v. Scent-secretors (Osmateria). Amongst other means with which insects are gifted for the annoyance of their foes and pursuers, are the powerful scents which many of them emit when alarmed and in danger. Concerning the internal organs by which these effluvia are secreted we possess but little information, but more notice has been taken of the external ones by which they are emitted. We may conclude in general, that the secretory organs are membranous sacs or vesicles, perhaps terminating in longer or shorter blind filiform vessels, sometimes secreting a fetid fluid, and at others a fetid gaseous effluvium. The Iulidæ, at least Iulus and Porcellio[626], cover themselves when alarmed, with a fluid of this kind, or emit one, for this faculty is not peculiar to the species noticed by Savi. I observed early in the year, when I handled Iulus terrestris, that it was covered with a slimy secretion, of a powerful scent, which stained my fingers of an orange colour. The spiraculiform pores that mark the sides of the animal are the outlets by which this fluid is emitted, and not spiracles as has been supposed: each of these orifices, as we learn from Savi, terminates internally in a black vesicle, which is the reservoir of the fluid[627]. The most remarkable insect for its powers of annoyance in this way, is one on that account called the bombardier (Brachinus crepitans), which can fire numerous volleys of stinking vapour at its assailants before its ammunition is exhausted[628]. M. Dufour has given a very particular account of the organ that secretes this vapour;—it consists of a double apparatus, one on each side, in the cavity of the abdomen, both formed of two distinct vessels. The first, which is the innermost, presents itself under two different aspects, according as it is contracted or dilated: in the former case it is a whitish, irregularly rounded, soft body, apparently glandular, placed under the last abdominal segments; communicating at one end with the reservoir, and terminating constantly at the other in a very long and slender filament: in the second case, or when it is dilated, it resembles an oblong, membranous, diaphanous sac, filled with air, then occupying the whole length of the abdomen, and appearing free except where it communicates with the reservoir. The second vessel or reservoir is a small, spherical, brown or reddish body, constant in its form, internally hollow, placed under the last dorsal segment, precisely above the rectum, and opening by a small pore into the anus[629]: so that the tail of this little beetle may be regarded as a battery mounted with two pieces of cannon, which our alert bombardier fires alternately without intermission till all his ammunition is expended. The ground-beetles (Eutrechina) in general have a pair of these anal scent-secretors, which discharge an acrid and caustic fluid, and sometimes a volatile one[630]. The external organ of the scent-secretors in Gyrinus consists of two minute hairy cylindrical retractile tubes, of a red colour[631]. Numerous insects of other tribes and genera emit scents from their anus, and from various other parts of the body, of which having before given you a very full account[632], I shall proceed to the consideration of the secretions themselves: but first I must observe, that in many cases, as in some of the cottony and powdery Aphides, Chermes, &c., the substance secreted appears to be a transpiration through the pores of the body, a kind of excretion from the superabundance of its fluid contents[633]. In many, however, this secretion transpires through appropriate orifices: thus in Chermes Abietis, which produces those curious galls resembling the cone of a fir[634], the flocoons of seeming cotton that cover it proceed from little oval concavities on its back, four of which are arranged in a transverse line on each dorsal segment of the abdomen: these concavities have minute tubercles probably terminating in a pore[635]. In Aphis Fagi the cottony flocoons are almost an inch long[636].
The secretions of insects may be considered under the following heads—Silk; Saliva; Varnish or Gum; Jelly; Oils; Milk; Honey; Wax; Poisons and Acids; Odorous fluids and Vapours; and Luminous matter.
i. Silk. This valuable product of insects, while in the silk-secretor, assumes in the Lepidoptera the appearance of a viscid gum, but the moment it is exposed to the air it hardens into a silken thread. It is remarkable for the following qualities:—it dries the instant it comes in contact with the air; it is then insoluble not only in water but in the most active solvents, and even heat has no effect upon it to melt or soften it: indeed, without these qualities it would be of no use to us[637]. As soon as it leaves the spinneret it becomes the thread we call silk, which being drawn through two orifices is necessarily double through its whole length. This thread varies considerably in colour and texture, as has been before stated[638], and sometimes resembles cotton or wool rather than silk. In spiders it is of a much softer and more tender texture than that of other spinning insects; and Mr. Murray seems to have proved that it is imbued, in the case of the gossamer, with negative electricity: in the sericterium the fluid that produces it is sometimes white or grey, and at others yellow[639]. A remarkable gnat (Ceroplatus tipuloides), living on an agaric, carpets its station of repose and its paths with something between silk and varnish, which it spins, not in a thread, but in a broad riband[640].
