INTRODUCTION 15 
tissue alone is not responsible for the extent of. the injury to the 
person or animal so affected. Borgstrom (1938, p. 396) stated 
‘“The severe effects must be due either to metabolic or toxie pro- 
ducts of the larvae formed during their activity in the wound.’’ 
Exudates from infested wounds contain practically no leucocytes 
and such wounds never show signs of pus formation. Although 
bacteria are consistently associated with myiasis lesions, when 
such bacteria are inoculated in pure culture into other wounds 
they fail to produce pyogenic infections. In two instances in 
which staphlococei were inoculated with americana larvae into 
wounds they produced an accumulation of pus which disappeared 
within two or three days. Borgstrom’s observations suggest that 
the leucopenia found in screwworm eases may be due to the 
destruction of leucocytes by the presence of larvae in such 
wounds. Wounds infected with americana larvae invariably 
develop a pure culture of a single species of bacterium, Proteus 
chandleri Borgstrom (1938). In no instance has any other organ- 
ism been found in the lesion from 24 to 48 hours after the 
introduction of americana larvae into a wound. 
Wounds with a pH of from 6.9 to 7.3 are most attractive to 
feeding and ovipositing adult blowflies. Wounds with an acid 
reaction heal quickly and are not so attractive. Wounds which 
produce a watery discharge are exceedingly attractive to gravid 
blowflies for a considerable period of time. 
Prior to 1933 subcutaneous myiasis in the southern part of 
North America was mostly attributed to Callitroga macellaria 
(F.) and control measures advocated were based in part upon 
the biology of that species. Actually macellaria is secondary in 
wounds and the larvae occur principally in decaying meats. 
The usual cause of serewworm injury and subcutaneous myiasis 
in southern North America is Callitroga americana,-a species 
previously confused with macellaria. Most discussions of screw- 
worm flies in the economic literature apply to americana. 
The impression generally prevails that secrewworm outbreaks 
are of only recent occurrence in North America and that such 
outbreaks date from the serious situation in the southern United 
States during 1933-1935. As a matter of fact screwworm flies 
have been rather well known in the Middle West for many years 
and an account written in 1882 describes nasal myiasis in the 
human so typically that it is repeated here in part. This account 
is from a letter addressed to C. V. Riley by Fred Humbert, M. D., 
Alton, Illinois. 
‘A farmer’s wife, 85 years of age, was attacked on Monday, 
Sept. 27, 1875, with a headache and a flushed face. From this 
