Fig. 200.—End of abdomen of Periplaneta americana, ♂, side view: c, cercopod; st, stilus; p, penis; t, titillator; d, “bird’s head” (clasper?); i, “oblong plate”; IX-XI, terminal segments; X, suranal plate; XI′, 11th sternite.—After Peytoureau.
In most larvæ of the Trichoptera, especially the Rhyacophilidæ and Hydropsychidæ, the last abdominal segment bears a pair of 2–jointed legs (cercopoda), ending in either one or two claws, which under various forms, sometimes forming long processes, persist in the pupa; and there appears to be a suranal plate, the vestige of the tenth uromere. In the pupa, judging by Klapálek’s figure of Leptocerus (248,9, 25), a pair of lateral spines arise which may in the imago form one of the pairs of appendages or styles. In the pupa of Œcetis furva his figure 289 shows two pairs of 1–jointed appendages arising from the last segment; whether the long dorsal or upper styles arise from the vestige of a more distal segment is not distinctly shown in Klapálek’s sketch. The origin of these elements of the genital armature evidently needs further study.
Whether the abdominal legs or so-called false or prop-legs of lepidopterous larvæ are genuine legs, homologous with those of the thorax and with the cephalic appendages, or whether they are secondary adaptive structures, is a matter still under discussion. That, however, they are true legs is shown by the embryology of the Lepidoptera, where there is a pair to each abdominal segment. It may also be asked whether the anal legs of lepidopterous larvæ are not the homologues of the 2–jointed anal appendages of caddis-worms.
Fig. 201.—Eriocephala calthella, ♂, side view: t, palpiform suranal plate; cl, claspers; s, inferior claspers; mxp, maxillary palpi; cx. coxa; tr, trochanter; sc, scutum; sc′, scutellum.
In Lepidoptera, notably the male of the very generalized Eriocephala calthella (Fig. 201), besides the broad unjointed claspers, which are curved upward and provided with a brush of stiff hooked setæ (this upper pair being perhaps modified cercopods), there is an accessory lower slenderer pair, while the suranal plate (t) is palpiform or clavate and also adapted to aid in the action of the claspers. The examination of the cercopods and rhabdopods in the Trichoptera and in a generalized lepidopterous form like this enables one to understand the morphology of the genital armature, since it consists, besides the suranal plate, which is often deeply forked (in Sphingidæ, Smith), of a pair of modified hook-like cercopoda, and in some cases (Eriocephala) of an additional pair of claspers which may be the homologues of the ephemerid rhabdopods. A pair of hooks, often strong and claw-like (harpes), are situated, one near the base on the inside of each clasper; they are especially developed in the Noctuidæ (Smith), and appear to be present in certain Trichoptera, but this remains to be proved. This complicated apparatus of claspers and hooks is utilized by those insects which pair while on the wing, and is wanting in such forms as Coleoptera and Hemiptera. Besides the forceps of Panorpa, there are two pairs of slender filiform appendages which need farther examination. In the Diptera, especially Tipulidæ, there is a pair of 2–jointed appendages or forceps, as in Limnophila (Osten Sacken). The male genital armature of Diptera appears to be on the same general plan as in Lepidoptera, but more complicated.
Fig. 202.—Male organs of generation of Athalia.—After Newport.
Notice should also be taken of the paired uncinate hooks which are modifications of the penis-sheath of the male of cockroaches (Phyllodromia), which Haase states appear to originate on the tenth ventral plate, and which probably “serve to open and dilate the vagina of the female, especially as a perforated penis, which is highly developed in Machilis, seems to be wanting in the Blattidæ.” (Haase.)