x. Odorous fluids and Vapours[686]. The powerful scents which different insects emit are extremely numerous, much more so indeed than the generality of Entomologists have been aware, for there is scarcely a scent odious or agreeable that may not be met with in the insect world. This you will be convinced of, by following a practice which I would recommend to you—that of smelling the insects you take. Some of these scents are peculiar to particular parts or organs, and some are exhaled generally by the whole body; some are emitted by a fluid secretion, and others are gaseous effluvia. On a former occasion I gave you a rather full account of these scents and their organs[687]; I shall relate here only what I there omitted. To begin with sweet odours. Many beetles emit an agreeable scent. The rose-scented Capricorn or musk-beetle (Cerambyx moschatus) has long been noted for the delicious scent of roses which it exhales; this is so powerful as to fill a whole apartment, and the insect retains it long after its death. Captain Hancock also informed me that another species of the same genus, C. sericeus, has in a high degree a scent resembling that of the cedar[688] on which they feed. Though most of the micropterous tribes (Brachyptera) have a fetid smell, yet there are some exceptions to this amongst them. One species (Philonthus suaveolens K. MS.) related to P. micans, which I once took, smelt precisely like a fine high-scented ripe pear; another, Oxytelus morsitans, like the water-lily; a third, O. rugosus, like water-cresses; and lastly, a fourth (P. fuscipes) like saffron[689]: Trichius Eremita, one of the Petalocerous beetles, is stated to have the scent of Russia leather; Geotrupes vernalis, in spite of its stercorarious food, of lavender-water[690]. Mr. Sheppard has observed that Dytiscus marginalis when recently taken smells not unlike liquorice: Bonnet mentions a caterpillar that had the scent of new hay. A little gall-fly (Cynips Quercus Ramuli) has the remarkable odour of Fraxinella: the larva of another species of this genus (C. Rosæ) has an odour which seemed to Reaumur as attractive to cats as that of Nepeta cataria or Teucrium Marum[691]: some Phalangia smell like walnut leaves[692]; and the various species of the genus Prosopis (Melitta * b. K.) have a very agreeable scent of Dracocephalum moldavicum[692].
We next come to fetid odours. These in numerous cases are known to be secreted and emitted by appropriate vessels and organs; they are often exhaled from a fluid secretion, of which, in the letter lately referred to, I gave you almost all the known instances. Savi, in his history of Iulus fœtidissimus, informs us that it emits a yellow fetid fluid from its supposed spiracles, which if applied in sufficient quantity imparts a red colour to the skin, to be removed neither by friction nor washing, but only disappearing by time; when removed from the black vesicles in which it is stored, it shoots into very transparent octahedral crystals[693].
I have before mentioned the coloured fluid which some insects emit when they are disclosed from the pupa, and that it probably exhales some powerful odour which attracts the males[694].
The great Hydrophilus, in its larva state, when first taken into the hand remains without motion; in a minute afterwards it renders itself so flaccid as to appear like a cast skin. Taken by the tail it contracts itself considerably, it then agitates itself briskly, and ejaculates with a slight noise a fetid and blackish fluid[695].
In other cases these odours are produced by gaseous vapours. That of the Bombardiers (Brachinus) is the most celebrated and remarkable. It is whitish, of a powerful and stimulating odour, very like that exhaled by nitrous acid. It is caustic, producing upon the skin the sensation of burning, and forming instantly upon it red spots which soon turn brown, and which, in spite of frequent lotions, remain several days. It turns blue paper red[696]. That amiable, intelligent, and unfortunate traveller Mr. Ritchie,—whose premature death, when attempting to penetrate to the interior of Africa, all lovers of Natural History so deeply lamented, and whose ardour in the pursuit of that science I had an opportunity of witnessing, when, in company with him, Messrs. Savigny, Du Fresne, and W. S. MacLeay in 1817, I visited the forest of Fontainebleau,—in a letter to the last-mentioned gentleman[697], relates that his companion M. Dupont, near Tripoli took a nest consisting of more than a thousand of a species of this genus. "I am making a few experiments," says he, "on the substance which they emit when they crepitate, but do not know whether I can collect enough to arrive at any conclusion. It made Dupont's fingers entirely black when he took them. It is neither alkaline nor acid, and it is soluble in water and in alcohol." From this we may conjecture that it formed crystals.
xi. Phosphorus. On this remarkable secretion I have so fully enlarged on a former occasion[698], that here I shall merely add a few observations which Mr. Murray obligingly communicated to me. He remarks that in a box in which glow-worms were kept—five luminous specks were found secreted by the animal, which seemed to glow and were of a different tinge of light. One put into olive oil at eleven p. m. continued to yield a steady and uninterrupted light until five o'clock the following morning, and then seemed, like the stars, to be only absorbed by superior effulgence. The luminous spherical matter of the glow-worm is evidently enveloped in a sac or capsule perfectly diaphanous, which when ruptured discloses it in a liquid form, of the consistency of cream. M. Macaire, he observes, in the Bibliothèque Universelle, draws the following conclusions from experiments made on the luminous matter of this animal;—that a certain degree of heat is necessary to their voluntary phosphorescence—that it is excited by a degree of heat superior to the first, and inevitably destroyed by a higher—that bodies which coagulate albumen take away the power—that phosphorescence cannot take place but in a gas containing no oxygen—that it is not excited by common electricity, but is so by the Voltaic pile—and lastly, that the matter is chiefly composed of albumen.
xii. Fat. There is one product found in the body of insects most copiously in their larva state, but more or less also in the imago, which may be called their fat. In the former it is a many-lobed mass, occupying the whole of the interior, except the space that is required for the muscles and the internal organs, which it wraps round and protects. It is contained in floating membranes, very numerous, which fill all the interstices, and assume the appearance sometimes of small globules, and sometimes of a thickish mucilage, which easily melts and inflames; in colour it is most commonly white, but sometimes yellow or green. It is imagined to be a kind of epiploon or caul, and is accumulated in the larva as a store of nutriment for the growth and development of the organs of the perfect insect while in the pupa state[699]. The blood in which the different organs float that is not required for their nutriment, is supposed to be expended in the formation of this substance. Marcel de Serres is of opinion that it is secreted from the chyle by passing through the pores of the dorsal vessel, formerly called the heart of insects[700].
Under this head I may mention what little is known with regard to the perspiration of these animals[701]. That a considerable quantity of fluid passes off from them when in the pupa state, is sufficiently proved by the loss of weight which they undergo, and by the experiments of Reaumur, who collected the fluid in closed glass tubes; and that in their perfect state they are constantly passing off perspirable matter by the pores of their skin or crust, is not only rendered probable by the succulent nature of their food and the absence of any urinary discharge, but is proved by what takes place in a swarm of bees. These insects, when crowded together in hot weather in a large mass, become heated to such a degree, and perspire so copiously, that those near the bottom are quite drenched with the moisture it produces, which so relaxes their wings that they are unable to fly[702].
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