iS 



NORTH AMERICAN DIPTERA. 



Bombyliidae, has ever had in the past a larger average 

 bodily size than is found among their living representa- 

 tives. On the other hand, those families composed to- 

 day chiefly of large forms are ones already past their 

 prime. These conclusions seem established for the larger 

 forms of life, and I believe that they are in the main also 

 applicable to insects. 



Many curious resemblances of external form are ob- 

 servable among diptera belonging to widely different fami- 

 lies and of remote relationships. Some of these, perhaps 

 many, have a protective value, in flies of like habits; or 

 they may have been the results of like environmental 

 conditions. 



Ceria. 



Cerioniydas. 



Conops. 



Fig. 3. Examples of mimetic resemblances in flies. Ceria (Syrphidae); 

 Ccriotnydas (Mydaidae); Conops (Conopidae). Slightly reduced. 



In figure 3 are given photographs of three wasp-like 

 flies of very different families and of considerable size, 

 all of them associated in the same fauna, and curiously 



