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, which is 

 the view adopted in this work, 

 this segment bears the cerci 

 when they are present. The 

 three pairs of appendages that 

 constitute the genitalia are 

 borne by the eighth and ninth 

 segments, two pairs being 

 borne by the ninth segment. 

 The outer pair of the ninth 

 segment constitute the sheath 

 of the ovipositor. See ac- 

 count of the genitalia of the 

 Orthoptera in Chapter eight. 



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 presemt. 



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



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



Fig. 90. — Ventral aspect of Machilis; c, cer- 

 cus; Ip, labial palpus; mf, median caudal 

 filament; mp, maxillary palpus; 0, oviposi- 

 tor; .s, s, styli. That part of the figure 

 representing the abdomen is after Oude- 

 mans. 



