The subject, then, of polypodous embryo insects is one of special interest, and has attracted much attention from Graber, Wheeler, Haase, and others. That these are genuine, though transitory appendages, is shown by the fact that certain pairs persist throughout adult life. The embryology of the Thysanura when worked out will throw much light on this subject, but we know that the spring (elater) of Collembola (and possibly the collophore) and the cerci of the winged insects are survivals of these limbs. That the three pairs of appendages forming the ovipositor, or sting, are most probably derived from these appendages is claimed by Wheeler (p. 167), and seems proved by the fact that Ganin and also Bugnion has detected three pairs of imaginal disks in the embryo of parasitic Hymenoptera. Hence the abdominal appendages may ultimately be found to arise in nearly all cases from imaginal disks like those giving origin to the cephalic and thoracic appendages.
As regards the Diptera, Pratt has observed that each of the three thoracic and eight abdominal segments of the embryo brachycerous Diptera (seen especially well in Melophagus) has two pairs of imaginal disks, a dorsal and a ventral pair. He thinks there is no doubt but that the ventral abdominal disks are homologous with the rudimentary appendages which appear in the embryos of all other insects, though not in the brachycerous dipters.
Appendages of the first abdominal segment (pleuropodia).—As early as 1844 Rathke observed in the embryo of the mole-cricket a pair of appendages on the 1st abdominal segment, which he described as mushroom-shaped bodies, and supposed to be embryonic gills. They are called pleuropodia by Wheeler, who, with Patten, Graber, and Nusbaum, ascribes a glandular function to them, while Wheeler suggests that they were odoriferous repugnatorial organs. In Blatta (Phyllodromia) they are of large size, in Melolontha enormous (Fig. 528, B) and filled with blood. Wheeler distinguishes as varieties, beside the mushroom-shaped appendages of Gryllotalpa and Hydrophilus, the reniform (Œcanthus), the broadly pyriform (Blatta), and the elongate pyriform (Mantis carolina). In the European Mantis they are most limb-like, with a digitiform continuation divided by a constriction into two sections. (Graber.) In Meloë they assume the shape of a stalked cup. (Nusbaum.) In the bee and in Lepidoptera the pleuropodia are not present, though the temporary appendages on the succeeding segments appear; Carrière, however, found them on the two first abdominal segments of very young larvæ of the wall-bee (Chalicodoma).
Their cellular structure is peculiar, and they are either formed by evagination or invagination, those of the latter type being subspherical and solid. Those of the former type have a cavity communicating by means of a narrow duct through the peduncle with the body-cavity (Blatta). No tracheæ, nerves, or muscles enter them, though blood-corpuscles have been seen in the cavities. “In some species the pleuropodia produce a secretion from the ends of their enlarged cells. This secretion may be a glairy albuminoid substance (Cicada, Meloë), a granular mass (Stenobothrus), a bundle of threads (Zaitha), or a thick, striated, cuticula-like mass (Acilius).” They attain their greatest size during the revolution of the embryo, and they are “mere rudiments of what were probably in remote ages much larger and more complex organs.” (Wheeler.)
Lameere has observed that in Phyllodromia the first pair of abdominal appendages, after becoming of considerable size, undergo an enlargement at their free end, become detached, and fall into the amnion.
Wheeler also calls attention to the homology of these pleuropodia with the 1st abdominal appendages of Campodea, shown by Haase to be originally glandular, but with at present a respiratory function. In the embryos of later, higher orders of insects, these appendages are in size and shape similar to those of the succeeding segments. (See also p. 164.)
Fig. 529.—Primitive band of Bombyx mori, showing the temporary legs on abdominal segments 2–11: A, early stage, in which the abdominal legs al2–al10 appear. B, later stage, when they are very faint and all except al3–al6 and al10 are about to disappear. C, the persistent abdominal legs al3–al6 and al10; st2, st9, the 2d and 3d pair of stigmata; sgl, silk duct.—After Tichomiroff.
Are the abdominal legs of larval Lepidoptera and phytophagous Hymenoptera true limbs?—The presence of these abdominal legs in the embryos of Sphinx (Kowalevsky), of Bombyx mori (Tichomiroff), and both Bombyx mori and Gastropacha quercifolia (except those of the first segment), as well as in Hylotoma, which has 11 pairs of such appendages, has suggested that the prop or prolegs of caterpillars and saw-fly larvæ are survivals of these outgrowths, and not secondary, adaptive structures. Opinions on this point vary. Balfour, and also, more recently, Cholodkowsky, hold that the prolegs are survivals of the embryonic appendages. Graber cautiously, after a lengthy and interesting discussion, says that the question cannot be, in the present state of our knowledge, solved. He, however, seems inclined to believe that the prolegs are not merely secondary structures, and that the rudiments of limbs may remain for a long time in a latent state before their final development. Korschelt and Heider are disposed to regard the abdominal appendages of Lepidoptera and Hymenoptera as true limbs, referring to Balfour’s statement that in the Crustacea there are different examples of the loss and later appearance of limbs, such as the loss of the mandibular palpi of the zoëa of decapods, and the loss in the zoëa of appendages in the Erichthus form of the Squilla larva corresponding to the third pair of maxillipedes and first two pairs of legs of Decapoda, and which are afterwards reproduced; similar cases occurring in the Acarina. In the wasps and bees also, as is well known, the imaginal disks of the thoracic appendages appear, the legs themselves being suppressed in the larva (the imaginal disk probably existing in an indifferent state), to reappear in the pupa and imago. It does not, however, necessarily follow that the numerous pairs of hooked ventral tubercles of certain dipterous larvæ (Ephydra) are true appendages.
It seems to us that it is a strong argument for the view that these prolegs are survivals of primitive limbs, that from similar embryonic paired outgrowths on different segments arise the spring of Podurans, the anal cerci, and three pairs of appendages forming the ovipositor, and the anal legs of the Corydalus larva, as well as those of caddis-worms; at least five abdominal segments throughout the class of insects as a whole bearing appendages in the adult.