METAZOA INSECTA. 403 



head and thorax whereby the insect is fitted for its aerial 

 life. The unusual habit of catching its food while on the 

 wing has caused the head to become extremely mobile. 

 It seems, indeed, as if the dragon-fly could turn this part 

 of its body completely round without the slightest injuri- 

 ous effect. The head is aided by the long slender legs 

 (PI. 976, fig. i) which have lost their function of walking 

 and changed their position, being much farther forward 

 on the thorax than in other insects, as seen in the side 

 view (fig. 2). Calvert l states that the first pair of legs 

 are usually employed in holding the food while it is 

 devoured. 



These peculiar habits have doubtless brought about the 

 complex structure of the thorax. The prothorax (fig. 

 i,/) which in the larva (PI. 971, fig. i) was about the 

 size of the other two segments of the thorax, has become a 

 tiny ring, while the mesothorax (PI. 976, figs. 1,2, ms) and 

 the metathorax (figs, i, 2, mt) are large and consolidated. 

 These contain the powerful muscles that control the 

 action of the wings. The two pairs of wings (fig. i) are 

 similar in size and structure, and each pair is free from 

 the other, which is unusual in fast fliers. To understand 

 fully what a complicated machine it is that produces the 

 flight of dragon-flies, one needs to examine von Lenden- 

 feld's figures 2 of the thorax and wings of these insects. 

 It is indeed surprising that such complexity is found to 

 exist among the more generalized orders of this class of 

 animals. 



Authorities differ in regard to the tracheal system of 

 the Odonata, but according to Calvert 3 there are two 

 pairs of spiracles in the thorax and a pair in each of the 

 abdominal segments from the second to the eighth inclu- 



1 Loc. cit., p. 162. 



2Sitzungsb. k. Akad. d. Wiss. Wien, LXXXIII, Th. i, 1881, 

 pp. 289-380, pis. 1-7. 



3 Trans. Amer. Ent. Soc., Phila., XX, 1893, pp. 161, 170. 



