I SO CHARLES T. BRUES. 



recognized. Extending these observations to include other insects 

 also, it seems possible to distribute all these cases into three 

 categories as follows : 



1. Wings having essentially a pupal character, /. t\, developing 

 as normal wings up to the pupal stage but failing to expand. 



2. Wings essentially normal, except for their smaller size and 

 less complex venation ; sometimes even developing a color pat- 

 tern, or possessing unique and quite distinctive characters. 



3. Wings consisting of little more than a hollow bag and giv- 

 ing no clue from their appearance as to the probable wing struc- 

 ture of their ancestors. (Comparable in a way to the halteres 

 of the Diptera.) 



Of these three groups, the first would seem to indicate the 

 most recently acquired brachypterous condition, and the third 

 apparently the one just preceding the totally apterous state. The 

 latter should therefore be phylogenetically the oldest. 



THE TREND OF PHYLOGENETIC DEVELOPMENT IN SUBAPTEROUS 



FORMS. 



In flying insects where the wings are of supreme importance 

 their structure is very constant for each form ; but as soon as 

 they become vestigial to such an extent that they are no longer 

 available for their only function, that of flight, they are a useless 

 burden, so that once this stage has been reached natural selection 

 should rapidly remove them entirely. This is no doubt the case, 

 for insects with wings just too short for use in flight are very 

 rarely met with. Of these exceptions two groups can be de- 

 fined : 



1 . Forms where the wings have suddenly developed characters 

 which make them of use in some other direction. 1 



2. In groups which seem to be undergoing rapid and remark- 

 able phylogenetic changes, e. g., the Phoridae among the Dip- 

 tera. 



The great preponderance of the forms which have wings of 

 the third type (vide supra] shows that there is a rather sharply 

 defined point where they become so vestigial that they are no 



1 Examples of this can be seen in widely separated groups. A notable case is 

 that of a chalcid fly, Eupelmus rhizophelus, considered in the sequel. 



