NOVEMBEK 1, lb!»8.] 



KNO WLE DGE 



215 



passive state than in a condition of activity. The means 

 adopted to meet the need are varied, but the majority of 

 cases may be grouped under two lieads : either the insect 

 buries itsi'lf in the ground, or, remaining ixliove ground, 

 envelopes itself in a silken covering or cocoon. AVhile the 

 former method, from the nature of the case, presents few 

 peculiarities and calls for little remarii, the latter is full 

 of interest in consequence of the variety of form and the 

 remarkable adaptations and contrivances which the cocoons 

 exhibit ; we therefore propose briefly to discuss the most 

 interesting of these specimens of msect architecture which 

 are to be met with in the woods, fields, and hedges of our 

 own country. We naturally look to the Lepidoptera for 

 our chief, though not for our only, examples ; the most 

 notably cocoon-constructing section of this order is the 

 group called Bombyces, including the moths known in 

 popular phraseology as "tigers," "ermines," " eggers," 

 and " emperors." 



But before referring to specific instances in detail, a few 

 words are necessary as to the organ by which the material 

 used in these constructions is produced. There is a pair 

 of glands which are found only in the larviP, and disappear 

 in the adult insect, and in these the silk is secreted as a 

 gummy substance. They are bent and coiled tubes, lying 

 at the sides of the body, and they sometimes extend for a 

 considerable distance from the head towards the tail. 

 Their structure is similar to that of salivary glands, but 

 they do not communicate with the mouth, and con- 

 sequently the silk does not, as at first sight it appears to 

 do, issue from the mouth. The ducts from the glands 

 unite into a common canal, which opens u^jon a minute 

 papilla a little below the mouth, and from this the secre- 

 tion exudes. Though glutinous at first, it has the property 

 of drying and hardening as soon as exposed to the air, 

 thus losing its stickiness. In this, as well as in other 

 respects, it difi'ers from the silk of spiders, which is secreted 

 by glands at the opposite end of the body, and which, as 

 serving the purpose of a snare, remains more or less sticky 

 after exposure. 



The cocoons of difi'erent species vary in colour from 

 white, through shades of yellow, brown, and grey, to 

 almost black ; but this is probably not entirely due to 

 difi'ereuces in the natural tint of the silk, but to processes 

 it undergoes after having been spun, or to the admixture 

 of foreign matter. The larva always forms its cocoon 

 round itself, so that it is usually an entirely closed covering, 

 with no indication of a way of entrance or exit. As it is 

 most commonly constructed amongst twigs, dead 

 leaves, etc., or in crevices and corners, and under ledges, 

 a few threads are first run promiscuously among sur- 

 rounding objects to serve as a sort of scatiblding or 

 mooring, and then a much more compact structure is 

 fashioned in the centre of these, so as closelyto envelope 

 the body of the spinner. 



It is difficult to say what have been the influences that 

 have determined the shape of the cocoon in each species ; 

 certainly neither the shape of the caterpillar nor that of 

 the chrysalis is the factor of prime influence, since we find 

 that caterpillars of similar shape make very difl'erent 

 cocoons, while there are caterpillars of very dissimilar 

 appearance which make cocoons more or less alike ; and 

 there seems no particular reason why a shajie that suits 

 one would not equally well suit another. Moreover, it 

 sometimes happens, as with the emperor moth to be 

 presently described, that the small end of the chrysalis 

 lies in the big end of the cocoon, so that no attempt is 

 made to secure what tailors would call a " good fit." The 

 protection afforded by the enclosure of the pupa in a 

 cocoon is not necessarily entirely in the direction of the 



disappointment of insectivorous enemies — in fact, as to the 



extent to which this sort of protection may be necessary 

 there is great lack of definite information, and whatever 

 opinion may bo formed will be more or less conjectural ; 

 we are not in a position to say exactly, from actual obser- 

 vation or experiment, what insectivorous animals would 

 be glad to feed upon these pupa', but are baulked in their 

 desire by the presence of the cocoon. Besides this some- 

 what hypothetical advantage, however, there are the 

 obvious benefits of the degree of fixity the cocoon gives to 

 a body which, if lying free and loose on the ground, would 

 be at the mercy of any disturbance in its surroundings, 

 and of the shelter afforded by enclosure in a non-eon- 

 ducting and probably damp-proof medium amidst changing 

 climatic conditions, espec.'ally when its use is required 

 during the winter season. But there is abundant scope for 

 inquiry in all these directions, and if any of our readers 

 feel inclined to investigate the subject for themselves, they 

 will doubtless be rewarded with discoveries. 



Amongst the numerous cocoons formed by British insects 

 none is more remarkable than that of the emperor moth 

 {Saiurnid carpini) (Fig. 1). As a rule, cocoons are more or 

 less oval in outline and of similar 

 shape at each end, but in the present 

 instance we have a pear-shaped or flask- 

 shaped body, rounded and swollen at 

 one end, and with a large circular 

 opening at the other. A little distance 

 below the opening a set of stout threads, 

 almost like bristles, pass from the 

 inner wall of the cocoon all round in 

 an upward direction towards the centre 

 of the opening, where they meet, thus 

 closing the cocoon by a hollow, conical, 

 brush-like partition, which efl'ectually 

 bars entrance from without, but admits 

 of easy egress from within by merely 

 pushing the threads on one side. Thus, 

 by this very simple but exquisite con- 

 trivance, no obstacle is placed in the 

 way of the exit of the moth when 

 matured, although the precious con- 

 tents of the chamber are during the long continuance 

 of pupahood securely shut otf from any would-be robber 

 in the outer world. 



The colour of these cocoons is variable ; usually they are 

 some shade of deep brown, but sometimes they become of 

 a pale creamy white. It has been supposed by some 

 naturalists that this difl'erence is intentional, and that its 

 object is to enable the cocoon to harmonize more com- 

 pletely with its surroundings, and so more oifectually to 

 elude notice. Many careful experiments have been made 

 to determine this point, since if established it would be a 

 more remarkable physiological fact than the change of 

 colour in the surface layers of a caterpillar's skin, under 

 the influence of external colours, to which we alluded some 

 time ago. For we are not here dealing with a case of 

 concealment brought about by the attachment of foreign 

 objects of suitable colour selected from the surroundings, 

 a case which would impl}' no more than a certain degree 

 of intelligence and ingenuity on the part of the fabricator 

 of the cocoon ; nor is it an instance of the colour of the 

 caterpillar itself being so modified as to correspond more 

 closely to its surroundings ; but it would be an instance of 

 a living being retaining its own colour unchanged, but 

 possessing the power of modifying the colour of its deep- 

 seated secretions at will, or at any rate in response to 

 external optical stimulus merely, notwithstanding that 

 usually the act of secretion is one of the most unconscious 



Fig. 1. — Cocoon of 

 Emperor Moth, part 

 of outer Tvall being 

 remored at the top 

 to show conical bar- 

 rier of ilbres. 



