50 DEFINITION OF THE TERM IXSECT. 



of the trunk and its members ; at any rate they do not 

 replace the two anterior pair of legs of the hexapod 

 Altera. When merely used as wings, they commonly 

 consist of a fine transparent double membrane, strength- 

 ened by various longitudinal and transverse nervures, or 

 bones as some regard them, accompanied by air-vessels, 

 of which more hereafter, as well as of their kind and cha- 

 racters. I shall only observe, that insects are known 

 from all other winged animals, by having /our wings, or 

 what represent them, and this even generally in those 

 that are supposed to have only a pair. Another pecu- 

 liarity distinguishes the trunk of insects that you will 

 in vain look for in the vertebrate animals — these are one 

 or two pair of lateral spiracles or breathing pores. Though 

 the respiratory sacs, &c. of birds are almost as widely 

 dispersed as the tracheae and bronchise of insects 3 , yet 

 their respiration is perfectly pulmonary, and nothing like 

 these pores is to be discovered in them. 



The principal peculiarity of the third part of the body, 

 the abdomen, is its situation behind the posterior pair of 

 thoracic legs, and its rank as forming a distinct portion 

 of what represents the skeleton. In most insects it is so 

 closely affixed to the posterior part of the trunk as to 

 appear like a continuation of it, but in the majority of 

 the Hymenoptera and Diptera, and in the Araneidan 

 Arachnida, or spiders, it is separated by a deep incisure; 

 and in the first-mentioned tribe is mostly suspended to 

 the trunk by a footstalk, sometimes of wonderful length 

 and tenuity. In the Mammalia the male genital organs 

 are partly external ; but in insects as well as in many of 

 the vertebrate animals, except when employed, they are 

 * N, Diet. d'Hist, Nat, xxviii. ; compare 104 and 110. 



