104 



KNOWLEDGE 



[May 1, 1894. 



snares are made is less like that of insects, inasmuch 

 as it remains more or less viscid after exclusion, while 

 insect silk hardens at once on exposure to the air. If 

 the silk of spiders' webs did not retain its viscosity, 

 it would be of little use for entrapping victims to 

 supply the larder ; and, on the other hand, a permanent 

 viscosity in the silk of cocoons, whether the egg ones of 

 spiders or the chrysalis ones of insects, would be a draw- 

 back rather than an advantage, as the cocoon would 

 become encumbered with rubbish which would adhere to it, 

 and consequently we find no such property developed in 

 ihis connection. The position of the silk glands, too, is 

 quite difi'erent in spiders from what obtains in insects. In 

 the former they are situated in the abdomen, and the silk 

 issues from innumerable minute orifices situated on papillas 

 imder the tail end of the body ; whereas the silk glands of 

 insects are placed in the anterior part of the body, and the 

 ducts open by a single orifice on one small papilla just 

 beneath the lower lip and outside the mouth. 



Early in the last century, Reaumur made some inquiries 

 as to the comparative value of the silk of spiders and that 

 of silkworms for manufacturing purposes. The inquiries 

 were suggested by the achievement of a certain M. Bon, 

 of Languedoc, who had succeeded in making stockings and 

 mittens of spider silk. Reaumur found that the thread of 

 the spider's cocoon had about eighteen times the strength 

 nf that of the web, and further, that it would take about 

 five of the spider's strongest threads to equal one of the 

 silkworm's. He calculated also that it would require 

 nearly thirteen times as many spiders as silkworms to 

 make the same amount of silk, and since only the cocoon 

 silk would be available, of course all of these must be 

 females. The superiority of insect silk was thus un- 

 mistakably demonstrated ; while the further difficulty, that 

 the natural ferocity of spiders rendered it impossible to 

 keep large numbers of them together, put the idea of 

 profitably using their silk out of the question. 



We have already pointed out that the power of secreting 

 silk is possessed by insects in their larval state only, the 

 glands by which it is secreted being aborted in the adult. 

 The commercial importance of the silk derived from the 

 Chinese silkworm leads us, no doubt, to mentally associate 

 silk most closely with the caterpillars of moths and 

 butterflies, but its production is not confined to the order 

 Lepidoptera, though what is produced by insects outside 

 that order has no commercial value. Amongst the 

 Hymenoptera, for example, we find saw-flies and ichneumon 

 files making cocoons just as successfully as any lepidopterous 

 insect. The larvse of some ants also make a silken cover 

 within which to become pupse, and those of bees and wasps 

 cover the entrance to their cells with a silken cap, so that 

 they may pass into their resting condition undisturbed. 

 Some beetles, too, make neat little network or papery 

 cocoons, which are attached to the food-plant. The 

 caddis worms, again, which are the larvae of caddis-fiies, 

 members of tbe order Trichoptera, line their curiously- 

 constructed cases with silk, so as to form a smooth and 

 comfortable chamber. The same material is used to 

 fasten together the shells, grains of sand, bits of stick, or 

 dead leaves by which the case is ornamented outwardly, 

 as well as to make the grating by which the end of the 

 case is guarded when the enclosed insect becomes a pupa. 

 Lastly, even the common flea envelopes itself in a little 

 silken ball when it changes from a wriggling maggot into 

 a restful, dumpy chrysalis. Thus we see that the power 

 of secreting some sort of material to which the name 

 " silk " may fairly be applied is found in at least four 

 orders, though in only three of them is it widely prevalent. 



As one of the chief uses of silk is to construct cocoons 



for protecting the chrysalis, silk glands must not be 

 expected to be present in insects whose metamorphosis is 

 incomplete, and in which there is no such thing as a 

 chrysalis ; so that to dragon-flies, grasshoppers, crickets, 

 earwigs, bugs, &c., silk spinning is an imknown art. And 

 as we have seen, even amongst those insects whose meta- 

 morphosis is complete, there are many that pass through 

 their changes without the assistance of silk. It is un- 

 questionably in the order Lepidoptera that the silk glands 

 reach their highest perfection, and here the uses of the 

 secretion are manifold. Many larvis of moths hang 

 suspended by a silken thread when suddenly shaken from the 

 trees on which they are feeding ; others construct shelters 

 by fastening leaves together or building fixed galleries or 

 movable tubes, while social caterpillars employ their silk 

 to fabricate a nest which all may use in common. There 

 is a curious little moth which is a great pest in choco- 

 late warehouses, and which, living in enormous swarms 

 together, sometimes covers the whole of a wall with a thin 

 sheet of silken webbing to facilitate the movements of the 

 members of the colony. The same secretion, too, helps to 

 bind together the cocoa-nibs, and prevent them from 

 shifting as they become disturbed by the ravages of the 

 insects. It is of this destructive pest that Curtis speaks 

 in his " Farm Insects," when he draws the following 

 horrifying picture ; "I have known bushels of cocoa-nuts 

 which were, every one, worm-eaten and full of maggots, 

 with their webs, excrement, cast-oiJ skins, pupte, and 

 cocoons, all ground down to make chocolate, flavoured, I 

 suppose, with vanilla." 



Should a caterpillar wish to change its skin, it will often 

 spin a little raft of silk upon a leaf to serve for foothold 

 for the " claspers," while it crawls out of its old skin 

 through a slit in the neck. The hooks with which the 

 claspers are furnished being inserted firmly amongst the 

 meshes of the silken structure, a position of vantage is 

 gained, and the creature can use all its strength to pull 

 itself out from its old skin, which will be left collapsed on 

 the raft. And then, when the pupating time comes round, 

 the results of the use of silk are of the most varied 

 description. In some cases a distinct cocoon is made, 

 smooth and even inside, and more or less rough outside, 

 either white or of some shade of yellow or brown, the 

 texture varying from that of a thin and brittle papery film, 

 or a hard and chippy layer in which no threads can be 

 traced, to the loosest possible network of distinct threads. 

 If the caterpillar adopts a subterranean retreat, either a cell 

 is excavated in the loose earth, and the fragments of soil in 

 its walls cemented together by silk, or a complete and 

 tough silken lining is made, which easily comes away from 

 the surrounding earth when it is dug up. Such subter- 

 ranean cocoons may often be found round the roots of 

 trees, especially where the soil is bare and aftbrds easy 

 ingress to the burying caterpillars. 



A third method of utilizing the silk for pupation is to be 

 met with amongst the larvse of butterflies. These insects 

 are very chary of their silk, expending an exceedingly 

 small quantity of it on their pupating arrangements, and 

 rarely making even so much as a loose web. Sometimes 

 there is merely a small button of silk to serve as a support 

 for the tail of the chrysalis, which thus hangs head down- 

 wards from the underside of a leaf or a ledge of some 

 sort. The beautiful butterflies known as Vaiu'ssid(r, and 

 popularly called tortoiseshells, admirals, peacocks, &o., 

 pupate in this way, as also do the lovely spotted 

 fritillaries. The process of suspension by the tail is a 

 difficult one, and is carried out in a remarkable fashion. 

 The button of silk is first formed, the caterpillar laying 

 down layer after layer, each one over a smaller area than 



