Part 31

The last nest is made of some substance which is smooth, and hard as horn, brown within, and dark grey on the outside. The circular lid by which the enclosed insect escapes is shown open.

In the accompanying illustration, we have five remarkable pensile nests of insects, some British, and others exotic.

Fig. 1 represents the nest of a Pelopæus from Natal. It is made of dried cow-dung, and is fixed to straws. The length is from three to five inches, and there are sometimes found three or more in a row upon a single straw. The insect is about an inch in length, black-blue in colour, and with clouded wings. The abdomen is small, sharply pointed, and placed on a long footstalk.

At Fig. 2 is seen the nest of Pelopæus Flavipes, a North American insect, which is also fixed along its whole length to the supporting object, which is sometimes a wall, and sometimes, as in the illustration, a branch. It is made of mud, and the insects seem to have a sort of gregarious instinct, loving to fix their nests in rows, one above the other. There is only one larva in each cell. The Pelopæi are, by the way, allied to the English genus Ammophila.

Fig. 3 shows the nest of Anthidium cordatum, one of the solitary bees of Natal. It is made of vegetable fibres. The insect as well as the nest is represented of the natural size. It is black and shining, with the under part and sides and legs yellowish.

At Fig. 4 are seen three of the nests of Trypoxylon aurifrons, a Brazilian insect. They are built of mud, and are remarkable for their elegant shape, which looks as if it had been formed by the hand of the potter, and for the manner in which the mouth is turned over so as to form a distinct neck. The larvæ is fed with a store of spiders. The insect is represented of the natural size; its colour is black, and the face is covered with short golden hairs, a fact which has gained for it the name of aurifrons, or golden-fronted.

Our last example, Fig. 5, is the nest of an English insect, Eumenes coarctata. The insect is represented of its natural size. It is very pretty in colour as well as elegant in shape, being black, diversified with yellow bands and spots. The nest is made of clay, and is found upon the heath twigs. The larvæ of the Eumenes are fed with those of a species of Crambus. The insect is tolerably common in Surrey and Hampshire, and appears in July and August.

The three figures in the next illustration represent the cocoons of three species of the Bombycidæ, and are given in order to show the different modes by which they are fastened. The upper nest is hung by a slight cord, which spreads into a broad silken band wrapped round the branch for some distance. The right-hand figure shows a very remarkable cocoon suspended by a long footstalk affixed to a ring. The remarkable point in the construction of this ring is that it is very hard and horny, and is not fastened to the branch, but passes loosely round it, so that the cocoon swings backwards and forwards in the breeze. The cocoon is about two inches in length, and is covered with thick black veinings. The lowermost cocoon is most curiously fixed to the branch by bending the leaves round the exterior of the dwelling, and fixing them to it with silk. All these specimens were brought from Northern India.]

In all the nests of social caterpillars, care is taken to leave apertures for passing out and in. It is remarkable, also, that however far they may ramble from their nest, they never fail to find their way back when a shower of rain or nightfall renders shelter necessary. It requires no great shrewdness to discover how they effect this: for by looking closely at their track it will be found that it is carpeted with silk--no individual moving an inch without constructing such a pathway, both for the use of his companions and to facilitate his own return. All these social caterpillars, therefore, move more or less in processional order, each following the road which the first chance traveller has marked out with his strip of silk carpeting.

There are some species, however, which are more remarkable than others in the regularity of their processional marchings, particularly two which are found in the south of Europe, but are not indigenous in Britain. The one named by Réaumur the Processionary (Cnethocampa processionea, Stephens) feeds upon the oak; a brood dividing, when newly hatched, into one or more parties of several hundred individuals, which afterwards unite in constructing a common nest nearly two feet long, and from four to six inches in diameter. As it is not divided like that of the brown-tails into chambers, but consists of one large hall, it is not necessary that there should be more openings than one; and accordingly, when an individual goes out and carpets a path, the whole colony instinctively follow in the same track, though from the immense population they are often compelled to march in parallel files from two to six deep. The procession is always headed by a single caterpillar; sometimes the leader is immediately followed by one or two in single file, and sometimes by two abreast, as represented in the cut. A similar procedure is followed by a species of social caterpillars which feed on the pine in Savoy and Languedoc; and though their nests are not half the size of the preceding, they are more worthy of notice, from the strong and excellent quality of their silk, which Réaumur was of opinion might be advantageously manufactured. Their nest consists of more chambers than one, but is furnished with a main entrance, through which the colonists conduct their foraging processions.

CHAPTER XVIII.

