There are in different Insects more than one kind of diverticula and accessory glands in connexion with the oviducts or uterus; a receptaculum seminis, also called spermatheca, is common. In the Lepidoptera there is added a remarkable structure, the bursa copulatrix, which is a pouch connected by a tubular isthmus with the common portion of the oviduct, but having at the same time a separate external orifice, so that there are two sexual orifices, the opening of the bursa copulatrix being the lower or more anterior. The organ called by Dufour in his various contributions glande sébifique, is now considered to be, in some cases at any rate, a spermatheca. The special functions of the accessory glands are still very obscure.
Fig. 75.—Egg-tube of Dytiscus marginalis; e.c, egg-chamber; n.c, nutriment chamber; t.c, terminal chamber; t.t, terminal thread. (After Korschelt.)
The ovaries of the female are replaced in the male by a pair of testes, organs exhibiting much variety of form. The structure may consist of an extremely long and fine convoluted tube, packed into a small space and covered with a capsule; or there may be several shorter tubes. As another extreme may be mentioned the existence of a number of small follicles opening into a common tube, several of these small bodies forming together a testis. As a rule each testis has its own capsule, but cases occur—very frequently in the Lepidoptera—in which the two testes are enclosed in a common capsule; so that there then appears to be only one testis. The secretion of each testis is conveyed outwards by means of a slender tube, the vas deferens, and there are always two such tubes, even when the two testes are placed in one capsule. The vasa deferentia differ greatly in their length in different Insects, and are in some cases many times the length of the body; they open into a common duct, the ductus ejaculatorius. Usually at some part of the vas deferens there exists a reservoir in the form of a sac or dilatation, called the vesicula seminalis. There are in the male, as well as in the female, frequently diverticula, or glands, in connexion with the sexual passages; these sometimes exhibit very remarkable forms, as in the common cockroach, but their functions are quite obscure. There is, as we have already remarked, extreme variety in the details of the structure of the internal reproductive apparatus in the male, and there are a few cases in which the vasa deferentia do not unite behind, but terminate in a pair of separate orifices. The genus Machilis is as remarkable in the form of the sexual glands and ducts of the male as we have already mentioned it to be in the corresponding parts of the female.
Fig. 76.—Tenthredo cincta. a, a, testes; b, b, vasa deferentia; c, c, vesiculæ seminales; d, extremity of body with copulatory armature. (After Dufour.)
Although the internal sexual organs are only fully developed in the imago or terminal stage of the individual life, yet in reality their rudiments appear very early, and may be detected from the embryo state onwards through the other preparatory stages.
The spermatozoa of a considerable number of Insects, especially of Coleoptera, have been examined by Ballowitz;[[65]] they exhibit great variety; usually they are of extremely elongate form, thread-like, with curious sagittate or simply pointed heads, and are of a fibrillar structure, breaking up at various parts into finer threads.
External Sexual Organs.—The terminal segments of the body are usually very highly modified in connexion with the external sexual organs, and this modification occurs in such a great variety of forms as to render it impossible to give any general account thereof, or of the organs themselves. Some of these segments—or parts of the segments, for it may be dorsal plates or ventral plates, or both—may be withdrawn into the interior, and changed in shape, or may be doubled over, so that the true termination of the body may be concealed. The comparative anatomy of all these parts is especially complex in the males, and has been as yet but little elucidated, and as the various terms made use of by descriptive entomologists are of an unsatisfactory nature we may be excused from enumerating them. We may, however, mention that when a terminal chamber is found, with which both the alimentary canal and the sexual organs are connected, it is called a cloaca, as in other animals.
Parthenogenesis.