THE ABDOMEN. 



617 



FIG. 369. Simple abdomen of Japyn (Jl contra>teil 

 with the highly modified one of an ant, Cryptocerus (B). 

 The abdominal segments are numbered from before 

 backwards. (From Sharp.) 



organs which vary greatly in size under certain conditions and 

 at different times. Hence the sclerites are separated from one 

 another by soft areas and frequently overlap one another (Fig. 

 369). There is as a rule a tergum or notum and a 

 united together by a 

 soft pleural membrane, 

 the pleuron, which 

 typically bears a stigma. 

 or external opening 

 of the trachea! system. 

 A pleural sclerite is 

 sometimes present. 

 The first abdominal 

 segment is often modi- 

 fied and in many Hy- 

 menoptera it seems to 

 have passed over into the thorax (Fig. 3695). At the posterior 

 region also great modifications and often reductions of parts 

 occur ; usually one or more of the hinder segments are invagi- 

 nated into their predecessors and become modified in connexion 

 with oviposition and copulation. The number of abdominal 

 segments visible in an insect is therefore usually smaller than 

 the number which reallv exists. According to Hevmons twelve 



*. *. 



segments are found in cockroach and earwig larvae and the 

 same number in much later stages of some dragon-flies ; the 

 twelfth segment giving rise to the "anal-piece." This seems 

 to be the maximum number ; ten segments are perhaps the 

 commonest number, but there may be and usually appear to 

 be fewer. Heymons does not recognize the serial homology of 

 the first and last segments (acron and telson) of the body with 

 those between, and therefore he does not consider the supra- 

 anal and sub-anal laminae which when present lie above and 

 below the anus as terga or sterna. The podical plates of the 

 Cockroach are probably developments of the sub-anal sclerite. 



The appendages (v. note p. 616) are processes borne by the 

 segments, typically one pair by each segment, though in the 

 adult those of the abdominal segments are usually absent. They 

 are as a rule jointed and contain muscles, nerves, tracheae, 

 and blood. 



The appendages of the pre-antennal segment are absent in 



