Fig. 470.—A, spermatozoön of a beetle (Copris), partly macerated to show structure of flagellum, which consists of a supporting fibre (s.f) and a fin-like envelope (f); n, nucleus; a, a, apical body divided into two parts. B, anterior part of that of Calathus, with barbed head and finmembrane.—After Ballowitz, from Wilson.
The ejaculatory duct during coition conducts the sperm into the copulatory pouch of the female. In consequence of the stretching of the integumental membrane the end of the duct can be erected and again withdrawn. For this purpose the end of the duct is thickened and is said to be provided with powerful muscles. The evaginable terminal portion is covered by a strong chitinous membrane forming the penis or intromittent organ (Fig: 462, h), which is externally enveloped by a pair of chitinous lobes, which in many beetles are converted into a capsule. The ductus ejaculatorius of the honey-bee is inserted by two chitinous branches into the point of union of the two glandulæ mucosæ; it and the entire copulatory apparatus are devoid of muscles, though it is, however, well developed beneath the mucous glands. (Koschewnikoff.)
The accessory glands of the vasa deferentia are tubes whose secretions either directly mix with the semen, or in many cases form seminal packets (spermatophores). In Coleoptera, Lepidoptera, and Diptera there is usually one pair. In many insects there are several pairs, as in Hydrophilidæ and Elateridæ; they are branched in Hemiptera, and in Orthoptera bushy. The single glandular tubules are very long, and form a skein-like mass. In Orthoptera, in the larger number of accessory glands, two forms may be distinguished, which differ from each other in their contents (Siebold). In the cockroach (Fig. 461) these glands form the “mushroom” shaped gland of Huxley, which was at first regarded as the testis.
The spermatozoa.—These very minute bodies, the sexual homologues of the eggs, abound in the seminal fluid, and are formed in the follicles of the testes from a germinal layer or epithelium, as are the eggs. They are hair- or thread-like, usually consisting of a head, a body or middle-piece, and a long, thread-like tail (flagellum), which vibrates rapidly, causing the spermatozoön to move actively forwards (Fig. 470).
In beetles, according to Ballowitz, there are two main types of spermatozoa, connected, however, by intermediate forms. There is a double-tailed type, already described by Bütschli and v. la Valette St. George, and there are others which are single-tailed. Bütschli showed that in the double spermatozoön one tail-filament is straight and stiff, the other being undulating and contractile. Ballowitz describes this type in Calathus (Fig. 470, B), Chrysomela, and Hylobius, etc., and shows that the straight or supporting portion of the tail is elastic, but somewhat stiff, resistant to reagents, and without any fibrillar structure, while the contractile fringe consists of an extremely complicated system of fibrils (Fig. 470). The single-tailed type of spermatozoön, as seen, e.g., in Melolontha and Hydrophilus, has no supporting fibres. The tail is twisted in a spiral, corresponds to the contractile fringe of the double type, and exhibits a complicated fibrillar structure. The fringed type works its way ahead like the screw of a steamer.
Fig. 471.—C, anterior end of spermatid of a moth (Pygæra). D, young spermatozoön of the same; af, axial filament; c, centrosome; m, middle-piece or mitosoma; n, nucleus; p, paranucleus; e, envelope of the tail.—After Platner, from Wilson.
Each spermatozoön is a modified but complete cell, and the nucleus contains the chromatin, a deeply staining substance of the nuclear network and of the chromosomes and the supposed bearer of heredity.
Formation of the spermatozoön.—It arises from a primordial germcell called spermatogonium. This cell contains a large, pale nucleus and a dark body, the accessory nucleus of Bütschli. The spermatogonia subdivide, but at a certain period pause in their subdivisions, and undergo considerable growth. “Each spermatogonium is thus converted into a spermatocyte, which, by two rapidly succeeding divisions gives rise to four spermatozoa, as follows: The primary spermatocyte first divides to form two daughter-cells, known as spermatocytes of the second order, or sperm mother-cells. Each of these divides again—as a rule without pausing, and without the reconstruction of the daughter-nuclei—to form two spermatids or sperm-cells. Each of the four spermatids is then directly transformed into a single spermatozoön; its nucleus becoming very small and compact, its cytoplasm giving rise to the tail and to certain other structures.... As the spermatid develops into the spermatozoön, it assumes an elongated form, the nucleus lying at one end, while the cytoplasm is drawn out to form the flagellum at the opposite end.” (Wilson’s The Cell, from La Valette St. George.)