148 PHYSIOLOGY OF INSECTS. 



is shaded, E F, and the divided structure of this segment 

 is equally visible, the anterior portion being the mesotho- 

 racic scutum, the posterior the mesothoracic scutellum. 

 The next segment G H 9 left unshaded, may from its sin- 

 gular figure seem to possess somewhat arbitrary limits, but 

 the presence of the hind wing in its usual place, and the 

 hind leg at H, prove that the site of the segment is cor- 

 rectly marked, and its limits can scarcely be otherwise 

 than those indicated ; in this instance the anterior division 

 or metathoracic scutum is much the smaller plate, and the 

 posterior or metathoracic scutellum is the larger section. 

 The fifth segment, I, is said by some writers to be merely 

 a section of the fourth, but there appears no ground for 

 this supposition, and its intervention between the meta- 

 thorax and podeon, together with its displaying a dorsal 

 surface only, as we have seen is the case with the propo- 

 deon of the locust, are sufficient to prove the propriety of 

 considering it distinct. If it be asked why each of the 

 other segments composing the thoracic mass is divided, 

 and this entire it may be replied that each of the others 

 bears at least two limbs, and this is entirely without them ; 

 and it may be laid down as a rule, scarcely liable to ex- 

 ception, that the presence of legs or wings causes a more 

 complicated structure in the segment which bears them. 

 The sections of the thoracic segments in a Sphex, al- 

 though they may be said to be really 

 identical with those of hornets, wasps, 

 bees, and other pedunculated insects, 

 yet undergo so great a change in form 

 M and situation that it is necessary fur- 

 ther to explain them. If we take a 

 hornet, and so place it as to look di- 

 rectly on its back, we shall observe a 

 large convex triangular plate, E 2, the 

 base of which extends from wing to 



