SCREW-WORM 519 



four days. The mature larva, which closely resembles that of 

 ('. anthropophaga, leaves the host in from 12 to 15 days, buries 

 itself to a depth of several inches in the ground, and pupates. 

 If a little moisture is present, the transformation into the adult 

 occurs in a little over three weeks. About two months is required 

 for the whole life cycle of this fly. 



Mjriasis of Wounds and of Natural Cavities of the Body 



A large number of flies, all of them related to the blowflies and 

 houseflies. occasionally deposit their eggs or newly hatched 

 larvae in neglected wounds when offensive discharges are exuding 

 from them. In severe cases infestations with maggots of these 

 flies may lead to a most horrible and loathesome death. 



The instinct of the female flies of all the species implicated is 

 to deposit offspring in places from which the odor of meat or of 

 decaying animal matter is emanating, regardless of where the 

 place may be. This instinct is, of course, of the highest value 

 to the species, since the larvse live upon the substances from which 

 such smells arise. It is an instinct analogous to that which 

 causes a mosquito to lay its eggs in water, or a horsefly to oviposit 

 in objects overhanging water — an unknowing but accurate 

 intuition on the part of the parent to provide for the welfare of 

 its young. 



Screw-worm. — One of the most important species in this 

 connection is the American screw-worm fly, Cochliomyia (or 

 Chrysomyia) macellaria, which occurs throughout America from 

 Canada to Patagonia, though abundant only in warm countries. 



The adult fly (Fig. 250A) is a handsome insect, slightly larger 

 than a housefly, of a metallic blue-green color with three dark 

 stripes on the thorax. It belongs to the family Muscidae, and 

 to the same section as the ordinary blowflies. The adults con- 

 gregate about carcasses of dead animals on which they ordinarily 

 deposit their offspring and on which the larvse feed. Records 

 differ as to whether the eggs hatch within the body of the parent 

 or after being deposited, and it is probable that during the early 

 part of the season and in cool climates the eggs are deposited, 

 while under other circumstances the living maggots are born. 

 The number produced by a single fly may be several hundreds, 

 but they are deposited with amazing rapidity. The maggots 



