STRUCTURE OF INSECTS. 5 



Having thus determined the limits of the class of Insects, a slight 

 sketch of their structural characters, and their distribution into orders, 

 will next occupy our attention. 



II. General Structure of Insects. 



The word Insect., derived from the Latin (m and seco, sectum), 

 indicates one of the chief characters of this group of animals ; the 

 body in which is not only composed of a continuous series of seg- 

 ments, articulating with each other, but is also often divided or cut 

 into three very marked portions, to which the names, head {caput), 

 thorax, and abdomen have been applied. Unlike the higher animals, 

 insects are not internally furnished with a bony skeleton ; but, to 

 supply its want, the external envelope is ordinarily of a corneous-like 

 texture, to which, on its inner surface, the muscles are attached, and 

 which encases the viscera and other internal organs. 



This external Envelope is, therefore, the analogue of the skeleton of 

 the mammalia, the external organs of locomotion being attached to 

 it.- In its chemical composition, this envelope is peculiar, consisting 

 of a substance which has been named Chitine, which is found only in 

 the teguments of articulated animals. To this is added a certain 

 quantity of albumen, an animal matter, certain salts,, and an oil of 

 variable colour, which last is generally disposed near the external sur- 

 face, and supplies the varying colours so much admired in this tribe 

 of animals. 



The Number of Segments, whereof the body of insects is composed, 

 appears, at first sight, to be very variable, but this is more apparent 

 than real ; being caused either by the soldering together of certain of 

 these segments, or the rudimental state of others, in consequence of 

 the superior developement of the adjacent segments. It is, however, 

 to be observed that, owing to the non-developement of wings and 

 generative organs in tlie larva, the body, in this state, is more regu- 

 larly segmented, and the segments more equally sized, than in the 

 imago state. It has been supposed that the typical nianhev of segments 

 is thirteen ; but some late observations upon the larva; of Hymenop- 

 terous insects, and ujion the common earwig {Trans. Ent. Soc. vol. i. 

 p. 157.), have led me to consider that the number is greater. From 

 the elaborate researches of Audouin and MacLeay into the structure 

 of the thorax of hexapod insects, it has been considered that each of 



B 3 



