I. Organs of Secretion. In general, these are membranous vessels that float in the blood or nutritive fluid, and secrete from it a peculiar substance. They may be denominated according to their products—Silk-secretors, Saliva-secretors, Varnish-secretor, Jelly or Gluten-secretor, Poison-secretor, and Scent-secretors.
i. Silk-Secretors (Sericteria). These organs are most remarkable in the caterpillars of the nocturnal Lepidoptera or moths, especially in that tribe called Bombyces, to which the silkworm belongs: but this faculty is not confined to these insects, but is shared by many other larvæ in different Orders; and in one instance at least, by the imago. In general, the outlet of the silk-secretors is at the mouth; sometimes, however, as in the larva of Myrmeleon and the imago of Hydrophilus, its exit is at the anus. The first is the organ which in the silk-worm provides for us that beautiful substance from which the animal takes its name. There are always two of these vessels, which are long floating tubes, growing slender towards the head of the insect, where they unite to form the spinneret (fusulus) before described[589], which renders the silk. Their lower extremity also is commonly more slender than the middle, and is closed at the end. These organs are usually very much convoluted and twisted[590]. According to Ramdohr[591], they consist of two transparent membranes, between which is found a yellow or transparent jelly. The greater the quantity of silk employed by the caterpillar in the construction of its cocoon, &c., the longer are the silk-secretors. Those of the silkworm are a foot long[592], while those of the larva of the goat-moth are little more than three inches[593].
Other insects spin silk with the posterior extremity of their body. In the great water-beetle (Hydrophilus piceus) the anus is furnished with two spinnerets, with which it spins its egg-pouch[594]; these are in connexion, probably, with the five long and large vessels containing a green fluid, described by Cuvier[595], which surround the base of each branch of the ovaries. The larva of Myrmeleon, which also spins a cocoon with its anus, differs remarkably in this respect from other insects, since its reservoir for the matter of silk is the rectum; this is connected with a horny tube, which the animal can protrude, and thus agglutinate the silk and grains of sand that compose its cocoon[596].
The web of spiders is also a kind of silk remarkable for its lightness and extreme tenuity. It is spun from four anal spinnerets, which never vary in number; two longer organs peculiar to some species have been mistaken for additional ones, but Treviranus affirms that they are merely a kind of anal feeler. Their structure, as far as known, has been before described[597]. The web is secreted in vessels varying in form. In some (Clubiona atrox) they consist of two larger and two smaller ones, at the base of which lie many still more minute[598]. The four larger vessels are wide in the middle, branching at top, and below terminating in a narrow canal leading to the spinnerets[599]. Treviranus thinks the fluid contained in the lower minute vessels different from that furnished by the larger ones—but for what purpose it is employed has not been ascertained.
ii. Saliva-secretors (Sialisteria). These are organs, rendering a fluid to the mouth or stomach, that are found in many insects, especially those that take their food by suction, as the Hemiptera, Lepidoptera, and Diptera, though they are not confined to the perfect insect, being also in some cases visible in the larva. Swammerdam was one of the first that discovered them, and he suspects that they may be salival vessels; though he, as well as Ramdohr, thinks they are the same with the silk vessels of the caterpillar[600]; an opinion which Herold has sufficiently disproved, by showing that at one period of the insect's life they co-exist[601], and Lyonet discovered a very conspicuous pair in the caterpillar of the Cossus, co-existent with the silk-secretors[602]. But the physiologist who has given the fullest account of these organs is Ramdohr:—I shall therefore extract chiefly from him what I have further to communicate with respect to them.
