522 rHiLosoPHY of zoology. 



contact of two flat surfaces. The head is frequently con- 

 tracted behind, and the trunk in front, to diminish the 

 uniting surfaces, but such joint admits but of very imper- 

 fect motion. 



The muscles of the head take their rise near the abdomi- 

 nal edge of the trunk, and, entering the occipital hole, be- 

 come attached to its margin. Those which move the head 

 upwards take their rise on the upper part of the trunk ; 

 while those which depress it arise from the under side. 

 These last are the largest. Those which arise from the late- 

 ral parts of the trunk give to the head its lateral motions. 



The characters derived from the head, which are used 

 in classification, are chiefly taken from the markings of its 

 surface, the inequalities of its margin, its size, and shape. 

 They are, in general, obvious and permanent. 



The Trunk, as an organ of support to the other mem- 

 bers, may be considered as the most important organ of 

 the body. The terms, however, which are employed to 

 designate its different parts are neither appropriate nor well 

 defined. Linnaeus, in reference to this organ, gives the 

 following enumeration of its parts : " Truncus, inter caput 

 et abdomen, pedatus, thorace supra dorso, postice scutello, 

 subtus pectore sternoque."" By modem authors, it is usual- 

 ly divided into the thorax and breast. 



The thorax is the second ring of the body, and is united 

 on the fore part with the head, and behind with the third 

 ring or breast. In many kinds, this part is minute, while, 

 in others, it occupies a large portion of the body. On its 

 ventral aspect in insects, it bears the fore legs, or first pair, 

 and between these is the thoracic sternum, frequently in the 

 form of a keel, and terminating behind by a spinous pro- 

 cess, which rests upon the pectoral sternum, as in the ge- 

 nera Elater and Dytiscus. It may be considered as the 

 onlv fixed part of the liody, giving origin to the nniscles of 



