As Hercules, after he had slain the Nemean lion, made a doublet of its skin, so the larva of another insect (Hemerobius Chrysops, a lace-winged fly with golden eyes,) covers itself with the skins of the luckless Aphides that it has slain and devoured. From the head to the tail, this pygmy destroyer of the helpless is defended by a thick coat, or rather mountain composed of the skins, limbs, and down of these creatures. Reaumur, in order to ascertain how far this covering was necessary, removed it, and put the animal into a glass, at one time with a silk cocoon, and at another with raspings of paper. In the first instance, in the space of an hour it had clothed itself with particles of the silk: and in the second, being again laid bare, it found the paper so convenient a material, that it made of it a coat of unusual thickness[380].
Insects in general are remarkable for their cleanliness;—however filthy the substances which they inhabit, yet they so manage as to keep themselves personally neat. Several, however, by no means deserve this character; and I fear you will scarcely credit me when I tell you that some shelter themselves under an umbrella formed of their own excrement! You will exclaim, perhaps, that there is no parallel case in all nature;—it may be so;—yet as I am bound to confess the faults of insects as well as to extol their virtues, I must not conceal from you this opprobrium. Beetles of three different genera are given to this Hottentot habit. The first to which I shall introduce you is one that has long been celebrated under the name of the beetle of the lily (Lema merdigera, Cantaride de' Gigli, Vallisn.) The larvæ of this insect have a very tender skin, which appears to require some covering from the impressions of the external air and from the rays of the sun; and it finds nothing so well adapted to answer these purposes, and probably also to conceal itself from the birds, as its own excrement, with which it covers itself in the following manner. Its anus is remarkably situated, being on the back of the last segment of the body, and not at or under its extremity, as obtains in most insects. By means of such a position, the excrement when it issues from the body, instead of being pushed away and falling, is lifted up above the back in the direction of the head. When entirely clear of the passage, it falls, and is retained, though slightly, by its viscosity. The grub next, by a movement of its segments, conducts it from the place where it fell to the vicinity of the head. It effects this by swelling the segment on which the excrement is deposited, and contracting the following one, so that it necessarily moves that way. Although, when discharged, it has a longitudinal direction, by the same action of the segments the animal contrives to place every grain transversely. Thus, when laid quite bare, it will cover itself in about two hours. There are often many layers of these grains upon the back of the insect, so as to form a coat of greater diameter than its body. When it becomes too heavy and stiff, it is thrown off, and a new one begun[381].—The larvæ of the various species of the tortoise-beetles (Cassida, L.) have all of them, as far as they are known, similar habits, and are furnished besides with a singular apparatus, by means of which they can elevate or drop their stercorarious parasol so as most effectually to shelter or shade them. The instrument by which they effect this is an anal fork, upon which they deposit their excrement, and which in some is turned up and lies flat upon their backs; and in others forms different angles, from very acute to very obtuse, with their body; and occasionally is unbent and in the same direction with it[382]. In some species the excrement is not so disgusting as you may suppose, being formed into fine branching filaments. This is the case with C. maculata, L.[383].—In the cognate genus Imatidium, the larvæ also are merdigerous; and that of I. Leayanum, Latr., taken by Major-General Hardwicke in the East Indies, also produces an assemblage of very long filaments, that resemble a dried fucus or a filamentous lichen.—The clothing of the Tineæ, clothes-moths and others, and also of the case-worms, having enlarged upon in a former letter[384], I need not describe here.
Some insects, that they may not be discovered and become the prey of their enemies when they are reposing, conceal themselves in flowers. The male of a little bee (Heriades[385] Campanularum), a true Sybarite, dozes voluptuously in the bells of the different species of Campanula—in which, indeed, I have often found other kinds asleep. Linné named another species florisomnis on account of a similar propensity. A third, a most curious and rare species (Andrena[386] spinigera), shelters itself when sleeping, at least I once found it there so circumstanced, in the nest-like umbel of the wild carrot. You would think it a most extraordinary freak of Nature, should any quadruped sleep suspended by its jaws, (some birds however are said, I think, to have such a habit, and Sus Babyroussa one something like it,)—yet insects do this occasionally. Linné informs us that a little bee (Epeolus[387] variegatus) passes the night thus suspended to the beak of the flowers of Geranium phæum: and I once found one of the vespiform bees (Nomada[388] Goodeniana) hanging by its mandibles from the edge of a hazel-leaf, apparently asleep, with its limbs relaxed and folded. On being disengaged from its situation it became perfectly lively.
There is no period of their existence in which insects usually are less able to help themselves, than during that intermediate state of repose which precedes their coming forth in their perfect forms. I formerly explained to you how large a portion of them during this state cease to be locomotive, and assume an appearance of death[389]. In this helpless condition, unless Providence had furnished them with some means of security, they must fall an easy prey to the most insignificant of their assailants. But even here they are taught to conceal themselves from their enemies by various and singular contrivances. Some seek for safety by burying themselves, previously to the assumption of the pupa, at a considerable depth under the earth; others bore into the heart of trees, or into pieces of timber; some take their residence in the hollow stalks of plants; and many are concealed under leaves, or suspend themselves in dark places, where they cannot readily be seen. But in this state they are not only defended from harm by the situation they select, but also by the covering in which numbers envelop themselves; for, besides the leathery case that defends the yet tender and unformed imago, many of these animals know how to weave for it a costly shroud of the finest materials, through which few of its enemies can make their way;—and to this curious instinct, as I long since observed, we owe one of the most valuable articles of commerce, the silk that gives lustre to the beauty of our females. These shrouds are sometimes double. Thus the larvæ of certain saw-flies spin for themselves a cocoon of a soft, flexible, and close texture, which they surround with an exterior one composed of a strong kind of net-work, which withstands pressure like a racket[390]. Here nature has provided that the inclosed animal shall be protected by the interior cocoon from the injury it might be exposed to from the harshness of the exterior, while the latter by its strength and tension prevents it from being hurt by any external pressure.
