n. The process of hatching
This has been observed only in a few cases, and careful observations as to the exact manner in which the embryo breaks the egg-shell and frees itself from the amnion are much needed. Also the rapid changes of form from that of the embryo within the egg-shell, and that which it immediately assumes after breaking forth from the shell and membranes, have yet to be observed; for these will undoubtedly be found to have special phylogenetic significance. Indeed, the phylogenetic importance of the latest embryonic changes in insects just entering on the nymph or the larval stages is very great, though little attention has as yet been bestowed upon the matter.
As regards the changes at the time of hatching, Wheeler tells us that the cockroach (Phyllodromia), shortly after leaving its narrow place in the egg-capsule, undergoes a peculiar change in shape. Before hatching, and when confined in the egg-shell, the body is about one-third as wide as thick; but soon after breaking out of the chorion its body is much flattened, its dorso-ventral diameter being only about a third as great as its greatest breadth. This shows that the flattened shape of the body of cockroaches, which adapts them for their life under bark and stones, is a very late inheritance, and that these insects have descended from those with more cylindrical bodies. The end of the body, also, which in the egg is bent underneath the abdomen, is, after hatching, bent dorsally, as indicated by the anal stylets, which now point directly upwards and outwards. The spines and claws are developed shortly before hatching. In the Locustidæ (Xiphidium, etc.) Wheeler has observed that the pleuropodia, or 1st pair of abdominal temporary embryonic appendages, are shed during hatching. All the other embryonic appendages have also disappeared, except those which persist and have rapidly become modified to form the cercopods, or the ovipositor.
In locusts, as we have observed[[85]] in the case of Melanoplus spretus, the egg-shell bursts open at the head end, when the nymph, immediately after extricating itself from the egg, casts off a thin pellicle (the amnion), as we have also noticed in the case of the larvæ of the flea, currant saw-fly, and other insects. Before the amnion is cast off, the young nymph is almost motionless, but by slight movements of the body draws itself, in about five minutes, out of the amnion. The exact process of extraction is as follows: While it lies motionless, it puffs out the thin, loose skin connecting the back of the head with the front edge of the prothorax. The distention of this part probably ruptures the skin, which slips over the head, the body meanwhile curved over until the skin is drawn back from the head; when the latter is thrown back, it withdraws its antennæ and legs, and the skin is in a second of time pushed back to near the end of the abdomen; finally, it draws its hind tarsi out of the skin, and in a moment or two more the young locust frees itself, kicks away the cast skin, which resembles a little white crumpled pellet, and which has also been compared to a diminutive mushroom, and walks actively off,—sometimes, however, with the cast skin adhering to the end of the abdomen. Before the shedding of the amnion the body and legs are soft and flabby; immediately after, it walks firmly on its legs. All the eggs hatched—at least one or more hundreds—at about the same time, i.e. before 11 A.M.
Fig. 553.—Locust just before the amnion is cast, enlarged.—Emerton del.
The nymph of Stagmomantis carolina also sheds an amnion-skin, like that of the locust; but the embryo before casting it off is much elongated, and probably, like the European Mantis religiosa, the curious elongated embryos have the same singular habit of suspending themselves by threads, as shown in Fig. 554.
The account by Pagenstecher of the first ecdysis of the European Mantis was so extraordinary that we asked Professor Cockerell to collect the eggs of our Stagmomantis in New Mexico and send them to us. This he has kindly done, writing that he can “hardly recognize a true moult, since all that is cast off is the egg-membrane. In short, Pagenstecher’s account must be not a little fanciful, unless our insect differs very much in its development from Mantis religiosa. The main change is that after leaving the egg the thorax enormously elongates, producing a bulging out, and thrusting the head forward.” Our observations on the alcoholic specimens fully corroborate Cockerell’s conclusions. Pagenstecher’s figure of the embryo appears to be inaccurate. Sharp states that the hatching nymphs remain suspended for some days until the “first change of skin is effected.” This so-called “skin” is evidently the amnion.
The 17–year Cicada, after hatching, is enveloped by the amnion, from which it soon extricates itself, and then drops deliberately to the ground, “its specific gravity being so insignificant that it falls through the air as gently and as softly as does a feather.” (Riley.)
Other insects, as caterpillars, have room enough to turn around within their shell and to eat their way through the walls of the chorion.