They are variously constructed blind vessels, that are present in almost all insects that take their food by suction, but are mostly wanting in those that masticate it. They have been found, however, in Cryptorhynchus Lapathi, Chrysopa Perla, and Iulus terrestris. The most usual number of the saliva-secretors is two[603]; but sometimes, as in the first of the last-named insects, there is only one[604]; in others (Pentatoma Baccarum) there are three, the exterior one consisting of a pair of reservoirs connecting with the gullet by a single capillary tube[605]; in Pentatoma prasina there appear to be four[606]; in Nepa cinerea, even six—the exterior double pair in this insect, under a powerful lens, is found to consist of spherical vesicles, resembling somewhat a bunch of currants[607]; and in Syrphus arcuatus they are covered with four rows of similar ones[608]. In the flea they consist of two pair of spherical reservoirs, each of which is connected with a short tube, which uniting with that of the other forms a common capillary one connecting with the mouth or gullet[609]; these organs sometimes terminate below in slender vessels;—thus, in Nepa, the inner pair terminates in a single vessel of this description[610], and in Tabanus and Hemerobius apparently in many[611]. It admits of a doubt however, as was lately observed, whether in the Hemiptera, which have usually more than a pair of these organs, some are not rather food-reservoirs as in the Diptera.
The saliva-secretors open either into the instruments of suction themselves (Tabanus, Musca); or into the entrance of the gullet (Pentatoma, &c.); or, lastly, into that of the stomach (Syrphus, Bombylius). Those which lie at the entrance of the stomach consist only of a blind uniform tube[612]; but there is commonly to be distinguished in those that open into the mouth, a reservoir, varying in shape in different species, and terminating in a capillary tube, or tubes, at one or both extremities[613]. In Bugs, two pair of these vessels are often present, one of which opens into the stomach (Reduvius), or gullet (Pentatoma), but the other into the instruments of suction[614]. In the Diptera they open into the stomach when the insect feeds only upon the nectar of flowers (Syrphus), and into the proboscis when it feeds upon both animal and vegetable juices (Tabanus, Musca). The function of the fluid secreted by these organs is to moisten or dilute the food before it is received by the instruments of suction and passed to the stomach[615]. When a common house-fly applies its proboscis to a piece of sugar, it is easy to see that it moistens and dissolves it by some fluid.
iii. Varnish-secretor (Colleterium). In butterflies, moths, and several other insects, one or more vessels called blind vessels open into the oviduct, concerning the use of which, physiologists are not agreed. In the cabbage butterfly there is a pair of ovate ones, or rather a bilobed one, each lobe of which externally terminates in long perplexed convolutions, not easily traced, filled with a yellow fluid, which Reaumur and Herold think is used for varnishing or gumming the eggs, so that they may adhere to the leaves on which they are deposited: it may probably serve likewise for other uses[616]. Another vessel is also to be found in the above butterfly, which enters the oviduct above this, filled with a thick white fluid, the function of which is, probably, to lubricate the passage[617]. A similar organ is found in Phryganea grandis[618].
iv. Jelly-secretor (Corysterium). This is a remarkable organ, related to the preceding, which secretes the jelly of Trichoptera, some Diptera, &c.; this organ in the former, at least in Phryganea grandis, is of an irregular shape, with four horns or processes[619].
Poison-secretor (Ioterium). This organ, which is most conspicuous in the Hymenoptera Order, has not received much notice, except in the case of the Hive-bee and the Scolia: in the former, it is an elliptical membranous vesicle or reservoir, furnished at its lower extremity with a tube which renders to the sting, and at the other by a blind, long, filiform, secretory, vessel, which according to Swammerdam divides into two terminal blind branches[620], though Reaumur could detect but one[621]; in this vessel the poison is secreted and stored up. In Scolia there are two secretory vessels, which enter the reservoir in the middle on each side[622]. In the Scorpion, we learn from Marcel de Serres that the poison-secretor is clothed externally with a horny thickish membrane, containing two yellowish glands, composed of an infinity of spherical glandules, terminating in a canal, enlarged towards its base so as to form a reservoir, and leading to the extremity of the sting[623]. Connected by a slender tube with each mandible in spiders is a vessel with spiral folds, which seems properly to belong to this head—though Treviranus calls it a saliva-vessel[624]—since in the Mygale avicularia and other spiders, the effect of the bite is said to be so venomous as to occasion considerable inflammation, and sometimes death[625].