STRUCTURES OF SPIDERS.

Modern naturalists do not rank spiders among insects, because they have no antennæ, and no division between the head and the shoulders. They breathe by leaf-shaped gills, situated under the belly, instead of spiracles in the sides; have a heart connected with these; have eight legs instead of six; and eight fixed eyes. But as spiders are popularly considered insects, it will sufficiently suit our purpose to introduce them here as such.

The apparatus by which spiders construct their ingenious fabrics is much more complicated than that which we have described as common to the various species of caterpillars. Caterpillars have only two reservoirs for the materials of their silk; but spiders, according to the dissections of M. Treviranus, have four principal vessels, two larger and two smaller, with a number of minute ones at their base. Several small tubes branch towards the reservoirs, for carrying to them, no doubt, a supply of the secreted material. Swammerdam describes them as twisted into many coils of an agate colour. We do not find them coiled, but nearly straight, and of a deep-yellow colour. From these, when broken, threads can be drawn out like those spun by the spider, though we cannot draw them so fine by many degrees.

From these little flasks or bags of gum, situated near the apex of the abdomen, and not at the mouth, as in caterpillars, a tube originates, and terminates in the external spinnerets, which may be seen by the naked eye in the larger spiders, in the form of five little teats surrounded by a circle, as represented in the following figure.

We have seen that the silken thread of a caterpillar is composed of two united within the tube of the spinneret, but the spider's thread would appear, from the first view of its five spinnerets, to be quintuple, and in some species which have six teats, so many times more. It is not safe, however, in our interpretations of nature to proceed upon conjecture, however plausible, nor to take anything for granted which we have not actually seen; since our inferences in such cases are almost certain to be erroneous. If Aristotle, for example, had ever looked narrowly at a spider when spinning, he could not have fancied, as he does, that the materials which it uses are nothing but wool stripped from its body. On looking, then, with a strong magnifying glass, at the teat-shaped spinnerets of a spider, we perceive them studded with regular rows of minute bristle-like points, about a thousand to each teat, making in all from five to six thousand. These are minute tubes which we may appropriately term spinnerules, as each is connected with the internal reservoirs, and emits a thread of inconceivable fineness. In the following figure, this wonderful apparatus is represented as it appears in the microscope.

We do not recollect that naturalists have ventured to assign any cause for this very remarkable multiplicity of the spinnerules of spiders, so different from the simple spinneret of caterpillars. To us it appears to be an admirable provision for their mode of life. Caterpillars neither require such strong materials, nor that their thread should dry as quickly. It is well known in our manufactures, particularly in rope-spinning, that in cords of equal thickness, those which are composed of many smaller ones united are greatly stronger than those which are spun at once. In the instance of the spider's thread, this principle must hold still more strikingly, inasmuch as it is composed of fluid materials that require to be dried rapidly, and this drying must be greatly facilitated by exposing so many to the air separately before their union, which is effected at the distance of about a tenth of an inch from the spinnerets. In the following figure each of the threads represented is reckoned to contain one hundred minute threads, the whole forming only one of the spider's common threads.

Leeuwenhoeck, in one of his extraordinary microscopical observations on a young spider not bigger than a grain of sand, upon enumerating the threadlets in one of its threads, calculated that it would require four millions of them to be as thick as a hair of his beard.

Another important advantage derived by the spider from the multiplicity of its threadlets is, that the thread affords a much more secure attachment to a wall, a branch of a tree, or any other object, than if it were simple; for, upon pressing the spinneret against the object, as spiders always do when they fix a thread, the spinnerules are extended over an area of some diameter, from every hair's-breadth of which a strand, as rope-makers term it, is extended to compound the main cord. The preceding figure exhibits this ingenious contrivance.

Those who may be curious to examine this contrivance will see it best when the line is attached to any black object, for the threads, being whitish, are, in other cases, not so easily perceived.

Shooting of the Lines.

It has long been considered a curious though a difficult investigation, to determine in what manner spiders, seeing that they are destitute of wings, transport themselves from tree to tree, across brooks, and frequently through the air itself, without any apparent starting point. On looking into the authors who have treated upon this subject, it is surprising how little there is to be met with that is new, even in the most recent. Their conclusions, or rather their conjectural opinions, are, however, worthy of notice; for by unlearning error, we the more firmly establish truth.

