498 



DISEASES OF CATTLE. 



Buffalo gnats are more troublesome in bright, sunny weather than 

 when it is cloudy, and animals which have not shed their winter coats 

 suffer more from their attacks than those with smooth coats. Cattle 

 kept in darkened stables are not molested. The application of one 

 of the fly repellants already mentioned (p. 495) will help to protect 



animals from buffalo gnats. The 

 burning of smudges is also a useful 

 means of protecting stock from the 

 attacks of these flies. 



Screw "Worms. 



Screw worms (fig. 6) are the mag- 

 gots of a fly ( Chrysomyia macellaria), 

 so called from their fancied resem- 

 blance to a screw. The adult fly (fig. 

 7) is about one-third of an inch long, 

 with a bluish-green body, red eyes, 

 and with three dark longitudinal 

 stripes on the back (thorax). At- 



FiG. 5.— Buffalo gnat. (Prom Bureau of 

 Entomology.) 



Fie. 6. — Screw worm (larva of Chrysomyia macel- 

 laria). (From Bureau of Entomology.) 



tracted by odors of decay it deposits 

 its eggs, 300 to 400 at a time, in cuts, 

 sores, castration wounds, etc. The 

 bursting of a tick on the skin com- 

 monly results in screw-worm infec- 

 tion at that point. The eggs hatch 

 in a few hours and the larvae or 

 maggots, or so-called screw worms, 

 begin to burrow into the flesh and 

 continue burrowing and feeding 

 from four to six days, after which 

 they leave the wound and crawl 

 into the eai^th, there transforming 

 into the quiescent pupal stage. 

 After this stage has lasted for one 



to two weeks, the mature fly appears. From two to three weeks are 



therefore required for the entire life cycle. 

 Besides cattle, the screw-worm fly attacks sheep, horses, hogs, and 



man. In the case of hogs it is generally the ears which are affected. 



Fig. 7. — Screw-worm fly {Chrysomyia macel- 

 laria). (From Bureau o£ Entomology) 



