 499 and 500, gs), which may form a covering membrane which extends over a large part of the egg, or may envelop the entire outer surface. In some cases micropyles are scattered over the entire surface of the egg, but usually the perforation is situated at the end, and is often guarded by raised processes, either one or several, like bristles, or toadstools, etc., these being especially characteristic of the eggs of certain Hemiptera (Nepa, Fig. 501, a, and Ranatra), or the region is variously sculptured, as in the eggs of butterflies. In the micropylar apparatus of Œcanthus the papillæ have a distinct lumen (Fig. 499), or a channel for the ingress of the male filament.
Fig. 502.—Diagrammatic median section through egg of Musca in stage of fertilization (incorporating the figures of Henking and Blochmann): ch, chorion; d, dorsal; v, ventral side of the egg; dh, yolk-membrane; do, nutritive yolk; g, gelatinous cap over the micropyle (m); K, outer layer of plasma (Keimhautblastem); p, male and female pronucleus before copulation; r, directive body (Richtungskörper).—After Korschelt and Heider.
Another use of the micropylar apparatus noticed by Ayers in the egg of the tree-cricket is that it “serves as a thick, roughened plate, against which the insect may push when ovipositing, without injury to the egg, and without danger that the ovipositor slips from its place.” In Chrysopa eggs the micropyle forms a conspicuous button-like knob, resembling the finely milled head of a certain kind of screw.
Internal structure of the egg.—The egg-contents are surrounded by an outer layer of protoplasm or formative yolk, which is separate from the inner parts of the egg (Fig. 502, do), the latter being mostly composed of the nutritive yolk-element. The superficial protoplasmic layer, called by Weismann Keimhautblastem (K) is, in a few cases, afterwards entirely lost, but in most instances forms a very thin layer of clear protoplasm, slight in extent compared with the yolk-mass within.
The eggs of insects are rich in yolk, only certain eggs, such as those of the Aphides and the egg parasites (Proctotrypidæ) being poor in yolk. The eggs of heterometabolous insects have been said by Brauer to contain relatively more yolk than those of the Metabola, particularly the Diptera; though, as Wheeler observes, this rule has some exceptions, the eggs of the 17–year Cicada being very numerous and small.
This he thinks is a greater advantage to the insect than the production of a few large eggs, “when we consider the extremely long period of larval life and the vicissitudes to which the larvæ may be subjected during all this time.” “Similarly, Meloë angusticollis produces a large number of very small eggs, while the eggs of the smaller beetles (Doryphora, e.g.) are much larger. But Meloë is a parasitic form, and probably only a few of its many offspring ever succeed in gaining access to the egg of the bee.”
In the eggs of Chrysopa the yolk-granules are remarkably small, so that the primitive band is in strong contrast to the yolk in color and density. When crushed, the yolk does not flow out as a liquid, but in a pasty mass, and we have questioned whether, as in the eggs of Limulus, whose yolk is solid with fine granules, the denseness of the yolk is not connected in the way of cause and effect with their exposed situation.
The central or yolk-mass (Fig. 502, do) consists chiefly of rounded masses of yolk, with fat-globules, between which extends a fine network of protoplasm.