INTRODUCTION 35 
another, such variation has been considered to be of possible 
specific value and such differences have been noted in the 
specific descriptions. 
VARIATION 
_ The general impression seems to. prevail among students of 
Diptera that few characters of color and chaetotaxy are suf- 
ficiently constant to be useful in the recognition of the different 
species of Calliphoridae. This feeling, however, is apparently 
based upon cursory examinations of series of specimens sup- 
posed to represent a single species but exhibiting very different 
metallic colorings, the mass of synonymy published in catalogs, 
and, at least in part, upon published papers dealing with 
‘‘variation’’ in Lucilia and Calliphora. That variation occurs in 
characters employed in this paper to distinguish between species 
of Calliphoridae cannot be doubted; but careful study of an 
immense mass of material has demonstrated that certain char- 
acters, ordinarily ignored as too variable, are definitely usable. 
Usually adults of Diptera acquire normal color within a few 
hours after emergence; but in many Calliphoridae the typical 
coloring may not develop for 24 hours, especially in humid 
weather. In some of the species of Phaenicia the typical color- 
ing may take on a bronzy hue after 2 or 3 days, and this 
change may become even more pronounced later. Such changes 
are most obvious when green or blue is the predominant color. 
Specimens may change in color even after they have been killed, 
although such changes are slight in the case of deep blue or 
yellowish species, provided the specimens are fully developed 
when killed. . 
It is true that some variation exists in the chaetotaxy of mus- 
eoid flies (See Townsend, 1914, p 112). It seems only necessary 
to state here that most of the characters used were selected from 
those proved to be relatively constant in long series of specimens 
reared from egg masses deposited by single females, which, 
in some eases, were from second- and third-generation rearings. 
When such series were not available, characters were chosen 
that had been found to vary little in long series of closely related 
species. ss 
It has been known for many years that large individuals of 
a species have more strongly developed bristles and hairs than 
do small individuals of the same species. In many cases weak 
bristles tend to become hairlike (or are rarely absent) and may 
be overlooked unless searched for carefully. The actual position 
of certain bristles may vary slightly in some specimens of a 
