Graber has observed the same relations in the pupa of Chironomus, the efferent genital tubes in both sexes being separate, so that there are two vaginal passages and two penes present. Palmén comments on these relations in the dipterous insects, remarking that during metamorphosis certain parts of the terminal abdominal segments are reduced, while others are hypertrophied; hence the points of insertion of the cords referred to becoming the openings of the vasa are carried within the abdomen; and this part of the integument becomes an unpaired section. In these insects, also, there is an unpaired vesicula seminalis, but its morphological nature (whether formed from the integumental duct or the fused vasa deferentia) can only be settled after special investigation.
In the Lepidoptera, also, it has been shown by Herold, Suckow, Bessels, and recently with full details by Jackson, that the paired larval oviducts are at first solid, but become tubular early in pupal life. A little later, their cavities open into that of the azygos or unpaired oviduct. The paired oviducts open in the female caterpillars on the hind edge of the 7th abdominal segment, afterwards uniting with the unpaired vagina of the 8th segment, which is developed from the hypodermis.
Jackson adds that there are three stages traceable in the evolution of the genital ducts of Lepidoptera: “an ephemeridal stage, which ends towards the close of larval life; an orthopteran stage, indicated during the quiescent period preceding pupation; and a lepidopteran stage, which begins with the commencement of pupal life.”
As a summary of these results it appears that the genital organs of insects consist of two morphologically different elements: 1. the primitive internal paired structures (testes with the vasa deferentia; ovaries with the ovarian tubes), and 2. integumental structures (Fig. 464). In the most primitive winged insects (Ephemeridæ) the latter structures are only represented by the two external sexual openings, the entire reproductive system being paired. The paired parts become in the more highly differentiated forms united into single parts, while, a, a common integumental division, grows in, forming the ductus ejaculatorius, or the vagina; or, b, the inner passages anastomose together, i.e. the openings fuse together; or, c, both of these cases occur at once; or, finally, we have d, where the superfluous paired parts by reduction become single.
Fig. 464.—Evolution of the unpaired from the paired sexual organs of insects: A-E, male organs. The parts arising by invagination of the integument indicated by thick black lines. A, an Ephemerid. B, Forficula auricularia. C, nymph of Orthoptera in general. D, Œdipoda. E, Cetonia aurata. F, female organs of Æschna.—After Palmén, from Lang.
The male ducts open behind the 9th, the female passages of Ephemerids behind the 7th abdominal segment, those of other insects behind the 8th, except in the Stylopidæ (Strepsiptera), in which they open much in front.
Figure 464 graphically shows their relation. In the Odonata (F) the chitinous lining or integumental invagination extends inwards where the two oviducts begin, in the Coleoptera (E) the vagina, bursa copulatrix, and receptaculum seminis being lined by a thick chitinous layer. While in Perla the two seminal ducts pass directly into the copulatory organ, in the Coleoptera they open into the unpaired ductus ejaculatorius at a distance from the copulatory organ.
The morphological results obtained by Palmén, and for the Lepidoptera by Jackson, were apparently confirmed from an embryological point of view by Nusbaum, from observations on the development of the sexual passages in two genera of Pediculidæ, and are as follows:—
1. The prevalent impression that the larval ducts unite with each other and give origin to the whole system of sexual ducts is incorrect; they form only the vasa deferentia or the oviducts.