farmers' bulletin 857. 



The eggs are deposited almost exclusively 

 upon dead animals and in wounds. The 

 flies show a marked tendency to visit fresh 

 carcasses rather than old ones; this is true 

 also to some extent with the infestation of 

 wounds. The eggs hatch in from less than 

 four hours when the weather is moist and 

 1 * warm to about two days when surroundings 



I ""~3|' ''^'^ l^ss favorable. The condition which ex- 



ists in fresh wounds is A^er}^ favorable for 

 incubation, and the eggs probably hatch 

 within three hours after deposition in such 

 situations. 



As soon as the eggs hatch, the young larvte 

 Ijeo'in Denetrating the carcass, or if on liv- 

 W^ ' % ing animals they start burrowing into tlie 



gP^ 1 flesh. While undergoing development m a 



wound the larvte tear the softer portions of 

 exposed flesh with their stout jaws and excrete 

 an irritating substance which probably assists 

 in breaking doAvn the tissue. When the mag- 

 gots are working in a deep wound they release 

 their hold on the flesh at intervals and come 

 to the surface, where they lie with the breath- 

 ing pores exposed. If disturbed Avhile at the 

 surface, they return at once to the deeper 

 l^ortions. The injury to the infested ani- 

 mal is not due entirely to the eating away 

 of the tissues, but also to the toxic material 

 resulting from the presence of the maggots 

 in the wound, which may result in general 

 poisoning of the system of the animal. 

 The worms grow very rapidly and in living animals are mature 

 and drop from the wound in from four to five days. In carcasses 

 they do not mature quite so rapidly (6 to 20 days) unless the weather 

 is hot and damp. The grown maggots burrow into the ground to a 

 depth of from 1 to 4 inches and soon contract, becoming first yellow- 

 ish and then brown, ,the outside skin forming a hard protective 

 covering. This is known as the pupa stage. The flies emerge from 

 these pupa^ in from 3 to 14 days and soon are ready for egg laying. 

 (See fig. 5.) The entire life cycle is completed in from one to four 

 weeks, depending on the temperature and liumidity. 



IG. 5. — Screw-worm flies 

 on weed near earcass. 

 All vegetation arounrt this 

 weed was covered in a 

 similar manner. Consid- 

 eralily reduced. 



