180 Annals Entomological Society oj America [Vol. XIII, 



dead plant and animal matter, this material is teeming with life. 

 The basic processes of amonification, nitrogen fixation, and the 

 synthesis of proteins are being carried on by these micro- 

 organisms and the beetle which feeds upon this decaying matter 

 profits by their activity. It has been shown, in the case of some 

 insects, that under certain conditions the rate of growth is in 

 direct proportion to the number of micro organisms in the food. 

 One has but to review the food lists of the beetles to note in 

 how many cases they are known to feed upon fungi or upon 

 substances which may well contain micro organisms. When 

 this subject has been investigated further we may come to a 

 newer and more rational undertsanding of the life cycle of the 

 Coleoptera. 



7. THE LIFE CYCLE OF THE DIPTERA. 



C. L. Metcalf, Ohio State University. 



In attempting a summary such as this, one is almost dis- 

 mayed by the great gaps in our knowledge of even the 

 commonest species. Doubtless many of the gaps in what follows 

 could be filled by a more careful scrutiny of the literature; 

 many more, I am sure, could be filled from unpublished records 

 and observations of the members of this Society; but when all 

 this is recorded we shall find that very much more investigation 

 must be performed before we can so much as give a com- 

 prehensive statement for all the species of a single family or 

 for a single species of each family. 



I do not believe that the order Diptera is surpassed, either 

 within or without the class Insecta, for variety of habits and 

 complexity of bionomics ; and it seems to me an impossible task 

 to present in a brief paper anything like a satisfactory picture 

 of the life-cycle of the flies. 



Not only is data wanting for more than a fraction of a per 

 cent of the species; but, moreover, in the families where our 

 knowledge is more complete, the most impressive thing is that 

 there is no agreement or Ufiiformity of habit. Where uniformity 

 appears in my statements it is possibly because we know only 

 a few of the many species in that group. 



