36 Tur BLOWFUIES oF NortH AMERICA 
reared series, but this happens rarely and does not seriously 
interfere with the use of the bristle arrangement as a distinguish- 
ing character. 
In a number of reared series of several species obtained from 
widely separated sources in the United States, reduction and 
duplication of bristles occurred in the dorsocentral or acrostichal 
series in slightly less than 5 percent of the individuals. Dupli- 
eation occurred nearly twice as often as reduction. Such varia- 
tion occurred on one side or the other, rarely on both sides in 
the same specimen. This matter is discussed more fully under 
sections dealing with Phormia regina and Phaencia sericata. 
In cases where variation seems to be significant, this fact 
has been noted in the description of the species; characters show- 
ing too wide variation have not been used in the taxonomic 
treatment of the species concerned. Only a few species have 
been ‘separated upon characters of chaetotaxy alone, although 
it would have been possible to do so in many instances. 
Just how much reliance may be placed on the position of 
bristles on the legs is questionable at the present time. A given 
number may remain constant in a single species, but the actual 
position of one or more of these may differ slightly among speci- 
mens which otherwise appear identical. Whether these differ- 
ences are specific cannot be ascertained now, and such characters 
have not been employed in the separation of species. 
As a rule, several characters have been given in the keys for 
the separation of each species. Usually one or two of the key 
characters pertain to chaetotaxy and one or more to propor- 
tional measurements of certain parts. Finally, a supplementary 
color character is often given. Although it is felt that any one 
of the characters will usually separate the species concerned 
satisfactorily, all characters mentioned should be fully considered 
before arriving at a definite conclusion as to the identity of a 
specimen. 
In several supposedly cosmopolitan species small differences 
can be noted among Australian, African, and North American 
specimens, but such differences are minute and may possibly be 
of no real significance. They can be distinguished in a long series 
but not always in individual specimens, so I have not attempted 
to differentiate between these forms. For example, the Phaenicia 
sericata of Europe and North America appear to have slightly 
different average frontal widths. Protophormia terrae-novae 
from Europe agrees with the Alaskan terrae-novae treated here, 
but specimens of this species from the southern part of its dis- 
tribution appear to have slightly different buccal and eye-height 
