14 . THE BLOwF.LIES oF NortH AMERICA 
flies do not cause lesions in man and animals. Gravid females of 
this group usually deposit their eggs upon decaying flesh. They 
may be attracted to wounds which already exist and they may 
deposit eggs sometimes on morbid tissue. The larvae then live 
in and extend such lesions. 
Tertiary Flies may cause Accidental Myiasis. Larvae of such 
flies are found only in very advanced eases of cutaneous myiasis. 
Gravid females of this group normally deposit their eggs in 
decaying organic material and their larvae are of relatively rare 
occurrence in wounds. 
Old pustular suppurating lesions may contain many species of 
secondary or tertiary blowflies in the maggot stage. These mag- 
gots may be ealliphorine, chrysomyine, sarcophagine, or even 
species of Musca, Muscina, or Stomoxys. Many of these species 
may be beneficial in wounds because lesions of the suppurative 
type seem to heal rapidly due to the presence of saprophagous 
larvae. This phenomenon may be attributed in part to secretions 
of urea, ammonium carbonate, and allantoin by the larvae, to 
mechanical effects which apparently promote growth of granu- 
lating tissue, to the stimulation of secretions within the wound 
which destroy large numbers of bacteria and inhibit their growth, 
and to the removal of dead tissues from the lesion which makes 
conditions less favorable for the development and increase of 
pathogenic organisms. 
Subcutaneous Myiasis. Several species of blowflies are respon- 
sible for subcutaneous myiasis in North America. One such 
species (Callitroga americana Cushing and Patton) causes the 
death of a number of persons each year. Borgstrom (1938) 
reported over 55 eases of human myiasis in southeastern United 
States during 1935. Cushing and Patton (1933) reported 8 per- 
cent mortality in 179 cases of myiasis caused by this species which 
were studied by them. During the summers of 1934 and 1935 
we of the Bureau.of Entomology and Plant Quarantine labora- 
tories in Georgia saw a large number. of human eases of nasal, 
auricular, ocular, and cutaneous myiasis, all of which were due 
to the larvae of americana. 
This species causes innumerable cases of myiasis in livestock 
and the death of large numbers of domestic and wild animals 
almost every year in southern North America. The most acute 
of all insect problems concerning livestock in southern North 
America is due to this blowfly. 
Myiasis cases produced by larvae of americana are often 
extremely destructive and may lead to death unless prompt cor- 
rective measures are applied. Apparently the destruction of 
