THYSANURA 



221 



The abdomen consists of eleven segments. The eleventh segment 

 bears the cerci, which are filiform and many-jointed except in th(j 

 Japygidge, where they are forceps-like. In the 

 Machilidee and the Lepismatids the eleventh 

 abdominal segment bears a long, many-jointed 

 median caudal filament; styli and eversible 

 ventral sacs are also usually present ; these vary 

 in number in difi'erent genera. 



The styli are slender appendages (Fig. 221, 

 s). Each stylus consists of two segments, a 

 very short basal one and a much longer termi- 

 nal one. The maximimi number of styli is 

 found in Machilis (Fig. 221), where they are 

 borne by the second and third thoracic legs 

 and the second to the ninth abdominal seg- 

 ments. In Lepisma there are only three pairs; 

 these are borne by the seventh, eighth, and 

 ninth abdominal segments. 



The abdominal styli are borne by large 

 plates, one on each side of the ventral aspect 

 of each abdominal segment. These plates are 

 termed coxites, as they are believed to be flat- 

 tened coxae of abdominal legs which have otherwise disappeared. 



A result of the large size and position of the coxites is a reduction 

 in the size of the sternum in the abdominal segments. This is well 

 shown in Machilis (Fig. 221) ; in the first seven abdominal segments, 

 there is in each a median triangular sclerite; this is the stemimi; in 

 the eighth and ninth segments no sternum is visible. 



Fig. 224. — One of the 

 paragnatha of Ma- 

 chilis. (After Car- 

 penter.) 



Fig. 225. — Cross-section of an abdominal segment of Machilis showing the 

 styli and the ventral sacs. The ventral sacs of the left side are retracted ; those 

 of the right side, expanded. (After Oudemans.) 



In the families Machilidas and Lepismatidas the females have an 

 ovipositor, which consists of two pairs of filiform gonapophyses aris- 

 ing from between the coxites of the eighth and ninth aiadominal 

 segments respectively. 



