ENTOMOLOGY

178 SANITARY ENTOMOLOGYtories. All of the species, except Wohlfahrtia magnifica Schiner, arecarrion breeders although the adult flies are attracted to various kindsof food, especially those with strong pungent odors as come from the.cooking of cabbage or turnips. . A few develop occasionally in humanexcrement; normally, however, the decomposition of animal matter hasthe strongest attraction for them and in many regions it is with greatdifficulty that animals can be slaughtered without having the meatcontaminated by their presence in large numbers (see plate II). Garbagecontaining meat and bone will attract and breed them.AMERlcA.-The screw-worm fly occurs throq.ghout the United States,but is of little importance as a pest except in the Southwest where insome sections it is a veritable scourge to the raisers of livestock.The life history of this species will serve as an illustration for thisgroup of flies in the United States. The eggs are deposited on carrion,especially on animals which have died recently. These hatch in a fewhours into maggots which enter the tissues rapidly and become mature inabout six to twenty days. In living animals development seems to berather more rapid. Pupation takes place in the soil from the surface tothree or four inches deep and the flies emerge in from three to fourteendays. The total development period from attack to adult has been foundto vary from seven to thirty-nine days. The activity of this species isconfined to the warmer part of the year, usually from about April firstto November first in the Southern States. The black blow fly, Phormiaregina, on the other hand, appears Jl}ore resistant to cool weather andbecomes most numerous in the southern region during early spring andlate fall. This is ,also true to a large extent with the large hairy blow­-flies. These latter entirely disappear during the summer months in thesouthern latitudes.Infestations of screw-worms in animals occur on any portion of thebody where there is broken skin or even on sound skin where blood spotsoccur. For the most part, however, the infestations follow mechanicalinjury or where ticks have been crushed on the host. In man practicallyany part of the body may be attacked, but the most commontype of myiasis is nasal. This is especially true in .Central and SouthAmerica. Such infestations are usual~y associated with malignantcatarrh or bleeding from the nose, and practically always with careles~modes of living. The larvre enter the nose and penetrate the tissue,rapidly producing extensive cellulitis and usually accompanied by considerableserous or bloody discharge. If not detected for two days theinjury is likely to ·be very serious. The frontal and ethmoid sinuses maybe entered and the cartilage and even the bone attacked. Often thetissues of the nose and beneath the eyes begin to collapse and sometimesexcavation reaches to the surface, giving permanent disfiguration. This