!6 NORTH AMERICAN DIPTERA. 



voidy's shade would turn pale with envy in the contem- 

 plation of some of the proposed genera of the modern 

 culicidologists. 



STRUCTURE OF FLIES. 



The word diptera, by which the two-winged flies are 

 known, signifies two wings, the chief characteristic of 

 the adult insects. All diptera, if they have any wings, 

 have but a single pair. The hind pair of other insects is 

 really represented by a small organ on either side, back 

 of the true wings, consisting of a short, slender stem, 

 terminating in a knob. The precise function of these 

 'halteres' or 'balancers', as they are called, if they have 

 any, is not known ; that they have a secondarily acquired 

 use is probable, since they are always in vibration 

 during flight; an orienting or balancing function has 

 been ascribed to them. The halteres are very character- 

 istic of the order, always present in the winged forms, 

 and usually present even in those in which the true wings 

 are aborted or functionless, though rarely they are en- 

 tirely absent. Not all flies are winged, as has been al- 

 ready intimated ; rarely, among both the more general- 

 ized and the more specialized groups they are imperfectly 

 developed or wholly wanting. Their absence is not of 

 very great classificatory importance, never more than 

 generic, and sometimes doubtfully that. Sometimes the 

 male has wings and the female is wingless ; but the num- 

 ber of wingless forms of any kind is very small. 



In size as well as in shape, flies vary not a little. As 

 compared with the lepidoptera, orthoptera, neuroptera, 

 and even the hemiptera, flies are relatively small insects. 

 The largest specimen of a fly of which I have knowledge 

 is that figured herewith natural size, pertaining to an 

 indeterminable species of Mvdas from South America. 

 The length of this specimen from the tip of the antennae 



