Fig. 463.—Upturned end of body of Ephemera, with two egg-masses (o) issuing at the same time from the double oviducts; q, anus.—After Réaumur.
Palmén suggests that the Ephemerids represent, in respect to the reproductive system among insects, a very primitive type of organization, and he concludes that the inner sexual organs of insects are built up of two different morphological elements; i.e. (a) internal primitive paired structures (testes with vasa deferentia, ovaria with oviducts), and (b) integumental structures, such as the ductus ejaculatorius and vagina.
In the younger larvæ the vasa deferentia form slender cords along which are situated the seminal glands; these cords are inserted in the integument on the hinder edge of the 9th sternite, where afterwards, during the last moult, the copulatory organs grow out. In the older larvæ the sperm collects in the cavities of these cords. Their walls become expanded, and this section then functions as vesiculæ seminales. The ends of the cords remain contracted and act as ductus ejaculatorii. Common unpaired glandular structures are not present. At the last moult the copulatory organs reach their complete development, and the ducts become open externally.
The oviducts in the larva are at first slender, string-like, and bear the egg-follicles. As soon as the eggs pass out of the follicles and collect in the oviducts, the walls of the latter become stretched, and this portion forms two uterus-like structures. The terminal division of the two passages forms their vaginal portions. But since there is no common vagina, there are no unpaired glands and no receptaculum seminis. The two ducts become open after the last shedding of the skin.
Palmén adds that this paired or double nature of the sexual glands and their external ducts in this group of insects occurs in some Myriopoda (Fig. 3, E, F) and a few Arachnida (Fig. 3, C, D, the outlets being in this class unpaired), numerous Crustacea, and most worms; and as already stated it is very marked in Limulus, where the paired outlets are in both sexes very simple and wide apart (Fig. 3, A). In the worms the paired genital ducts are modified segmental organs. As we have seen, in the young male Lepisma there are two male genital openings. Hence this double nature of the genital passages in the may-flies seems to be very primitive.
In the Dermaptera, also, the genus Labidura was found by Meinert to have two independent ductus ejaculatorii, opening externally in double external slit-like processes (penes). The two ducts arise from a single seminal vesicle, which is either paired (L. advena), or forms a common passage (L. gigantea). In Forficula (Fig. 464, B) only one ejaculatory duct persists, the other is obliterated, and one of the penes is atrophied, the other assuming a position in the middle line of the body. Thus the single ejaculatory duct and seminal vesicle arise from the primitive vasa deferentia, and not from the integument of the body, as is the case in the following examples.
According to the researches of Dufour, Loew, etc., most species of Orthoptera (Œdipoda), Libellula, Perla, Panorpa, Rhaphidia, Myrmeleon, Sialis, and Trichoptera (Hydropsyche) have double vasa deferentia and seminal vesicles, and two ejaculatory ducts. The male genital passages of Rhaphidia have a double opening, Loew describing “the two seminal vesicles as lying near each other and at last uniting in a common passage, with an external opening, which, however, must be very short, since I could only once clearly observe it.” This opening is a deep invagination of the external integument, at the bottom of which the two ducts open independently of each other. In such insects, Palmén states that the single ejaculatory duct morphologically arises by an invagination of the integument.
In another group, forming, as regards the genital apparatus, a step next above the Ephemeridæ, viz. the Perlidæ, the oviducts open near each other at the bottom of a median single “vagina,” situated between the 7th and 8th abdominal segment; it is covered beneath by a valve-like, enlarged sternite of the preceding segment, and Palmén homologizes it with the ovi-valvula of some Ephemeridæ. He regards this bell-shaped vagina as a cup-like, deep, intersegmental fold, which projects into the body-cavity and there receives the two ducts.
This differentiation in the Perlidæ may be regarded as the type for several groups of insects. But in others occur a complication which in some degree modifies the type. Thus the invagination arises out from one segment alone, but several segments during metamorphosis may become so reduced that the ventral portions of all may be invaginated to form the vagina. Thus in the larva of Corethra, according to Leydig, and also Weismann, the two testes are attached by two cords to the integument; the hinder ones are inserted independently, and share in the development of the outlets.