Chapter XXII: Letter XIV: Habitations of Insects (2)

As many of the habitations which I have been describing, fit the body of the insects as close as a coat, they might perhaps with more propriety be called _clothes_. This is certainly the most appropriate designation of the abodes of some species of Tineæ (the clothes' moths), which not only cover themselves with a coat, but employ the very same material in its composition as we do in ours, forming it of wool or hair curiously felted together. Like us, they are born naked, but not like us helpless at that period, scarcely have they breathed before they begin to clothe themselves; thus contradicting Dr. Paley's assertion, that "the _human_ animal is the only one which is naked, and the only one which can clothe itself[808]:" and wisely inattentive to change of fashion, the same suit serves them from their birth to mature age. The shape of their dress is adapted to that of their body--a cylindrical case open at both ends. The stuff of which it is composed is the manufacture of the larva of the moth (_Tinea_), which incorporates wool or hair artfully cut from our clothes or furniture, with silk drawn from its own mouth, into a warm and thick tissue: and as this would not be soft enough for its tender skin, it also lines the inside of its coat with a layer of pure silk. Since this suit of clothes during the earliest age of the insect accurately fits its body, you will readily conceive that it will frequently require enlarging. This the little occupant accomplishes as dexterously as any tailor. If the case merely requires lengthening, the task is easy. All that is needful is to add a new ring of hair or wool and silk to each end. But to enlarge it in width is not so simple an affair. Yet it sets to work precisely as we should, slitting the case on the two opposite sides, and then adroitly inserting between them two pieces of the requisite size. It does not, however, cut open the case from one end to the other at once: the sides would separate too far asunder, and the insect be left naked. It therefore first cuts each side about half way down, and then after having filled up the fissure proceeds to cut the remaining half: so that, in fact, four enlargements are made, and four separate pieces inserted.--The colour of the habit is always the same as that of the stuff from which it is taken. Thus, if its original colour be blue, and the insect previously to enlarging it be put upon red cloth, the circles at the end and two stripes down the middle will be red. If placed alternately upon cloths of different hues, its dress will be parti-coloured like that of a Harlequin.--The injury occasioned to us by these insects is not confined to the quantity of materials consumed in clothing and feeding themselves. In moving from place to place they seem to be as much incommoded by the long hairs which surround them, as we are by walking amongst high grass; and accordingly, marching scythe in hand, with their teeth they cut out a smooth road, from time to time reposing themselves, and anchoring their little case with small silken cables.

If, as I hope, you are induced to investigate the manners of these insects, you have but to leave an old coat for a few months undisturbed in a dark closet, and you may be pretty certain of meeting with an abundant colony.

Not merely wool or hair, but another substance analogous to one employed in our dress, is adopted for their clothing by other insects. The larva of a fly which lives on the seeds of willows, makes itself a very beautiful case of their cottony down, not only impervious to wet and cold, but serving, if accidentally blown into the water, which from the situation of these trees frequently happens, as a buoyant little barge which is wafted safely to the shore[809].

