DIPT ERA. 253 



The possession of this structure is of great advantage 

 to the insect, since it increases its chances of obtain- 

 ing food. 



The fly has only one pair of functional wings, and 

 this characteristic has given the name of Diptera (8t's, 

 double; TrrepoV, wing), meaning two-winged, to the 

 order. These wings (Fig. 197, 7e/' ; Fig. 196) are 

 strong organs, and are borne by the large, muscular 

 mesothorax, so that the flight of the insect is swift, 

 the fastest horse not being able to outstrip it. The 

 point of insertion of the wings is indicated in PL XII., 

 Fig. 198, «/'. Attached to each wing, and moving 

 with it, are two small, flat scales, or winglets, called 

 alulets (PI. XII., Fig. 197, sc~), the use of which is 

 unknown. The second pair of wings (Figs. 196, 197, 

 7£/" ; Fig. 198, 7£'", indicates the point of insertion) are 

 reduced to a pair of knobbed balancers, or halteres. 

 It is interesting to note in this connection the decrease 

 in size and efficiency of the hind wings in passing 

 from the Lepidoptera to the Diptera. Hammond ^ has 

 shown that there is not only this reduction, but also a 

 corresponding change in the segment which bears the 

 posterior wings, and the muscles which move them. 

 In the Lepidoptera the fore and hind wings are more 

 nearly equal than in the other two orders ; in the 

 Hymenoptera the posterior wings are much reduced 

 in size, as we have seen, and in the Diptera they are 

 only represented by the diminutive halteres, while 

 the metathorax, in the specialized forms, is so small it 

 cannot be seen from above, and the metathoracic 

 muscles exist only as remnants. The buzzing of the 



1 Joiirn. Linn. Soc, London, Vol. XV., p. 16, 1880-81. 