1. One of the earliest notions upon this subject is that of Blancanus, the commentator on Aristotle, which is partly adopted by Redi, by Henricus Regius of Utrecht, by Swammerdam, by Lehmann, and by Kirby and Spence. "The spider's thread," says Swammerdam, "is generally made up of two or more parts, and after descending by such a thread, it ascends by one only, and is thus enabled to waft itself from one height or tree to another, even across running waters; the thread it leaves loose behind it being driven about by the wind, and so fixed to some other body." "I placed," says Kirby, "the large garden spider (Epeira diadema) upon a stick about a foot long, set upright in a vessel containing water.... It let itself drop, not by a single thread, but by two, each distant from the other about the twelfth of an inch, guided, as usual, by one of its hind feet, and one apparently smaller than the other. When it had suffered itself to descend nearly to the surface of the water, it stopped short, and by some means, which I could not distinctly see, broke off, close to the spinners, the smallest thread, which still adhering by the other end to the top of the stick, floated in the air, and was so light as to be carried about by the slightest breath. On approaching a pencil to the loose end of this line, it did not adhere from mere contact. I therefore twisted it once or twice round the pencil, and then drew it tight. The spider, which had previously climbed to the top of the stick, immediately pulled at it with one of its feet, and finding it sufficiently tense, crept along it, strengthening it as it proceeded by another thread, and thus reached the pencil."

We have repeatedly witnessed this occurrence, both in the fields and when spiders were placed for experiment, as Kirby has described; but we very much doubt that the thread broken is ever intended as a bridge cable, or that it would have been so used in that instance, had it not been artificially fixed and accidentally found again by the spider. According to our observations, a spider never abandons, for an instant, the thread which she despatches in quest of an attachment, but uniformly keeps trying it with her feet, in order to ascertain its success. We are, therefore, persuaded that when a thread is broken in the manner above described, it is because it has been spun too weak; and spiders may often be seen breaking such threads in the process of netting their webs. (J. R.)

The plan, besides, as explained by these distinguished writers, would more frequently prove abortive than successful, from the cut thread not being sufficiently long. They admit, indeed, that spiders' lines are often found "a yard or two long, fastened to twigs of grass not a foot in height.... Here, therefore, some other process must have been used."

2. Our celebrated English naturalist, Dr. Lister, whose treatise upon our native spiders has been the basis of every subsequent work on the subject, maintains that "some spiders shoot out their threads in the same manner that porcupines do their quills; that whereas the quills of the latter are entirely separated from their bodies, when thus shot out, the threads of the former remain fixed to their anus, as the sun's rays to its body." A French periodical writer goes a little farther, and says, that spiders have the power of shooting out threads, and directing them at pleasure towards a determined point, judging of the distance and position of the object by some sense of which we are ignorant. Kirby also says, that he once observed a small garden spider (Aranea reticulata) "standing midway on a long perpendicular fixed thread, and an appearance caught" his "eye, of what seemed to be the emission of threads." "I, therefore," he adds, "moved my arm in the direction in which they apparently proceeded, and, as I had suspected, a floating thread attached itself to my coat, along which the spider crept. As this was connected with the spinners of the spider, it could not have been formed" by breaking a "secondary thread." Again, in speaking of the gossamer-spider, he says, "it first extends its thigh, shank, and foot, into a right line, and then, elevating its abdomen till it becomes vertical, shoots its thread into the air, and flies off from its station."

Another distinguished naturalist, Mr. White of Selborne, in speaking of the gossamer-spider, says, "Every day in fine weather in autumn do I see these spiders shooting out their webs, and mounting aloft: they will go off from the finger, if you will take them into your hand. Last summer, one alighted on my book as I was reading in the parlour; and running to the top of the page, and shooting out a web, took its departure from thence. But what I most wondered at was, that it went off with considerable velocity in a place where no air was stirring; and I am sure I did not assist it with my breath."

Having so often witnessed the thread set afloat in the air by spiders, we can readily conceive the way in which those eminent naturalists were led to suppose it to be ejected by some animal force acting like a syringe; but as the statement can be completely disproved by experiment, we shall only at present ask, in the words of Swammerdam--"how can it be possible that a thread so fine and slender should be shot out with force enough to divide and pass through the air?--is it not rather probable that the air would stop its progress, and so entangle it and fit it to perplex the spider's operations?" The opinion, indeed, is equally improbable with another, suggested by Dr. Lister, that the spider can retract her thread within the abdomen, after it has been emitted. De Geer very justly joins Swammerdam in rejecting both of these fancies, which, in our own earlier observations upon spiders, certainly struck us as plausible and true. There can be no doubt, indeed, that the animal has a voluntary power of permitting the material to escape, or stopping it at pleasure, but this power is not projectile.