The habitations which we have hitherto been considering, are formed by larvæ that live on land, but others equally remarkable are constructed by aquatic species, the larvæ of the various _Phryganeæ_ L., a tribe of four-winged insects which an ordinary observer would call moths, but which are even of a distinct order (_Trichoptera_), not having their wings covered by the scales which adorn the lepidopterous race. If you are desirous of examining the insects to which I am alluding, you have only to place yourself by the side of a clear and shallow pool of water, and you cannot fail to observe at the bottom little oblong moving masses resembling pieces of straw, wood, or even stone. These are the larvæ in question, well known to fishermen by the title of _Caddis-worms_, and which, if you take them out of the water, you will observe to inhabit cases of a very singular conformation. Of the larva itself, which somewhat resembles the caterpillars of many _Lepidoptera_, nothing is to be seen but the head and six legs by means of which it moves itself in the water, and drags after it the case in which the rest of the body is inclosed, and into which on any alarm it wholly retires. The construction of these habitations is very various. Some select four or five pieces of the leaves of grass, which they glue together into a shapely polygonal case; others employ portions of the stems of rushes, placed side by side so as to form an elegant fluted cylinder; some arrange round them pieces of leaves like a spirally-rolled ribband[810]; others inclose themselves in a mass of the leaves of any aquatic plants united without regularity; and others again form their abode of minute pieces of wood either fresh or decayed[811]. One, like the _Sabellæ_[812], forms a horn-shaped case composed of grains of sand, so equal in size, and so nicely and regularly gummed together, the sides throughout being of the thickness of one grain only, that the first time I viewed it I could scarcely persuade myself it could be the work of an insect. The case of _Leptocerus bimaculatus_, which is less artificially constructed of a mixture of mud and sand, is pyriform, and has its end curiously stopped by a plate formed of grains of sand, with a central aperture[813]. Other species construct houses which may be called alive, forming them of the shells of various aquatic snails of different kinds and sizes even while inhabited, all of which are immoveably fixed to it, and dragged about at its pleasure--a covering as singular as if a savage, instead of clothing himself with squirrels' skins, should sew together into a coat the animals themselves. However various may be the form of the case externally, within it is usually cylindrical and lined with silk; and though seldom apparently wider than just to admit the body of the insect, some species have the power of turning round in it, and of putting out their head at either end[814]. Some larvæ constantly make their cases of the same materials; others employ indifferently any that are at hand; and the new ones which they construct as they increase in size (for they have not the faculty, like the larva of the moth, of enlarging them) have often an appearance quite dissimilar to that of the old. Even those that are most careless about the nature of the materials of their house, are solicitously attentive to one circumstance respecting them, namely, their specific gravity. Not having the power of swimming, but only of walking at the bottom of the water by aid of the six legs attached to the fore part of the body which is usually protruded out of the case, and the insect itself being heavier than water, it is of great importance that its house should be of a specific gravity so nearly that of the element in which it resides, as while walking neither to incommode it by its weight, nor by too great buoyancy; and it is as essential that it should be so equally _ballasted_ in every part as to be readily moveable in any position. Under these circumstances our Caddis-worms evince their proficiency in hydrostatics, selecting the most suitable substances; and, if the cell be too heavy, glueing to it a bit of leaf or straw; or, if too light, a shell or piece of gravel. It is from this necessity of regulating the specific gravity, that to the cases formed with the greatest regularity we often see attached a seemingly superfluous piece of wood, leaf, or the like.

A larva of one of the aquatic _Tipulariæ_ lives in cases somewhat similar to those of some _Phryganeæ_. Several of these of a fusiform shape and brown colour, composed partly of silk and partly perhaps of fragments of leaves, and inhabited by a red larva apparently of a _Chironomus_, were found by Reaumur upon dead leaves in a pool of water in the Bois de Boulogne[815].

In concluding this head I may observe, that here might have been described the various abodes which solitary larvæ prepare for themselves previously to assuming the pupa, and intended for their protection in that defenceless stage of existence; but as I shall have occasion again to refer to them in speaking of the larva state of insects, I shall defer their description to that letter, to which they more strictly belong.

* * * * *

From the next division of the habitations of insects--those formed by solitary _perfect_ insects for their own accommodation--I shall select for description only two, both the work of spiders, and alluded to in a former letter, which indeed, with the exception of the inartificial retreats made by the _Grylli_, _Cicindelæ_, and perhaps a few others, are the only ones properly belonging to it.

The habitation of one of these (_Cteniza cæmentaria_) is subterraneous, not a mere shallow cavity, but a tube or gallery upwards of two feet in length and half an inch broad. This tunnel, so vast compared with the size of the insect, it digs by means of its strong jaws in a steep bank of bare clay, so that the rain may readily run off without penetrating to its dwelling. Its next operation is to line the whole from top to bottom with a web of fine silk, which serves the double purpose of preventing the earth that composes the walls from falling in, and, by its connexion with the door of the orifice, of giving information to the spider of what is passing above. You doubtless suppose that in saying _door_ I am speaking metaphorically. It could never enter into your conception that any animal, much less an insect, could construct any thing really deserving of that name--any thing like our doors, turning upon a hinge, and accurately fitted to the frame of the opening which it is intended to close. Yet such a door, incredible as it may seem, is actually framed by this spider. It does not indeed, like us, compose it of wood, but of several coats of dried earth fastened to each other with silk. When finished, its outline is as perfectly circular as if traced with compasses; the inferior surface is convex and smooth, the superior flat and rough, and so like the adjoining earth as not to be distinguishable from it. This door the ingenious artist fixes to the entrance of her gallery by a hinge of silk, which plays with the greatest freedom, and allows it to be opened and shut with ease; and as if acquainted with the laws of gravity, she invariably fixes the hinge at the _highest_ side of the opening, so that the door when pushed up shuts again by its own weight. She has not less sagaciously left a little edge or groove just within the entrance, upon which the door closes, and to which it fits with such precision, that it seems to make but one surface with it. Such is the astonishing structure of this little animal's abode; nor is its defence of its subterraneous cavern less surprising. If an observer adroitly insinuates the point of a pin under the edge of the door, and elevates it a little, he immediately perceives a very strong resistance.--What is its cause?--The spider, warned by the vibrations of the threads which extend from the door to the bottom of her gallery, runs with all speed to the door, fastens its legs to it on one side, and on the other to the walls, and, turning upon its back, pulls with all its might. Thus the door is alternately shut or opened, as the exertions of the observer or of the spider prevail. It is easy to guess which will in the end conquer; and the spider, when it finds all resistance ineffectual, betakes itself to flight, and retreats. If, to make a further experiment, the observer fastens down the door so that it cannot be forced open, the next morning he will find a new entrance, with a new door formed at a small distance; or, if he take the door entirely away, another will be constructed in less than twelve hours.

