Net-work cocoon.
These are additional instances of the remarks we formerly made, that caterpillars which spin a slight web are transformed into perfect insects in a much shorter period than those which spin more substantial ones. Thus the cream-spot tiger (Arctia villica, Stephens) lies in chrysalis only three weeks, and therefore does not require a strong web. It is figured above, along with another, which is still slighter, though more ingeniously woven, being regularly meshed like net-work.
A very prettily-netted cocoon is constructed by the grub of a very small grey weevil (Hypera rumicis), which is not uncommon in July, on the seed spikes of docks (Rumices). This cocoon is globular, and not larger than a garden pea, though it appears to be very large in proportion to the pupa of the insect, reminding us not a little of the carved ivory balls from China. The meshes of the net-work are also large, but the materials are strong and of a waxy consistence. Upon remarking that no netting was ever spun over the part of the plant to which the cocoon was attached, we endeavoured to make them spin cocoons perfectly globular by detaching them when nearly finished; but though we tried four or five in this way, we could not make them add a single mesh after removal, all of them making their escape through the opening, and refusing to re-enter in order to complete their structure. (J. R.)
The silk, if it may be so termed, spun by many species of larvæ is of a still stronger texture than the waxy silk of the little weevil just mentioned. We recently met with a remarkable instance of this at Lee, in the cocoons of one of the larger ichneumons (Ophion Vinulæ? Stephens), inclosed in that of a puss-moth (Cerura Vinula)—itself remarkable for being composed of sand as well as wood, the fibres of which had been scooped out of the underground cross-bar of an old paling, to which it was attached. But the most singular portion of this was the junction of the outer wall with the edges of the hollow thus scooped out, which was formed of fibres of wood placed across the fibres of the bar nearly at right angles, and strongly cemented together, as if to form a secure foundation for the building.
In this nest were formed, surreptitiously introduced into the original building, five empty cells of a black colour, about an inch long, and a sixth of an inch in diameter; nearly cylindrical in form, but somewhat flattened; vertical and parallel to one another, though slightly curved on the inner side. The cells are composed of strong and somewhat coarse fibres, more like the carbonized rootlets of a tree than silk, and resembling in texture a piece of coarse milled cloth or felt, such as is used for the bases of plated hats. It is worthy of remark, that all these cells opened towards one end, as if the caterpillars which constructed them had been aware that the wall of the puss-moth, in which the flies would have to make a breach, was very hard, and would require their united efforts to effect an escape. The importance of such a precaution will appear more strikingly, when we compare it with the instance formerly mentioned (page 215), in which only one ichneumon had been able to force its way out. (J. R.)
Nest of Puss-moth, inclosing five cocoons of an Ichneumon. Natural size.
It appears indispensable to some grubs to be confined within a certain space in order to construct their cocoons. We saw this well exemplified in the instance of a grub of one of the mason-bees (Osmia bicornis), which we took from its nest, and put into a box with the pollen paste which the mother bee had provided for its subsistence. (See pages [45], [46].) When it had completed its growth, it began to spin, but in a very awkward manner-attaching threads, as if at random, to the bits of pollen which remained undevoured, and afterwards tumbling about to another part of the box, as if dissatisfied with what it had done. It sometimes persevered to spin in one place till it had formed a little vaulted wall; but it abandoned at the least three or four of these in order to begin others, till at length, as if compelled by the extreme urgency of the stimulus of its approaching change, it completed a shell of shining brown silk, woven into a close texture. Had the grub remained within the narrow clay cell built for it by the mother bee, it would, in all probability, not have thus exhausted itself in vain efforts at building, which were likely to prevent it from ever arriving at the perfect state—a circumstance which often happens in the artificial breeding of insects. This bee, however, made its appearance the following spring. (J. R.)
Besides silk, the cocoons of many insects are composed of other animal secretions, intended to strengthen or otherwise perfect their texture. We have already seen that some caterpillars pluck off their own hair to interweave amongst their silk; there are others which produce a peculiar substance for the same purpose. The lackey caterpillar (Clisiocampa neustria, Curtis) in this manner lines its cocoon with pellets of a downy substance, resembling little tufts of the flowers of sulphur. The small egger, again (Eriogaster lanestris, Germar), can scarcely be said to employ silk at all,—the cocoon being of a uniform texture, looking, at first sight, like dingy Paris plaster, or the shell of a pheasant’s egg; but upon being broken, and inspected narrowly, a few threads of silk may be seen interspersed through the whole. In size it is not larger than the egg of the gold-crested wren. It has been considered by Brahm a puzzling circumstance, that this cocoon is usually perforated with one or two little holes, as if made by a pin from without; and Kirby and Spence tell us that their use has not been ascertained.[ED] May they not be left as air-holes for the included chrysalis, as the close texture of the cocoon might, without this provision, prove fatal to the animal? Yet, on comparing one of these with a similar cocoon of the large egger-moth (Lasiocampa quercus), we find no air-holes in the latter, as we might have been led to expect from the closeness of its texture. We found a cocoon of a saw-fly (Trichiosoma), about the same size as that of the egger, attached to a hawthorn-twig, in a hedge at New Cross, Deptford, but of a leathery texture, and, externally, exactly the colour of the bark of the tree. During the summer of 1830 we found a considerable number of the same cocoons. These were all without air-holes. The egger, we may remark, unlike the dock-weevil or the bee-grub just mentioned, can work her cocoon without any point of attachment. We had a colony of these caterpillars in the summer of 1825, brought from Epping Forest, and saw several of them work their cocoons, and we could not but admire the dexterity with which they avoided filling up the little pin-holes. The supply of their building material was evidently measured out to them in the exact quantity required; for when we broke down a portion of their wall, by way of experiment, they did not make it above half the thickness of the previous portion, though they plainly preferred having a thin wall to leaving the breach unclosed. (J. R.)
Several species of caterpillars, that spin only silk, are social, like some of those we formerly mentioned, which unite to form a common tent of leaves (see pages [351], [352], &c.). The most common instance of this is in the caterpillars which feed on the nettle—the small tortoise-shell (Vanessa urticæ), and the peacock’s eye (V. I.). Colonies of these may be seen, after midsummer, on almost every clump of nettles, inhabiting a thin web of an irregular oval shape, from which they issue out to feed on the leaves, always returning when their appetite is satisfied, to assist their companions in extending their premises. Other examples, still more conspicuous from being seen on fruit-trees and in hedges, occur in the caterpillars of the small ermine-moth (Yponomeuta padella), and of the lackey (Clisiocampa neustria), which in some years are but too abundant, though in others they are seldom met with. In the summer of 1826, every hedge and fruit-tree around London swarmed with colonies of the ermine, though it has not since been plentiful; and in the same way, during the summer of 1829, the lackeys were to be seen everywhere. We mention this irregularity of appearance that our readers may not disappoint themselves by looking for what is not always to be found. It is probable that in 1830 the lackeys will be few, for, notwithstanding the myriads of caterpillars last summer, we saw only a single moth of this species, and out of a number of chrysalides which a young friend had in his nurse-boxes, not one moth was bred.