Fig. 527.—Rudiments of the appendages of the embryo of Hydrophilus: an, antenna; md, mandibles; mx1, 1st, mx2, 2d maxilla; vk, clypeal region; m, mouth; p1-p3, legs; p4-p9, rudiments of abdominal appendages, 1–9; st, stigma; a, anus.—After Heider, from Lang.
Usually at the time of origin of the stomodæum a projection arises at the anterior edge of the primary head-region, the so-called forehead (Fig. 527, vk), which is the common rudiment of the clypeus and labrum. In many cases (certain Coleoptera and Lepidoptera) these rudiments first assume the form of paired hooks (see Figs. 83, 102, 104, 105, of Graber’s Keimstreif der Insekten, also Figs. 529 and 546), which afterwards, by fusion in the median line, become single, though notched in the middle; but in the more generalized Blatta and Mantis, as well as in bees, the rudiment is single at the outset.
The view advanced by Patten, and also by Carrière, that the labrum is a first pair of antennæ, is scarcely tenable, and we quite agree with Korschelt and Heider in regarding the clypeo-labral region as homologous with the upper lip of Crustacea, and, we may add, of Merostomes and of Trilobites.
It should be observed that in many insects, in their earlier embryonic state, directly behind the mouth arises, from paired rudiments, what seem provisional lower lip structures (not to be confounded with the 2d maxillæ of insects). This under lip structure was first discovered by Bütschli in the bee (his inner or 2d antennæ), and afterwards by Tichomiroff in Lepidoptera. Heider, in his work on Hydrophilus, describes it as the “lateral mouth-lips,” while, more recently, Nusbaum has observed it in Meloë. This under lip structure may be regarded as analogous to the paragnaths of Crustacea, although to attempt to homologize it with these seems useless. (Korschelt and Heider.)
Completion of the head.—Sufficient attention has not been paid to this subject by embryologists. The head is at first, dorsally, mostly composed of the head-lobes, or antennal segment only, and the dorsal or tergal portion of the oral appendages develop at a later period. We have observed in the embryo of dragon-flies (Æschna) that the tergites of the mandibles and first maxillæ are simultaneously fused with the head-lobes, while the much larger tergal region of the 2d maxillæ remains for some time separate from the anterior part of the head, and is continuous with the thoracic segments, and it is only just before hatching that this segment becomes fused with the rest of the head (Fig. 36). In a sense, the 2d maxillary segment when it is free from the head reminds us of the foot-jaw, or 5th segment of chilopod myriopods (see also p. 53).
g. The appendages
As we have seen, nearly or quite simultaneously all the limbs as a rule bud out from each side of the median line of the primitive band. They arise as saccular evaginations or outgrowths of the ectoderm, directed a little backwards. They are at first filled with mesoderm cells, and in the Orthoptera diverticula of the cœlom-sac are taken up into the rudimental limbs, as in Peripatus and Myriopoda. (Graber, Cholodkowsky.) As the antennæ, mouth-parts, legs, and abdominal appendages are all alike at first, their strict homology with one another is thus demonstrated. In insects never more than a single pair of limbs is known to arise from one segment.
The cephalic appendages.—The antennæ evidently arise from the hinder edge of the procephalic lobes (Fig. 527, an). As in Limulus, the first pair of appendages are at first postoral (Fig. 528, at), afterwards moving forward owing to changes in the relative proportions of the parts of the head, and they are in all respects, in their development and position in relation to the segment from which they arise, homologous with the appendages succeeding them.
The occurrence of rudiments of a pair of preantennal appendages in Chalicodoma which is claimed by Carrière, needs confirmation, as other embryologists have not observed them.