174 INSECTS 



surface on even a living animal. Most of the flies of 

 this series are attracted by foul odors which, to them, 

 is an indication of a suitable place for eggs; hence it 

 is not altogether unusual to have a female oviposit 

 into the open mouth or into the nostrils of a sleeping 

 human afflicted with catarrh or some other trouble 

 giving rise to foul breath. Eggs of this kind are usually 

 ready to hatch when laid and sometimes already hatched 

 within the abdomen of the female; hence it is a matter 

 of only a very short time for the young larva to make 

 its way along the mucous membrane, in which it may 

 exist for a day or two without giving rise to much dis- 

 comfort. After this it bores into the soft tissue of the 

 palate and into the cavities and sinuses of the head, 

 giving rise to intense pain, high fever and often death, 

 if the character of the trouble has not been at once 

 recognized and prompt treatment made. Yet this 

 form of parasitism is incidental or accidental, and shows 

 principally how slight and easy is the step from the 

 beneficial scavenger to the injurious parasite. When 

 the eggs are deposited on or near an open wound or 

 sore surface, the larvae bore into the exposed tissue and 

 feed upon the living flesh, which of course becomes 

 much inflamed, ulcerates and attracts yet other flies 

 of the same character, unless the matter is promptly 

 looked after. The screw-worm flies are common enough 

 throughout the middle, southern and central states, 

 but are most troublesome in the south and the south- 

 west where, during some years, much loss among domes- 

 tic animals has resulted from their attacks. The adults 

 are stout flies almost two-fifths of an inch long, 

 metallic bluish in color, with three blackish longitudinal 

 stripes on the upper side of the thorax. 



A very similar but somewhat smaller species is the 

 blue-bottle fly, much more common in the northern 



