178 SANITARY ENTOMOLOGY 



tories. All of the species, except Wohlfahrtia magnifica Schiner, are 

 carrion breeders although the adult flies are attracted to various kinds 

 of food, especially those with strong pungent odors as come from the 

 cooking of cabbage or turnips. A few develop occasionally in human 

 excrement ; normally, however, the decomposition of animal matter has 

 the strongest attraction for them and in many regions it is with great 

 difficulty that animals can be slaughtered without having the meat 

 contaminated by their presence in large numbers (see plate II). Garbage 

 containing meat and bone will attract and breed them. 



America. — The screw-worm fly occurs throughout the United States, 

 but is of little importance as a pest except in the Southwest where in 

 some sections it is a veritable scourge to the raisers of livestock. 

 The life history of this species will serve as an illustration for this 

 group of flies in the United States. The eggs are deposited on carrion, 

 especially on animals which have died recently. These hatch in a few 

 hours into maggots which enter the tissues rapidly and become mature in 

 about six to twenty days. In living animals development seems to be 

 rather more rapid. Pupation takes place in the soil from the surface to 

 three or four inches deep and the flies emerge in from three to fourteen 

 days. The total development period from attack to adult has been found 

 to vary from seven to thirty-nine days. The activity of this species is 

 confined to the warmer part of the year, usually from about April first 

 to November first in the Southern States. The black blow fly, Phormia 

 regina, on the other hand, appears more resistant to cool weather and 

 becomes most numerous in the southern region during earl}^ spring and 

 late fall. This is also true to a large extent with the large hairy blow- 

 flies. These latter entirely disappear during the summer months in the 

 southern latitudes. 



Infestations of screw-worms in animals occur on any portion of the 

 body where there is broken skin or even on sound skin where blood spots 

 occur. For the most part, however, the infestations follow mechanical 

 injury or where ticks have been crushed on the host. In man practi- 

 cally any part of the body may be attacked, but the most common 

 type of myiasis is nasal. This is especially true in Central and South 

 America. Such infestations are usually associated with malignant 

 catarrh or bleeding from the nose, and practically always with careless 

 modes of living. The lai'vae enter the nose and penetrate the tissue, 

 rapidly producing extensive cellulitis and usually accompanied by con- 

 siderable serous or bloody discharge. If not detected for two days the 

 injury is likely to be very serious. The frontal and ethmoid sinuses may 

 be entered and the cartilage and even the bone attacked. Often the 

 tissues of the nose and beneath the eyes begin to collapse and sometimes 

 excavation reaches to the surface, giving permanent disfiguration. This 