The habitation thus singularly formed and defended is not at all used as a snare, but merely as a safe abode for the spider, which hunts its prey at night only; and, when caught, devours it in security at the bottom of its den, which is generally strewed with the remains of coleopterous insects[816].--From some curious observations of M. Dorthes on this species in the second volume of the _Linnean Transactions_, it appears that both the male and female spider and as many as thirty young ones occasionally inhabit one of these galleries.--_Mygale Sauvagesii_ of Rossi, which is a distinct species found in Corsica, forms a similar habitation[817].

The galleries just described are the work of an European species not uncommon in the south of France; but similar ones are fabricated by _Thomisus venatorius_, an inhabitant of the West India islands, as well as by many other tropical species. I have seen one of these, which had been dug out of the earth, in the cabinet of Thomas Hall, Esq. F.L.S., that was nearly a foot in length, and above an inch in diameter, forming a cylindrical bag of dark-coloured silk, closed at the bottom, and accurately fitted at the top by a door or lid.

The habitation of _Argyroneta aquatica_, the other spider to which I alluded, is chiefly remarkable for the element in which it is constructed and the materials that compose it. It is built in the midst of water, and formed, in fact, of air! Spiders are usually terrestrial, but this is aquatic, or rather amphibious; for though she resides in the midst of water, in which she swims with great celerity, sometimes on her belly but more frequently on her back, and is an admirable diver, she not unfrequently hunts on shore, and, having caught her prey, plunges with it to the bottom of the water. Here it is she forms her singular and unique abode. She would evidently have but a very uncomfortable time were she constantly wet, but this she is sagacious enough to avoid; and by availing herself of some well-known philosophical principles, she constructs for herself an apartment in which, like the mermaids and sea-nymphs of fable, she resides in comfort and security. The following is her process. First she spins loose threads in various directions attached to the leaves of aquatic plants, which may be called the frame-work of her chamber, and over them she spreads a transparent varnish resembling liquid glass, which issues from the middle of her spinners, and which is so elastic that it is capable of great expansion and contraction: and if a hole be made in it, it immediately closes again. Next she spreads over her belly a pellicle of the same material, and ascends to the surface. The precise mode in which she transfers a bubble of air beneath this pellicle is not accurately known; but from an observation made by the ingenious author of the little work from which this account is abstracted, he concludes that she draws the air into her body by the anus, which she presents to the surface of the pool, and then pumps it out from an opening at the base of the belly between the pellicle and that part of the body, the hairs of which keep it extended. Clothed with this aërial mantle, which to the spectator seems formed of resplendent quicksilver, she plunges to the bottom, and, with as much dexterity as a chemist transfers gas with a gas-holder, introduces her bubble of air beneath the roof prepared for its reception. This manœuvre she repeats ten or twelve times, until at length in about a quarter of an hour she has transported as much air as suffices to expand her apartment to its intended extent, and now finds herself in possession of a little aërial edifice, I had almost said an enchanted palace, affording her a commodious and dry retreat in the very midst of the water. Here she reposes unmoved by the storms that agitate the surface of the pool, and devours her prey at ease and in safety. Both sexes form these lodgings. At a particular season of the year the male quits his apartment, approaches that of the female, enters it, and enlarging it by the bubble of air that he carries with him, it becomes a common abode for the happy pair[818].--The spider which forms these singular habitations is one of the largest European species, and in some countries not uncommon in stagnant pools.

I am, &c.

FOOTNOTES:

[758] _Melitta._ *. a. K.

[759] Grew's _Rarities of Gresham Colledge_, 154. Kirby, _Mon. Ap. Angl._ i. 131. _Melitta._ *. a.

[760] Curtis _Brit. Ent. t._ 61.

