General Review. 



In the pages preceding an attempt has^een made to pre- 

 sent the most important points now known about the biology 

 of water bugs. . It has been arranged to most readily determine 

 what is known on any given form. To facilitate future work, 

 the sexual differences of the various groups are figured. These, 

 together with the drawings of the nymphs, permit a ready 

 analysis of a survey of the pond at any time. The general 

 reader is referred to the chapters on Ecologic and Economic 

 Relations, and Form and Function, for matters of popular 

 interest. 



In the developmental studies there have appeared certain 

 points of note. The developmental changes of these bugs fall 

 into two categories : First, characters acquired by the adult ; 

 second, characters lost by the adult. 



Under the first may be mentioned the fact that ocelli, as a 

 rule, when present at all, are adult characters. The toad bug 

 is an exception to this, the nymphs possessing ocelli. The 

 tarsus when two or three segmented is usually an adult charac- 

 ter, the nymphs having one-segmented tarsi. Sometimes the 

 adult possesses more segments in the antennse, as in Corixids. 

 The sexual dimorphism of the adults, such as the asymmetrj^ 

 of male Corixids, etc., is, of course, attained only in the adult. 



On the other hand, in some instances of adaptation the 

 adults lose structures present in the nymph. The best illus- 

 tration, perhaps, is that of the Belostomatids, which have a 

 single claw on front tarsus. The nymphs have two, one of 

 which may get relatively smaller as later instars are reached. 

 Another case is that of the hind claws of Notonecta and the 

 Corixids. The first instar nymphs have two long claws, while 

 those of adult are setaform and obscured by the swimming 

 fringe. These various changes are fundamental and signifi- 

 cant. In determining phylogenetic relationships, the internal 

 organization, combined with developmental changes, are most 

 important in forms with incomplete metamorphosis. No 

 staple arrangement of the Heteroptera can be made until much 

 more is known of the biology of the group. 



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