184 



FIELD ZOOLOGY. 



little humps or knobs, called halteres, rudiments of the 

 wings which, in the fly ancestor under far different cir- 

 cumstances, probably were present but are not now needed 

 and are only suggested by these tiny vestiges which 

 probably play the part of balancers in flight. The 

 tendency toward non-differentiated forms as we go 

 downward in the scale explains why the wings in the 



FIG. 73. Mouth parts of a horse fly. md, mandible; mx, maxilla; mxl, max- 

 illary lobe; mxp, maxillary palpus; hyp, hypopharynx; Ib, labrum; ep, epi- 

 pharynx; li t labium; la, labellum. (Kellogg.') 



lower insects are increasingly equal in size and similar 

 in venation, while in the higher insects there is the 

 apparent tendency to let one pair of wings get the ascen- 

 dancy over the other pair. So here, in the Diptera, we 

 have the hind pair of wings giving way altogether to the 

 front pair, and we have a two-winged animal which gives 

 evidence in other ways of being high in the animal scale. 



The order includes house flies, carrion flies, flower 

 flies, horse flies, bluebottles, bee flies, pomace flies, hessian 

 flies, bot flies, robber flies, midges, gnats, dance flies, 

 punkies, and the mosquito tribe. (Fig. 74.) 



The mouth parts are not fitted for biting, but the 

 mandibles, when present, together with the maxillae, are 



