PARASITES FREE DURING THEIR WHOLE LIFE. 121 



cies which are here represented, and which differ as much 

 by their external characters as by their mode of life. 



Another fly also attacks horses and cattle, and occa- 

 sionally even man, the Asilus crabroniformis, whose 

 wounds sometimes draw blood. Martins, the birds of 

 the twilight, which fly in flocks above the houses, 



Fig. 11.— Ox-fly. 



Fig. 12.— Antenna of Ox-fly. 



describing circles and uttering shrill cries, are usually 

 infested by many vermin, among which we find a fly of 



Fig. 13.— Blue Fly. 



considerable size, which looks much like a spider, the 

 Ornithomya hirundinis. It moves about among the 



