194 LECTURE XVI. 



gerous prolegs of the Anellides. In the flying insects there are 

 developed from the dorsal arches of the middle and third segments, 

 locomotive appendages which constitute the wings {g-,j)' 



It must not be supposed that the parts of the thorax which have 

 just been described are naturally or uniformly separate, and moveably 

 connected with one another ; they are more commonly confluent, but 

 in different degrees in difl'erent families ; so as more or less to obscure 

 the primitive traces of their original distinctness, which can only be 

 demonstrated, as has been done by Macleay, Audouin, Burmeister, 

 and others, by an extended comparison of the thorax in the whole 

 class of Insects, or by tracing its development and modifications during 

 the various stages of the metamorphoses When the composition of 

 the thorax of an insect is thus studied, it is found to be made up of 

 not less than fifty-two pieces, which have for the most part received, 

 and necessarily, distinct names in Entomology, and many of them, 

 very unnecessarily, more names than one. 



The abdomen is usually formed of a greater number of segments, 

 always nine in the larva, which retain a greater degree of mobility 

 upon each other ; but it supports no locomotive appendages in the 

 hexapod insects. 



The tissue of the external skeleton is of a dense, resisting, but 

 light material ; it looks and feels like horn, but it has for its base a 

 peculiar substance called " chitine," which is insoluble in caustic, 

 potash, and retains its form like charcoal when submitted to a red 

 heat. The articulated appendages consist, like the segments of the 

 trunk, of hollow cases or tubes of the same firm and slightly flexible 

 substance; which tubes contain the muscles, nerves, and other soft 

 parts in their interior. The integument is softer and more yielding 

 in larvae, flies, and most parasitic insects. It consists of three layers, 

 epidermal, pigmental, and dermal, the derm and epiderm more 

 closely resemble each other in physical properties than in other 

 animals : they are separated and cemented together by sometimes 

 two distinct coloured layers of rete mucosum. The hairs, spines 

 and scales are processes of the epiderm, which often include a co- 

 loured substance. 



I may now proceed to a more particular description of the jointed 

 and aliform appendages of the skeleton. The first pair inserted 

 into the front or upper part of the head, are the antennae, which 

 present a vast variety of shapes and sizes in different insects, but 

 seem in all to have most intimate relation to the senses of touch 

 and hearing. Their precise function has not, however, yet been well 

 defined. The Entomologist avails himself of their various conforma- 

 tion to obtain characters for the distinction of families, of genera, or 



