The postoral appendages of the head are the mandibles and the 1st and 2d maxillæ, besides the supposed premandibular segment already referred to on pp. 50–54, which only temporarily exists.
The trophi or oral appendages are all alike at first, but soon differ in shape, acquiring their characteristic form shortly before the embryo leaves the egg. The mandibles of Œcanthus are said by Ayers at the time of revolution of the embryo to be slightly bilobed, and in his Fig. 5, Pl. 19, they are represented as deeply trilobed, but in general they are undivided. The 1st maxillæ are at this time distinctly trilobed. The 2d maxillæ are separate, and distinctly though unequally bilobed, becoming united shortly before birth. In the embryos of dragon-flies they are at an early date very large and long, and directed backwards, and are not fused together until just before hatching, when the extraordinary mask-shaped labium is fully developed.
Fig. 528.—Two embryonic stages of the primitive streak of Melolontha. A, younger stage, with rudiments of eight pairs of abdominal appendages (a1–a8). B, older stage, the primitive band now very broad: a, 1st abdominal appendage, in B sac-like; x, place of adhesive disc; g, brain; l, clypeo-labrum; s, lateral cord of the ventral nervous cord; other lettering as in previous figures.—After Graber, from Korschelt and Heider.
The distal parts of the labium, such as the ligula, palpifer, and palpus are elaborated before the mentum and submentum. Many details as to the final changes in the mouth-parts before hatching remain to be worked out.
The thoracic appendages.—The three pairs of legs arise at the same period and in the same manner in all insects; it is not until the end of embryonic life that they become jointed, and that the claws and onychia are developed. Especial attention has not yet been given to the details of the development of the parts of the last joint of the tarsus.
In many forms the antennæ are the first to appear, the mandibles, maxillæ, and legs appearing at a latter date, though simultaneously. It is thus in Stenobothrus, Hydrophilus, and Melolontha. In Lina, according to Graber, the mandibles precede the antennæ in appearance. In the Libellulidæ, according to Brandt, the legs first appear, then the jaws, and lastly the antennæ. This did not seem to be the case in the embryos of Æschna observed by us, although our observations were more superficial.
On the other hand, in those insects whose larvæ are footless, the rudiments of the legs are retarded and aborted just before hatching (fossorial Hymenoptera and Apidæ), or the rudiments of the legs are not developed at all.
The abdominal appendages.—These appear soon after the thoracic limbs, corresponding in most cases to the latter in shape and position, and their position in the embryo is a matter of the greatest interest. Von Rathke was the first embryologist to detect those of the first abdominal segment, in his examination of the development of Gryllotalpa. Long afterwards Bütschli detected them in the embryo of the honey-bee, observing a pair on each segment. Patten observed them in Trichoptera; Kowalevsky first perceived them in Lepidoptera, Tichomiroff confirming his observations. Graber, Ayers, and Wheeler have observed them in Orthoptera and Coleoptera, and the latter has detected them also in Hemiptera and Neuroptera; and while they do not arise in the embryos of Diptera and of Siphonaptera, they are to be looked for in any or all the lower or more generalized orders.
As the result of these discoveries of polypodous embryos occurring in all but the most specialized order (Diptera), it appears to be a rational deduction that the winged insects have descended from insects in which there were functional legs on each abdominal segment. Such an ancestor was the forerunner of the Thysanura, in which abdominal locomotive appendages still survive, though in a modified, more or less aborted condition. This polypodous ancestral form was apparently allied to Scolopendrella, which has a pair of functional legs on each abdominal segment.