The body varies in form from the most elongated and thread like to 

 short or circular, from the thickest to forms so flattened that they may 

 live in the narrowest crevices They are convex above and below, or 

 flattened or concave above and convex below. 



The head serves by its position to separate the two leading sub- 

 orders but aside from this it is subject to most extreme modification. It 

 is cylindrical, often elongate in Reduviidce and Nabidce, decidedly flatten- 

 ed in Corddce and allied families. In the Homoptera the front is very 

 often produced, the most extreme modification of this part occuring in 

 the Fulgoridce. 



The ryes are usually prominent organs, standing out at the sides of 

 the head; they serve rather as land marks— by which to locate the other 

 organs of the head — than as distinctive in themselves. 



The ocelli are commonly present and by their absence or by their 

 relation to the eyes serve to define some minor groups. 



The antenna are never particularly elongated, usually composed of 

 few joints and as a rule but slightly modified. They run from filiform in 

 the lower Homoptera to setiform in Cicada and allies: and are mostly 

 filiform, with the terminal joints either slender or else slightly enlarged 

 in the Hderoptera. In a few cases the joint next the last is much flatten- 

 ed and broadened or otherwise modified. They are truly capitate only 

 in the Lioiheida. 



The labrum is usually very small, but occasionally with the clypeus 

 furnishes good characters for separation of genera and species. The setae 

 of the mouth vary greatly in length, but are of little value in determin- 

 ing the relation of groups. 



The labium is an important structural element. It forms the sheath 

 for the setse, and is termed the beak or rostrum. It consists of three or 

 four joints except in the Pediculidae where it is apparently devoid of arti- 

 culation. By its origin at the front of the head indicating the horizontal 

 position, or at the sternal border of the head indicating the vertical posi- 

 tion, it serves to distinguish the Heteroptera and Homoptera and by its 

 separation from or apparent union with the sternum it serves to divide 

 the Homoptera into two minor groups. Furthermore the form of the 

 basal joint and the comparative length of the joints are points of great 

 value in determining relationships. 



The thorax in its modifications corresponds in general with the 

 habits of the groups, whether aquatic, terrestrial or aerial. 



The prothorax is often minute and weak, again enormously deve- 

 loped and overshadowing all other parts as in Membracidae. It is trans- 

 versely sutured in Reduviidae. 



