[LETTER XLII.]
INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED.
REPRODUCTION.
The reproductive organs of insects in their general denominations and functions correspond with those of the higher classes of animals; but as to number, proportions, and other particular details of their structure, they differ from them very considerably. I shall not now, however, enter at large upon this subject, but confine myself principally to the consideration of those organs in the female which are appropriated to the formation, fecundation, maturation, exclusion and deposition of their eggs, and other circumstances relating to that subject. The organs connected with this function are the Sperm-reservoir; the Oviduct; the Ovaries; and the Ovipositor.
I. The Sperm-reservoir (Spermatheca) is an organ connecting the vagina with the oviduct, which, according to Herold, receives the male sperm as into a reservoir[703], and fecundates the eggs in their transit through that passage. This vessel, which consists of a double tunic, in the cabbage-butterfly terminates the vagina, and is connected with the oviduct by a lateral undulating tube: in shape it is a rather irregular oblong, and is surmounted by a small orbicular vesicle, connected by a short tubular footstalk with the main reservoir[704]. A similar organ was discovered by Malpighi in the imago of the silkworm, who denominates it the uterus; to which indeed it seems analogous, and which he also regards as a reservoir for the sperm for the gradual fecundation of the eggs[705]. But in that fly the organ is of a rather different shape, and the interior vessel terminates in several spherical vesicles[706]. John Hunter by the most decisive experiments, such as covering the eggs of the unimpregnated moth, after exclusion, with the liquor taken from the spermatheca in those which had been impregnated, and rendering them fertile, he demonstrated that this organ was a reservoir for the spermatic fluid, to impregnate the eggs as they were ready for exclusion, and that coition and impregnation were not simultaneous[707]. It is not improbable that in all insects whose eggs are gradually laid, this provision for their gradual fecundation, if carefully sought for, might be detected[708]. Rifferschweils is of opinion, that in these cases the eggs are fertilized in their transit through the oviduct by sperm adhering to the folds of the cloacæ[709]: but this opinion seems less analogous to what takes place in other cases, with regard to the due preparation of the eggs for a safe and effectual transit[710].
II. The Oviduct (Oviductus) is the canal, always separate from the vagina, which receives the eggs from the ovary, transmitting them, often by a peculiar and complex instrument in which it terminates, to their proper station. This canal sometimes opens into the anal passage or cloaca, and at others, as in the cabbage-butterfly[711], is distinct, and lies between the sexual organ and the anus. In the Arachnida there are two oviducts[712].
III. The Ovaries (Ovaria) in insects are the viscera in which the eggs are generated and grow till they arrive at maturity, when they pass through the oviduct, and are extruded or deposited in their appropriate station. They vary considerably in their structure. In all however, except the Iulidæ, in which there is only a single ovary[713], the oviduct at its upper or inner extremity terminates in two branches, usually further subdivided into a number of smaller conical ones, which several ramifications constitute the ovaries, or egg-tubes as they are sometimes called: these tubes generally consist of a single membrane, and are joined to the oviduct by membranous rugose cloacæ[714]: in the Phalangia, however, there are two tunics; the outer one of a cellular substance, and the inner one consisting of spiral fibres like tracheæ—a kind of structure which renders them capable of great extension[715]. Rifferschweils considers the ovaries as formed upon two primary types.—First, flagelliform ovaries, consisting of conical tubes equal in length, and inserted at the same place at the end of the primary branches as in the Lepidoptera, the Bee, &c. Secondly, racemose ovaries, consisting of short conical tubes, so proceeding from the primary branches as to render the ovary racemose or pinnated, as in certain Neuroptera, Coleoptera, and Diptera[716]: but perhaps their structure will be better understood if they are divided into agglomerate ovaries and branching ovaries: in the first the egg-tubes form two bundles, in which the branches are not discernible, as in the Ephemera, the chamæleon-fly, and spiders[717]: and in the second the branches are distinct, as in the Lepidoptera and the majority of insects.
The number of branches varies in different genera and species. In Echinomyia grossa, a large fly, there are only the two primary branches[718]; in the common dung-beetle (Geotrupes stercorarius) these appear divided at their apex into fingers[719]: in Scolia, a Hymenopterous genus, and the butterfly of the nettle, there are three secondary branches on each side[720]: in many other Lepidoptera and the humble-bee there are four[721]; in the common louse there are five[722]; in the rhinoceros-beetle and the cockchafer, six[723]; in the wasp, seven[724]; eight in the cockroach[725]; twelve in the Carabi and the mealworm-beetle[726]; thirty in the large green grasshopper (Acrida viridissima[727]); thirty-two in the cheese-maggot-fly[728]; and in the hive-bee more than a hundred and fifty[729].