But of all the contrivances by which insects in this state are secured from their enemies, there is none more ingenious than that to which the may-flies (Trichoptera) have recourse for this purpose. You have heard before that these insects are at first aquatic, and inhabit curious cases made of a variety of materials, which are usually open at each end[391]. Since they must reside in these cases, when they are become pupæ, till the time of their final change approaches, if they are left open, how are the animals, now become torpid, to keep out their enemies? Or, if they are wholly closed, how is the water, which is necessary to their respiration and life, to be introduced? These sagacious creatures know how to compass both these ends at once. They fix a grate or portcullis to each extremity of their fortress, which at the same time keeps out intruders and admits the water. These grates they weave with silk spun from their anus into strong threads, which cross each other, and are not soluble in water. One of them, described by De Geer, is very remarkable. It consists of a small, thickish, circular lamina of brown silk, becoming as hard as gum, which exactly fits the aperture of the case, and is fixed a little within the margin. It is pierced all over with holes disposed in concentric circles, and separated by ridges which go from the centre to the circumference, but often not quite so regularly as the radii of a circle or the spokes of a wheel. These radii are traversed again by other ridges, which follow the direction of the circles of holes; so that the two kinds of ridges crossing each other form compartments, in the centre of each of which is a hole[392].
Under this head I shall call your attention to another circumstance that saves from their enemies innumerable insects:—I mean their coming forth for flight or for food only in the night, and taking their repose in various places of concealment during the day. The infinite hosts of moths (Phalæna, L.),—amounting in this country to more than a thousand species,—with few exceptions, are all night-fliers. And a considerable proportion of the other orders,—exclusive of the Hymenoptera and Diptera, which are mostly day-fliers,—are of the same description. Many larvæ of moths also come out only in the night after their food, lying hid all day in subterraneous or other retreats. Of this kind is that of Fumea pulla and Nycterobius, whose proceedings have been before described[393]. The caterpillar of another moth (Noctua subterranea, F.) never ascends the stems of plants, but remains, a true Troglodyte, always in its cell under ground, biting the stems at their base, which falling, bring thus their foliage within its reach[394].
The habitations of insects are also usually places of retreat, which secure them from many of their enemies:—but I have so fully enlarged upon this subject on a former occasion[395], that it would be superfluous to do more than mention it here.
I am now to lay before you some examples of the contrivances, requiring skill and ingenuity, by which our busy animals occasionally defend themselves from the designs and attack of their foes. Of these I have already detailed to you many instances, which I shall not here repeat; my history therefore will not be very prolix.—I observed in my account of the societies of wasps, that they place sentinels at the mouth of their nests. The same precaution is taken by the hive-bees, particularly in the night, when they may expect that the great destroyers of their combs, Galleria mellonella and its associates[396], will endeavour to make their way into the hive. Observe them by moonlight, and you will see the sentinels pacing about with their antennæ extended, and alternately directed to the right and left. In the mean time the moths flutter round the entrance; and it is curious to see with what art they know how to profit of the disadvantage that the bees, which cannot discern objects but in a strong light, labour under at that time. But should they touch a moth with these organs of nice sensation, it falls an immediate victim to their just anger. The moth, however, seeks to glide between the sentinels, avoiding with the utmost caution, as if she were sensible that her safety depended upon it, all contact with their antennæ. These bees upon guard in the night, are frequently heard to utter a very short low hum; but no sooner does any strange insect or enemy touch their antennæ, than the guard is put into a commotion, and the hum becomes louder, resembling that of bees when they fly, and the enemy is assailed by workers from the interior of the hive[397].
To defend themselves from the death's-head hawk-moth, they have recourse to a different proceeding. In seasons in which they are annoyed by this animal, they often barricade the entrance of their hive by a thick wall made of wax and propolis. This wall is built immediately behind and sometimes in the gateway, which it entirely stops up; but it is itself pierced with an opening or two sufficient for the passage of one or two workers. These fortifications are occasionally varied: sometimes there is only one wall, as just described, the apertures of which are in arcades, and placed in the upper part of the masonry. At others many little bastions, one behind the other, are erected. Gateways masked by the anterior walls, and not corresponding with those in them, are made in the second line of building. These casemated gates are not constructed by the bees without the most urgent necessity. When their danger is present and pressing, and they are as it were compelled to seek some preservative, they have recourse to this mode of defence[398], which places the instinct of these animals in a wonderful light, and shows how well they know how to adapt their proceedings to circumstances. Can this be merely sensitive? When attacked by strange bees, they have recourse to a similar manœuvre; only in this case they make but narrow apertures, sufficient for a single bee to pass through.—Pliny affirms that a sick bear will provoke a hive of bees to attack him in order to let him blood[399]. What will you say, if humble-bees have recourse to a similar manœuvre? It is related to me by Dr. Leach, from the communications of Mr. Daniel Bydder—an indefatigable and well-informed collector of insects, and observer of their proceedings—that Bombus[400] terrestris, when labouring under Acariasis[401] from the numbers of a small mite (Gamasus Gymnopterorum) that infest it, will take its station in an ant-hill; where beginning to scratch, and kick, and make a disturbance, the ants immediately come out to attack it, and falling foul of the mites, they destroy or carry them all off; when the bee, thus delivered from its enemies, takes its flight.