394: Mr. Bennett on the Anatomy of the Thorax in 



mon to the two pairs of wings when they both exist ; it occupies 

 in these orders a considerable portion of the capacity of the pos- 

 terior segment, which is thus prevented from affording attachment 

 on its internal surface to any but small auxiliary muscles of flight, 

 or those which are adapted to extend and fold the inferior wings. 



The Colcoptera and Orthoptera are the only orders in which the 

 posterior alar segment is the principal one, or that which bears 

 the true wings ; in the Coleoptera alone this segment also con- 

 tains the muscles of flight common to the wings and the elytra. 

 The middle segment in these orders, the place of which is almost 

 entirely occupied by the Metathorax, and which is capable of 

 affording attachment to very small auxiliary muscles alone, des- 

 tined to open and to close the elytra, takes only a very small share 

 in the action of flying. 



The pectoral concha or the sternal portions of the two posterior 

 segments of the thorax, corresponds in its functions with the 

 sternum, the ribs, and the clavicles of birds ; it consequently 

 affords a support to the wings, and a point of attachment to many 

 of the muscles concerned in flying. But as in insects the whole of 

 the muscles are internal, it is therefore necessary that all the parts to 

 which these must be affixed, should also be placed internally ; and 

 hence the analogous structure to the crest of the sternum in birds, 

 should be sought for on the internal surface of the parietes of the 

 thorax. This appears to be the Entosternum, an internal solid 

 part which is frequently bifurcated, and which is situated in the 

 middle of the sternal portion of the chest. It is met with in all 

 insects, and furnishes a strong point of attachment to numerous 

 muscles ; but it is peculiarly remarkable in many of the Coleop- 

 tera, in which its posterior portion has been called furculaire ot 

 furcular branches, from its forked appearance approaching very 

 nearly in form to that of the letter Y. 



The supports of the wings are always the most solid portion of 

 the framework of the trunk, and are frequently strengthened by 

 nervures or by other contrivances : in all insects their superior 

 extremity is curved inwards. These supports or thoracic clavicles, 

 are articulated sometimes immediately with the base of the wing 

 (which is situated above in a transverse direction, so as to be 



