196 



KNOWLEDGE. 



[September 1, 1894. 



taking the place of elephants, and the hornless rhinoceros 

 representing the two-horned woolly species which he had 

 probably been accustomed to hunt. 



INSECT SECRETIONS.-V. 



By E. A. Butler, B.A., B.Sc. 

 {Continued from page 178.) 



AMONGST butterflies and moths, the exciting 

 moment when the most wonderful change in their 

 whole history takes place — the moment when 

 they throw aside the deathlike torpor of the 

 limbless chrysalis and take on, to all appearances 

 suddenly, a complete locomotive equipment consisting of 

 six legs and four wings — is an occasion which, in conse- 

 quence of its exceptional requirements, is fruitful in the 

 production of characteristic liquids. When a chrysalis is 

 enclosed in a cocoon, it is obvious that some substance 

 must be produced by the newly emerging insect, which it 

 may use as a means of escaping from its enveloping shroud 

 of silk. It has no biting jaws, as it had just before it 

 entered on the pupa stage, so that gnawing its way through 

 is altogether out of the question, and it must make its 

 escape by using some solvent which will chemically affect 

 the silk, and either loosen its component threads at the 

 spot where the exit is to be made, or absolutely dissolve 

 them. In our articles on " Curious Cocoons," we have 

 already pointed out that it has recently been discovered 

 that, in one instance at least, this solvent has the remark- 

 able characteristic of being a strong caustic alkali — none 

 other, in fact, than potash itself. We refer to the instance 

 of the puss moth, whose exceptionally hard cocoon 

 necessitates an equally powerful solvent. That an alkaline 

 fluid was produced by this insect on its emergence was 

 indeed noticed nearly fifty years ago, but its exact nature 

 was not determined until quite recently. Analogy would 

 lead us to conclude that a liquid of somewhat similar 

 nature, or at any rate of similarly corrosive action, must 

 be produced by other moths which enjoy the shelter of 

 closed cocoons during their period of lethargy. 



Those who have kept caterpillars, and watched the 

 changes through which they pass, are familiar with the 

 fact that a newly emerged butterfly or moth discharges a 

 quantity of a reddish fluid from its digestive tube, so that 

 the ground in the immediate neighbourhood of such an 

 emergence, and the various objects around, including 

 often the old chrysalis shell, become bespattered with drops 

 of coloured liquid, which soon dry up and leave behind a 

 reddish or orange powdery deposit. This liquid is of an 

 acid character, since it reddens blue litmus paper when 

 placed upon it. It is often shot out in considerable 

 quantity and with great force, especially when the insect is 

 disturbed before it has become properly prepared for its 

 new style of life. 



Not only does this discharge take place when a chrysalis 

 becomes a perfect insect, but a similar incident often occurs 

 when the caterpillar becomes a pupa. The contents of 

 the alimentary canal are then voided in a semi-fluid con- 

 dition, though under ordinary circumstances the excrement 

 is compact, and comparatively dry and hard. In the case 

 o; the emperor moth {Saturniii carpini), i\Ir. Bateson has 

 made a careful examination of this discharge, specially 

 with a view of determining its relation to the colour of 

 the cocoon. While still contained within the alimentary 

 canal the liquid is green, but shortly after being ejected it 

 becomes reddish-brown. Microscopically examined, it is 

 seen to consist of a coloured liquid containing partially 



digested food. If it be filtered through blotting paper, a 

 clear green liquid is obtained, which turns reddish-brown 

 in the course of a few minutes. This is no doubt a 

 chemical change due to oxidation, resulting from exposure 

 to the air, as the following experiments tend to prove. If 

 a little be placed on a glass plate, the outer layer may be 

 seen to have changed colour, while the lower part, being 

 out of contact with the air, still remains green. If again 

 the liquid be placed in a tube, and then shaken up with 

 air, the darkening process is accelerated. Yellow nitric 

 acid, a body which easily yields up oxygen, destroys the 

 green colour at once, and turns the liquid reddish-brown. 

 The question whether this substance is a true secretion, or 

 merely a product of the digestion of the food, is not so 

 easy to determine, but that the latter is its true source 

 seems probable from the fact that no such colouring matter 

 can be discovered in the walls of the intestine, though one 

 would expect to find it there if it were a true secretion 

 from glands on the walls. 



But though these coloured liquids ejected by caterpillars 

 just before pupation may not really be connected with any 

 special secretory organs, there is an instance of a remark- 

 able product which has been traced back to a special 

 part of the alimentary canal, and is thus shown to be 

 of the nature of a secretion. The product referred to 

 is a powder formed by the caterpillar of the lackey moth 

 (CUsiocampa nenstria), a well-known flabby creature, striped 

 with reddish, orange and blue, and often found abundantly 

 on whitethorn hedges. This caterpillar fashions for itself 

 a delicate white silken cocoon, of loose and semi-transparent 

 texture. The part immediately surrounding the insect is 

 more compact than the rest, but even that remains suffi- 

 ciently thin for the outline of the chrysalis to be faintly 

 traced through it. When the caterpillar has completed 

 its task, it discharges from the end of the alimentary canal 

 a quantity of a pale yellow powder, which looks a good 

 deal like flowers of sulphur. This powder, lodging in the 

 meshes of the silk, of course renders the cocoon a good 

 deal more opaque, and thus may perhaps be a source of 

 protection to the insect. 



Mr. Poulton recently examined this powder chemically 

 and microscopically, and found that it consisted of minute 

 crystals of carbonate of lime. Dissections of the cater- 

 pillars showed that the ultimate source of the powder was 

 the malpighian tubules, the long thin tubes that open into 

 the intestine just behind its junction with the stomach. 

 It is, of course, initially obtained from the food, and appa- 

 rently accumulates in the tubules till the end of the feeding 

 life, when it is got rid of and made to subserve a possibly 

 useful function in giving greater opacity to the cocoon. 

 This would be of little avail if the creature remained in 

 the chrysalis condition throughout the winter, for the 

 winds and storms of that season would very soon shake 

 all the powder out. But as the insect remains a chrysalis 

 only for a very short time, the obscuration caused by the 

 powder may perhaps be of some slight advantage to it. 



Like the skunk amongst mammals, many insects are 

 distinguished for the strong odour that attends them, and 

 that usually proceeds from fluids which they secrete. la 

 a few instances the odour is agreeable to the human 

 olfactory sense, but generally it is quite the reverse. These 

 odours may in some cases be a means of sexual attraction, 

 and in others the liquids which produce them may cause 

 the insects to become nauseous and unpalatable, and so 

 protect them from their enemies. But it would be unsafe 

 to conclude that scents which are agreeable to human 

 beings are also necessarily a source of attraction to the 

 lower animals, and in fact often the very reverse is the 

 case, for many insects are strongly attracted by decaying 



