The meat-fly, as we have observed, hatches in the following manner. The embryo moves to and fro, the body twisting until the exochorion is ruptured; the egg-shell splits longitudinally, and in one or two seconds the larva pushes its way out through the anterior end, and in a second or two more extricates itself from the shell. The latter scarcely changes its form, and the larva slips out, leaving the amnion within.
Fig. 554.—Egg-case of Mantis with young escaping: A, the case with young in their position of suspension. B, cerci magnified, showing the suspensory threads.—After Brongniart, from Sharp.
In the case of a fossorial wasp, Specius speciosus, which carries Cicadæ into its burrow, laying an elongated egg on the body under the median thigh of its victim, the larva on hatching, Riley states, “does not emerge from the skin of the egg, but merely protrudes its head and begins at once to draw nourishment from between the sternal sutures of the Cicada.”
The hatching spines.—Animals belonging to quite distinct classes are provided late in embryonic life with hard knobs or spines, which are temporary structures for the purpose of breaking or cutting open the egg-shell, when it is too thick and solid to be ruptured by the movements of the embryo. The embryos of certain lizards, turtles, the blind worm and some snakes, of the crocodile, and even birds, as well as the duckbill and Echidna, are provided with them, always occurring, so far as we are aware, on the end of the upper jaw. In the Arthropoda similar structures have thus far only been met with in myriopods and insects, though an analogous structure on the cephalothorax of the embryo of phalangids has been observed by Balbiani. Metschnikoff describes and figures a low conical spine serving this purpose situated on the embryonal cuticle over the head of the advanced embryo of Strongylosoma, and one on the 3d pair of mouth-parts of Geophilus.
In the winged insects, the embryo of Forficula is said by Heymons to bear a single spine between the eyes, which serves as an egg-tooth. The embryo of the Hemerobiidæ, according to Hagen, “opens the egg with an egg-burster like a saw.” (Proc. Bost. Soc. Nat. Hist., xv, p. 247.) Riley states that the egg-burster, or ruptor ovi, as he calls it, of Corydalus cornutus, has “the form of the common immature mushroom,” and he adds that it is a part of the amnion, being “easily perceived on the end of the vacated shell.” Wheeler has observed three pairs of broad-based chitinous “hatching spines” used by Doryphora in rupturing its embryonic envelopes, and which are secreted by pyramidal thickenings of the hypodermis (Figs. 555, 556).
Fig. 555.—The three pairs of hatching spines (hsp) on the late embryo of Doryphora.—After Wheeler.
Fig. 556.—Rudiment of the hatching spine: eb, being a thickening of the ectoderm (ec) in embryo Doryphora after formation of the heart; s, serosa.—After Wheeler.