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The mesothorax contains in the higher Homopterii and in nearly all 

 Hiteroptera, a will developsd scutellum, situ.ited between the bases of 

 the fore wings Its form and size assist in distinguishing some import- 

 ant groups, riie mstathorax is often reduced in size, forming a thin 

 plate between mesothorox and abdomen. The legs vary in size and 

 length and in the structure of the tarsi. A part or all of them are ciliated 

 in most aquatic species: they are strong and spiny in most of the raptorial 

 groups: pulvilli are in some groups present between the claws of the 

 tarsi. The wings are quite characteristic. In HomopUra they are gener- 

 ally membranous and veined, or the fore ones as in Ctrcopidcr and allies 

 uniformly coriaceous. In the latter case they do not overlap at the tips 

 but unite on the median line as the elytra of Coleoptera. In Hikropkra 

 the fore wings are divisible into three parts: a basal thick portion, the 

 corium; a transparent apical portion, the membrane, which is veined: 

 and a portion bordering the scutellum, the clavus. In some groups, 

 notably the Ph\t(honidae there is a triangular part between the corium 

 and membrane, the cuneus. They usually lie Hat on the back, the 

 membranous portions overlapping each other. The hind wings present 

 le.ss differences in structure, in both groups: but their venation is often 

 important in distinguishing minor groups. 



The Parasita and Mallophaga as well as one se.x in many of the 

 other groups, and occasionally both sexes are wingless or have these or- 

 gans represented by mere rudiments. In I'hysanopkra they are very deli- 

 cate with a very broad and delicate fringe. 



The abdomen presents many modifications of form, some of which 

 are quite constant. It is quite commonly concave above for the recept- 

 ion of the wings, but seldom so concave as not to be filled by the flatly 

 folding wings. The spiracles are, except in Parasita, situated below the 

 margin. The margin is quite generally sharp and either horizontal or 

 slightly elevated. The genital organs are in some groups external and 

 furnish valuable characters for separating groups, but more commonly 

 they are hidden in both sexes so as to furnish little aid without dissect- 

 ion and frequently the distinction of the sexes is impossible by external 

 characters. In the higher Homopiera and in a few groups of the Heterop- 

 Ura the ovipositor of the female is received into a slit on the underside 

 of the abdomen and serves to readily distinguish the sexes and affords 

 useful characters in systematic arrangement. 



The following analytical tables will assist in placing the vaiious sub- 

 groups, and also show the arrangement which seems to ine at present 

 most satis factor)'. 



In the synopses (i'i. families many groups are given that rank for 



