INSECTA- 



349 



like the legs of the Crustacea and the ventral oars or setigerous 



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 different families, so as more or less to ob- 

 scure the primitive traces of their original distinctness, which can 

 only be demonstrated, as has been done by Macleay, Audouin, Bur- 

 meister, 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 compo- 

 sition 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, like the " cellulose " of 

 plants, is insoluble in caustic potash, and retains its form like charcoal 

 when submitted to a red heat ; but it contains nitrogen. The articu- 

 lated 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 is thickest and hardest in the 

 burrowing beetle tribe, in which the flexible property is limited to 

 the joints of the segments and their appendages. Uncalcified chitine 

 is always more or less elastic, and no tissue could be better adapted to 

 enable a light flying animal to resist external violence than that 

 which constitutes the seeming horny case of 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 ; but the pigmental 

 matter is often combined with the dense chitine. The hairs, spines, 

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

 coloured substance: the bulbs of the hairs are imbedded in the 