[761] _Mon. Ap. Angl._ i. 173. _Apis._ **. c. 2. α. From later observations I am inclined to think that these cells may possibly, as in the case of the humble-bee, be in fact formed by the larva previously to becoming a pupa, after having eaten the provision of pollen and honey with which the parent bee had surrounded it. The vermicular shape, however, of the masses with which the cases are surrounded, does not seem easily reconcileable with this supposition, unless they are considered as the excrement of the larva.

[762] _Apis._ **. d. 2. β. K.

[763] Reaum. vi. 39-50. _Mon. Ap. Angl._ i. 189. _Apis._ **. α. 2. β.

[764] _Ann. du Mus._ x. 236.

[765] Reaumur plausibly supposes that it has been from observing this bee thus loaded, that the tale mentioned by Aristotle and Pliny, of the hive-bee's ballasting itself with a bit of stone previously to flying home in a high wind, has arisen.

[766] Reaum. vi. 57-88. _Mon. Ap. Angl._ i. 179.

[767] _Apis._ **. c. 2. δ. K.

[768] _Apis._ **. c. 2. α. K.

[769] Reaum. vi. 139-148.

[770] Latr. _Hist. Nat. des Fourmis_, 297.

[771] Reaum. vi. 971-24. _Mon. Ap. Angl._ i. 157. Apis. **. c. 2. α.

[772] Reaum. vi. 251-7. _t._ xxvi. _f._ 1.

[773] Latr. _Fourmis_, 419.

[774] See above, p. 317--.

[775] Aikin's _Dictionary of Chemistry_, i. 455. What have probably been taken by Mr. Aikin for "kernels," in the imperforated nuts, are the cocoons of the inhabitants of these galls in the pupa state, which often extremely resemble the seeds of a capsule, as Reaumur (iii. 429.) has remarked.

[776] Reaum. iii. 417, &c.

[777] _Introd. to Botany_, 349.

[778] Reaum. iii. 474.

[779] Ibid. 479.

[780] Ibid. 501.

[781] Ibid. 479.

[782] _De Insectis_, 233 &c.

[783] Reaum. iii. t. 38. f. 2, 3.

[784] Ibid. iii. 448.

[785] Ibid. 455.

[786] De Geer, vi. 409.

[787] De Geer, vi. 421.

[788] Jacquin _Collect._ ii. 255.

[789] Reaum. iii. 427.

[790] Lyonet, _Anat. of Coss._ 9.

[791] P. 307, 392.

[792] Lewin's _Prodromus Entomology_ (sic!), p. 8.

[793] Bonnet, ix. 188.

[794] Reaum. iii. 100-120.

[795] Ibid. 146.

[796] Forsyth _on Fruit Trees_, 4to edit. 271.

[797] Goeze _Natur. Menschenleben und Vorsehung_. Anderson's _Recreations_, ii. 409. See above p. 16.

[798] Reaum. iii. 206. PLATE XVII. FIG. 9.

[799] Germar's _Mag. für Entomologie_, i. 40.

[800] x. 458.

[801] Reaum. iii. 183.

[802] The larvæ of the males intermix with the pieces of twigs, which are less closely and regularly arranged, bits of dried leaves and other light materials. See the excellent elucidation of the history of this tribe, whose mode of generation is so singular, by Von Scheven, in the _Naturforscher_ Stk. xx. 61, &c. also a valuable paper by Dr. Zincken genannt Sommer, in Germar's _Mag. für Ent._ i. 19-40.

[803] Reaum. iii. 148-9. T. 11. f. 10. 11.

[804] Fuessly, _Archiv._ 53. _t._ 31. Germar's _Mag. für Ent._ i. 136.

[805] See above, p. 165.

[806] Aristot. _Hist. Anim._ l. viii. c. 27.

[807] Reaum. iii. mem. 8.

[808] _Nat. Theol._ 230.

[809] Reaum. iii. 130.

[810] PLATE XVII. FIG. 10.

[811] Reaum. iii. 156-9.

[812] Sowerby's _Nat. Miscell._ _No._ ix. _t._ 51.

[813] De Geer, ii. 564.

[814] De Geer, ii. 564.

[815] Reaum. iii. 179.

[816] Sauvages _Hist. de l'Acad. des Sc. de Paris_, 1758, p. 26. Perhaps this, as well as _M. cæmentaria_, belongs to Latreille's genus _Cteniza_. _Familles Naturelles du Règne Animal_, 313.

[817] Latr. _Hist. Nat._ vii. 165.

[818] _Mémoire pour servir à commencer l'Histoire des Araignées Aquatiques_, 12mo.