How Insects Cause or Carry Disease 47 



well as on man makes it possible for them to act 

 as carriers of parasites that might under proper 

 conditions become of serious importance. Then, 

 too, the irritation caused by the bites of these in- 

 sects usually causes scratching which may result 

 in abrasions of the skin that open the way for 

 various harmful germs, particularly those causing 

 skin diseases. 



Coming now to the group containing the house- 

 flies and related forms we find a number that are 

 of interest on account of the suffering that they 

 may cause, particularly in their larval stages. 



The screw-worm flies (Chrysomyia macellaria) 

 are among the most common and important of 

 these (Fig. 25). These "gray flies," as they are 

 sometimes called, lay a mass of three or four 

 hundred eggs on the surface of wounds. The 

 larvae which in a few hours hatch from these make 

 their way directly into the wound where they feed 

 on the surrounding tissue until full grown when 

 they wriggle out and drop to the ground where 

 they transform to the pupa and later to the adult 

 fly. Of course their presence in the wounds is 

 very distressing to the infected animal, and great 

 suffering results. Slight scratches that might other- 

 wise quickly heal often become serious sores be- 

 cause of the presence of these larvae. 



