Vol. 1.] Woodivorth. — Wiug Veins of Insects. 93 



already described. In the venation we find these groups are 

 absolutely isolated from others by the basal structure and that 

 the Odonata are further distinguished by a combination of 

 characters on the wing disk. 



The phrase "combination of characters" is used to express 

 what is often designated facies. It consists of a general plan 

 of arrangement, —elastic enough to allow considerable varia- 

 tion, but only within certain limits both of kind and amount, 

 and ofti'n bound up l^y definite correlations, — and in addition 

 to this general plan a series of details, perhaps never all pres- 

 ent at one time, but having definite places and characters 

 when present. 



The Odonata are distinguished from the Protodonata by the 

 specialization of three sets of coiirdinatecl cross veins forming 

 connectives (Fig. 28). The most 

 conspicuous of these is known 

 as the arculus. The cross veins 

 in this connective may extend 

 entirely across the wing from bor- 



T , "i 1 ,1 . . T FIG. 28. Diagram of the important fea- 



der to border; the minimum dis- tmes of the wing of the odonata. 

 tance is from the primary to the .,. stigma'';^ «',^/od!a Si-. '*' "°'^"'' 

 first posterior. When the arculus 



is present, as in all Odonata, the independents never cross this 

 vein, but end in it. 



The next most conspicuous connective is the one at the 

 nodus, with the production of which it is quite likely associated. 

 This structure (Fig. 28) is found in all Odonata, and nothing 

 of exactly this type is found in any of the other orders of 

 insects. The connective at this point binds the primary to the 

 veins anterior to it, and it also extends behind the primary 

 and involves one or two of the independent veins. 



The third connective is weaker and nearer to the base; it is 

 sometimes so disguised as to be difficult to recognize. This is 

 particularly true in the groups with a well-developed triangle. 

 The character of the connective is much the same as the one 

 just described, only that it connects the hindmost independ- 

 ents with the first posterior. These three connectives are the 

 only constant characters in the venation of the Odonata dis- 

 tinguishing it from the Protodonata. 



In order to understand the development of these connectives,- 

 we should look, first, for the conditions determining the loca- 



