226 PTILOTA : IMAGO — EXTERNAL ANATOMY. 



covering is of a scaly or homy consistence. It is, however, 

 modified by the nature of the habits of the individual. 

 Thus the Aphides, Hemerobii, Ejjhenierce, and other short- 

 lived insects, have the skin very soft, whilst those which 

 generally take up their abode beneath stones, under the bark 

 of trees, in water, &c., are more strongly defended against 

 accidents, by the solidity of their envelope, than those which 

 live upon flowers. Those also, which reside beneath the 

 bark of trees, or under stones, have a flattened form, whilst 

 those which burrow into wood are cylindrical. 



The bod}^ of insects consists of a series of rings, connected 

 and articulating with each other by the assistance of muscular 

 membranes, whereby the insect is enabled to contract or 

 lengthen, narrow or dilate, its body, and so give to it all the 

 necessary movements. 



I have said that the external integuments of insects ex- 

 hibited a horny appearance ; nevertheless there is no real 

 analogy between the scaly covering of a beetle and horn, 

 their chemical properties being quite distinct, to be con- 

 vinced of which it is sufficient to apply them separately to 

 the flame of a lamp ; and the same distinction is proved by 

 placing them in a solution of potash, of the temperature of 

 boiling water, in which the horn will be dissolved, but the 

 covering of the beetle undergoes no change except that of 

 loss of colour. The base of the latter is found, by chemical 

 analysis, to consist of a peculiar substance, found only in the 

 integuments of annulose animals, which has been termed 

 chitine, and which forms the outer covering. Albumen also, 

 an animal extract, a small portion of carbonate of potash, 

 phosphate of lime and phosphate of iron, and an oil variable 

 in colour according to the colours of the organs from which 

 it is extracted, form parts of the insect integument, but 

 chitine is by far the greatest portion of the materials of 

 which it is composed. 



