52 



AN INTRODUCTION TO ENTOMOLOGY 



The spiracles. The external openings of the respiratory system 

 are termed spiracles. Of these there are two pairs in the thorax. 

 The first pair of thoracic spiracles open, typically, one on each side in 

 the transverse conjunctiva between the prothorax and the meso- 

 thorax ; the second pair open in similar positions between the meso- 

 thorax and the me'athorax. In some cases the spiracles have 

 migrated either forward or backward upon the adjacent segment. 

 For a discussion of the number and distribution of the spiracles, see 

 the next chapter. 



The peritremes. In many cases a spiracle is surrounded by a cir- 

 cular sclerite; such a sclerite is termed a peritreme. 



The acetabula or coxal cavities. In some of the more specialized 

 insects, as many beetles for example, the basal segment of the legs is 

 inserted in a distinct cavity; such a cavity is termed an acetabulum or 

 coxal cavity. When the epimera of the prothorax extend behind the 

 coxae and reach the prosternum, the coxal cavities are said to be 

 closed (Fig. 63) ; when the epimera do not extend behind the coxae 

 to the prosterum, the coxal cavities are described as open (Fig. 64) . 



The sclerites of a sternum. In the more generalized insects the 

 sternum of a wing-bearing segment may consist of three or four 

 sclerites. These have been designated, beginning with the anterior 

 one, the presternum (Fig. 

 61, Ps), the sternum or 

 eusternum (Fig. 61, S), 

 the sternellum (Fig. 61, 

 SI) , and the poststernellum 

 (Fig. 61, Psl). 



In the more special- 

 ized insects only one of 

 these, the sternum, re- 

 mains distinctly visible. 

 It is an interesting fact 

 that while in the speciali- 

 zation of the tergum 

 there is an increase in 

 the number of the scleri- 

 tes in this division of a 

 segment, in the specialization of the sternum there is a reduction. 



It is a somewhat unfortunate fact that the term sternum has been 

 used in two senses : first, it is applied to the entire ventral division of 

 a segment ; and second, it is applied to one of the sclerites entering 



Fig. 63. Prothorax of Harpalus, ventral aspect; 

 c, coxa; em, epimerum; es, episternum; /, 

 femur; n, pronotum; s, s, s, prosternum. 



