EXTERNAL ANATOMY OF INSECTS. 61 



the horse, these are purely organs of locomotion, and differ 

 in no respect from the other organs destined to the same 

 end; but in many animals we find them applied with per- 

 fect ease to either purpose. The adaptation of the same 

 organs to different purposes in the superior animals is obvious ; 

 consequently, in the inferior, fairly to be inferred. Conclu- 

 sions of this kind have been stigmatized as theoretical. Be it 

 so : theory may be sound as well as unsound. When theory 

 is a compound, of which facts are the ingredients, it is sound. 

 In the present instance, facts are the ingredients. Whether 

 the four portions of the head be primary or secondary parts, 

 — in other words, whether they be segments or sections of 

 segments, seems to hinge on another question ; viz. whether a 

 single segment can bear four feet ; for it seems scarcely to 

 admit of a doubt, that, in some annulate animals, the part 

 which is analogous to the head of tetrapterous hexapods has 

 four organs of progressive motion employed as feet. This 

 circumstance appears to me by no means more remarkable, 

 than that the third and fourth segment should each bear four 

 organs of progressive motion, two of them adapted to walking, 

 and two to flight. On these grounds I have considered the 

 four parts of the head as so many sections of a segment, and 

 consequently equivalent to the sections of succeeding segments. 

 To give them the same names, however, while a doubt remains, 

 would be objectionable ; more especially, as a nomenclature 

 sufficiently definitive has been long established, although in its 

 application confused and various. The parts of the head are 

 the skull, the lips, the feeler-jaws, and the mandibles. These 

 are the four sections of a segment. To simplify and conform 

 to received ideas, the three last must be treated of as the mouth, 

 of which, in tetrapterous hexapods, they constitute the com- 

 ponent parts. 



The skull of insects is compact, solid, and osseous. It has 

 a large opening in front, in which is situated the mouth ; 

 another behind, through which pass the oesophagus, spinal 

 cord, blood-vessels, muscles of connexion with the prothorax, 

 &c. ; and two smaller ones, generally in front, above that of 

 the mouth, in which are placed the antennge. There are two 

 compound eyes, one on each side, so closely soldered into the 

 skull, tiiat, in case of fracture, the sepai-ation does not take 

 place at the suture. Desvoidy well observed, that the eyes 