ii. Saliva. Many insects have the power of discharging from their mouth a fluid which seems in some degree analogous to the saliva of larger animals. Thus many, as Lepidoptera, Hemiptera, Diptera, &c., can dilute their food, and render it fitter for deglutition. I have seen a common fly when not employed in eating, emit a globule of fluid as big as a grain of mustard-seed from its proboscis, and retract it again. On a former occasion I observed to you that many predaceous, carnivorous, and some herbivorous beetles, when alarmed emit a drop of coloured acrid fluid from the mouth[641]. That this is not secreted in any of the ordinary salival vessels is evident from Ramdohr's dissections of those beetles[642], who, had there been such an organ, would doubtless have discovered it: but as the stomach of all of them is distinguished by those minute cœca or blind vessels, which he denominates shags (zotten)[643], perhaps these may be the secretors of this fluid, probably analogous to the gastric juice[644]; in which case its primary office would be the digestion of the food. We are not however warranted in considering every fluid effused from the mouth as saliva. The glutinous material with which wasps cement the woody fibres for their paper edifices[645]; that with which some sand-wasps moisten the sand which they scrape away, of which they form the singular tubes that lead to their nests[646]; and that with which the aphidivorous larvæ fix themselves previously to their becoming pupæ[647],—may be a secretion distinct from saliva; possibly intermediate between it and gum or the matter of silk, and secreted by peculiar organs. In the wasp, however, Ramdohr discovered nothing of the kind[648]; and in Syrphus, as before observed, the saliva-secretors are very peculiar in their structure, as if appropriated to the secretion of a peculiar fluid[649]. Something similar has been observed by Reaumur with regard to the larva of Crioceris merdigera, which forms its cocoon with a kind of froth produced from the mouth[650].
iii. Varnish or Gum. The eggs of various insects, when they leave the oviduct, are covered with a kind of varnish or gum by which they adhere to the substances that the young larvæ are to feed upon, or are placed in a proper position for their hatching in an appropriate station. Several instances of this have been already mentioned[651]; I shall therefore not enlarge further upon the subject. With regard to the secretion itself, little has been recorded except its colour, which has been before noticed. Some Lepidoptera also as we learn from Reaumur and Bonnet[652], use a varnish in the construction of their cocoons.
iv. Jelly or Gluten. This secretion is particularly conspicuous in the Trichoptera and some Diptera, serving as a bed or nidus for those eggs that are committed to the water,—upon which I have nothing to add to what has been before said[653]. Under this head also may be noticed the fluid, secreted in peculiar vesicles, that lubricates the oviduct and the passages of the sexual organs[654].
v. Oils. Oily substances are sometimes produced by insects. The common oil-beetle (Meloe Proscarabæus) when touched sends forth a drop of this kind of fluid, of an orange colour, from each joint of its legs[655]: something similar I have observed in Coccinella bipunctata: Ray mentions a locust taken in Spain which emits a yellow oleaginous fluid from between the claws of its fore legs[656]; but the precise nature of these substances has not been ascertained, nor whether they are secreted by peculiar organs.
vi. Milk. A milky fluid is produced by the larva of Chrysomela Populi. Willughby observed a similar effusion from pores in the upper surface of the body of Acilius sulcatus; and other insects emit it from other parts of their body[657].
vii. Honey. It is certain that honey is not an animal secretion; yet the saccharine matter collected from the nectaries of flowers, from which it is derived, seems to undergo some alteration in the stomach; for the consistence of honey is greater than that of any vegetable nectar, and its taste does not vary greatly, while that of the nectar in different plants is probably not the same. Reaumur also has observed, that each honey-cell in a bee-hive is always covered by a cream-like layer of a thicker consistence than the rest, which apparently serves to prevent the more liquid honey, which from time to time is introduced under it, from running out[658]. Now if honey were the unaltered nectar of plants, it is difficult to conceive how this cream could be collected in proper proportions. The last-mentioned naturalist likewise ascertained, that if bees, in a season in which the fields afford a scarcity of food, be supplied with sugar, they will from this substance fill their cells with honey which differs in no respect from the common sort, except that its flavour is a little heightened[659]:—a similar argument may be deduced from the circumstance of the bees imbibing the juices of fruits of various kinds as they are well known to do[660]. It seems therefore evident that the honey collected by bees undergoes some modification in their honey-stomach before it is regurgitated into the cells, and therefore may be regarded in some degree as a peculiar secretion.