THE EXTERNAL ANATOMY OF INSECTS 



77 



ments of the abdomen to which these appendages belong. One cause 

 of difference is that some writers regard the last segment of the abdo- 

 men as the tenth abdominal 

 segment while others believe it 

 to be the eleventh. This seg- 

 ment bears the cerci when they 

 are present. The genitalia are 

 borne either by the two or the 

 three segments immediately 

 preceding the last. If the last 

 'segment is the eleventh the 

 genitalia are, according to one 

 view, the appendages of the 

 eighth, ninth, and tenth seg- 

 ments; according to another 

 view, they are the appendages 

 of the ninth and tenth seg- 

 ments, those of the tenth seg- 

 ment being doubled. 



The genitalia of many in- 

 sects have been carefully fig- 

 ured and described and special 

 terms have been applied to 

 each of the parts. But as most 

 of these descriptions have been 

 based upon studies of repre- 

 sentatives of a single order of 

 insects or even of some smaller 

 group, there is a great lack 

 of uniformity in the terms 

 applied to homologous parts 

 in the different orders of in- 

 sects; such of these terms as 

 are commonly used are defined 



later in the characterizations of the several orders of insects. 



The cerci. In many insects there is a pair of caudal appendages 



which are known as the cerci; these are the appendages of the 



eleventh abdominal segment, the last segment of the body except in 



the few cases where a telson is present. 



The cerci vary greatly in form; in some insects, as in most Thy- 



sanura, in the Plecoptera, and in the Kphermerida, they are long and 



Fig. 90. Ventral aspect of Machilis: c,cer- 

 cus: Ip, labial palpus; mf, median caudal 

 filament; mp, maxillary palpus; r>, oviposi- 

 tor; s, 5, styli. That part of the ngure 

 representing the abdomen is after Oude- 

 mans. 



