THE EXTERNAL ANATOMY OF INSECTS 49 



abdominal segment. In the Acridiidae, for example, the sternum of 

 the first abdominal segment forms a part of the intermediate region 

 of the body, and in the Hymenoptera the entire first abdominal 

 segment pertains to this region. 



The alitrunk. — ^When, as in the Hymenoptera, the intermediate 

 region of the body includes more than the three true thoracic seg- 

 ments it is designated the dlitrnnk. 



The propodeum or the median segment. — ^When the alitrunk con- 

 sists of four segments the abdominal segment that f omis a part of it is 

 teniied the propodeum or the median segment. In such cases the true 

 second abdominal segment is termed the first. 



h. THE SCLERITES OF A THORACIC SEGMENT 



The parts of the thorax most generally recognized by entomologists 

 were described nearly a century ago by Audouin (1824) ; some addi- 

 tional parts not observed by Audouin have been described in recent 

 times, by the writer ('02), Verhoeff ('03), Crampton ('og), and 

 Snodgrass ('09, '10 a, and '10 h). The following account is based on 

 all of these works. 



In designating the parts of the thorax the prefixes pro, meso, and 

 meta are used for designating the three thoracic segments or corres- 

 ponding parts of them; and the prefixes pre and post are used to 

 designate parts of any one of the segments. Thus the scutum of the 

 prothorax is designated the proscutum; while the term prescutum is 

 applied to the sclerite immediately in front of the scutum in each of 

 the thoracic segments. This system leads to the UL^e of a number of 

 hybrid combinations of Latin and Greek terms, but it is so firmly 

 established that it would not be Vv^ise to attempt to change it on this 

 account. 



Reference has already been made to the division of a body-segment 

 into a tergum, two pleura, and a sterntun ; each of these divisions will 

 be considered separately; and as the maximum number of parts are 

 found in the wing-bearing segments, one of these will be taken as an 

 illustration. 



The sclerites of a tergum. — In this discussion of the external ana- 

 tomy of the thorax reference is made only to those parts that form 

 the external covering of this region of the body. The infoldings of 

 the body- wall that constitute the internal skeleton are discussed in the 

 next chapter. 



The notum. — In nymphs and in the adults of certain generalized 

 insects the tergum of each wing-bearing segment contains a single 



