182 THE BLOWFLIES OF NortH AMERICA 
lica; it is aberrant in either group. The second group includes 
only hirudo, which lacks reclinate frontoorbital -bristles in the 
female and has different habits in the larval stages. The third 
group includes metallica and its allies, these differing from each 
other in minor head proportions and in coloration. The true 
interpretation of the American species may have been shown by 
Shannon when he divided the forms into three species and 
grouped the rest of the segregates around these as varieties and 
subspecies. It is possible, however, that there are many more 
than three species and that the groups mentioned should have 
subgeneric rank. 
Biology, habits, and immature stages. Since the habits of all 
the species are more or less similar, all available notes are briefly 
summarized here. 
Members of both Apaulina and Protocalliphora are obligatory, 
bloodsucking parasites upon nestling birds. Eggs are deposited 
singly or in small masses in new nests. The larvae are nocturnal 
in habit and are apparently intermittent feeders. The larval 
period is from 10 to 13 days. When the larvae mature, they 
either pupate within the debris in the bottom of the nest or drop 
to the ground and pupate in the soil. The method of hibernation 
is not known. According to Dobroscky (1925) the species prob- 
ably overwinter as adults; they may be similar to Phormua in this 
respect. 
Certain types of bird nests seem to be preferred by females of 
Apaulina, those of passerine birds apparently being most attrac- 
tive. Mason (1936) was able to show quantitative figures proving 
the tree swallow to be the preferred host in New England and 
the blue bird the second choice. He suggested that these prefer- 
ences were due to differences in nest construction. There seems 
to be no question that hole-nesting species suffer greatly from 
these parasites and that nests built upon the ground or in the 
open rarely contain specimens of Apaulina. This explains, at 
least in part, the large percentage of hole-nesting species in the 
following host list. It is usually necessary to observe nestlings 
within nests in order to secure records of parasitism of those 
species which build in trees or in shrubbery. 
Apaulina metallica was found in large numbers in nests of 
Riparia riparia riparia in New York State by Dayton Stoner 
(1936). He stated that the larva of this fly is the most injurious 
external parasite of the bank swallow in this region. The larvae 
seemed to confine their activities largely or entirely to young 
birds in nests, particularly in late June and throughout July. 
Their occurrence appeared to vary considerably, and to be 
