356 INSECTA. 



bonring object, allows a minute drop of the glutinous secretion to 

 exude from its extremity, which, of course, adheres to the surface upon 

 which it is placed : the head of the Silkworm being then slowly with- 

 drawn, the fluid silk is drawn out in a delicate thread through the 

 aperture of the spinneret, its thickness being regulated by the size of the 

 orifice, and, immediately hardening by the evaporation of its fluid parts, 

 forms a filament of silk, which can be prolonged at the pleasure of the 

 animal until the contents of its silk-reservoirs are completely exhausted. 

 (922.) Such is the structure of the larva of a Lepidopterous insect ; ' 

 and the arrangement of its internal viscera, when arrived at maturity, 

 has been already described. We have yet, however, to mention the 

 series of phenomena observable during the progress of its growth, and 

 the mode of its expansion from the minute size that it exhibits on leaving 

 the egg to the full dimensions which it ultimately acquires. In order 

 fully to understand the circumstances connected with this part of our 

 subject, it is necessary to premise that the outer integument of most 

 larvaa is of a dense corneous texture, coriaceous in some parts, but quite 

 hard and horny in others. In the second place, it is but very slightly 

 extensible ; and, moreover, as is always the case with epidermic struc- 

 tures, it is not permeated by any vascular apparatus, and consequently 

 is absolutely incapable of growth when once formed. This epidermis 

 encases every portion of the larva : the body, the legs, the antennae, the 

 jaws, and all external organs are closely invested with a cuticular 

 envelope, such as, from its want of extensibility, would form an in- 

 superable obstacle to development, were there not some extraordinary 

 provision made to meet the necessity of the case. The plan adopted is, 

 to cast off at intervals the old cuticle by a process termed moulting an 

 operation which is repeated several times during the life of the insect in 

 its larva condition, and is accomplished in the following manner : The 

 caterpillar becomes for a few days sluggish and inactive, leaves off 

 eating, and endeavours to conceal itself from observation. The skin, or 

 more properly the cuticle, becomes loosened from the subjacent tissues, 

 and soon a rent appears upon the back of the animal, which gradually 

 enlarges in a longitudinal direction, and the imprisoned insect, after a 

 long series of efforts, at length succeeds in extricating itself from its old 

 covering, and appears in a new skin of larger dimensions than the one 

 it replaces, which, however, in all other particulars it closely resembles. 

 With the old epidermis the larva throws off all external appendages to 

 the cuticle : the horny coverings of the jaws, the corneae of the eyes, the 

 cases of the claws, are all removed ; and many writers have even found 

 attached to the exuviae an epidermic pellicle that had formed a lining to 

 the rectum, and delicate prolongations of the cuticle derived from the 

 interior of the larger ramifications of the air-tubes. Absurd, indeed, 

 have been the explanatious given by various writers of the nature of the 

 process under consideration. Swammerdam and Bonnet, nay, even our 



