ARTICULATA. 135 



"When the caterpillar incloses itself in a cocoon to take tlie helpless con- 

 dition of a pupa, various internal changes take place, tending towards the 

 organization of the future insect ; and even previous to this, the internal 

 organization of the larva had been gradually undergoing various changes 

 in the nervous and alimentary systems, unaccompanied by any external 

 change except that of size. Some pupa? are protected by a cocoon, and 

 some are not thus protected. Of the l^itter, some are suspended by the 

 posterior extremity, and others, as Pajnllo, attach themselves with the head 

 above, and a thread around the body to maintain it in its position. Some 

 susjDended pupae are finely marked with bright colors and golden spots, 

 whence the name of Chrysalis, which is seldom used, and aurelia, which is 

 obsolete. 



Agassiz has pointed out several curious analogies among the classes of 

 Articulata, composing the larvie of Lepidoptera (particularly those with 

 bristly tufts) to the Annelida ; and the pupa? in which the abdomen alone is 

 capable of motion (the head and thorax being united under a kind of cara- 

 pace) to the decapodous Crustacea, which are, on account of this affinity 

 with one of the conditions of insects, placed at the head of their class. He 

 places the Insects above the Crustacea, because the former leave the con- 

 dition in which they are covered by a carapace, and advance a step further. 

 Finally, this philosophical author places the Lepidoptera at the head of the 

 insects (as Swainson had done upon difterent grounds), because the larva 

 is mandibulate, and the adult insect perfectly haustellate, so that it advances 

 further from the larva condition than any of the orders. See Lectures on 

 Embryoiog}" ; and Proceed. Am. Assoc, Charleston, 1850. 



Imago. The perfect insect or imago appears when the case of the pupa 

 is split, and in the winged species, the wings, which were closely folded, 

 begin to expand and take their final shape. The imago differs from the 

 larva in having the body divided into the three principal divisions of head, 

 thorax, and abdomen. 



The integument of insects contains a peculiar principle, named chitine 

 by Odier, which constitutes a third or fourth of its bulk. It resembles bone 

 somewhat in its composition, as it contains phosphate of lime, and a trace 

 of other compounds found in bone. The chitine forms part of the dermis, 

 which is covered by a thin epidermis. Coccine is another chemical con- 

 stituent, found particularly in the genus Coccus. 



The head of insects is usually regarded as a single piece, but as the cor- 

 responding part in the Crustacea is considered to be made up of minor parts, 

 it has been proposed to extend the same theory to the class under con- 

 sideration, either allowing as many theoretical segments as there are kinds 

 of appendages, or allowing two kinds of appendages to some of them. Of 

 these two modes of viewing the^question, the former is preferred in studying 

 the Crustacea, and should therefore have the preference in this class. 



The organs concerned in manducation (enumerating them from above) 

 are, the upper lip or lahrum., the mandihulcp., or upper jaws, tlie maxillcG or 

 lower jaws, the tongue or I'tgida, and the lower lip or labium. Brulle 

 divides the ligula into an upper organ or ei:)i])liarynx.^ and a lower one or 



339 



