264 THE BLOWFLIES OF NorTH AMERICA 
third segment of antenna three times as long as second. Wing 
with costal sections 2 to 6 in the proportion 65: 43:78: 33:4. 
Abdomen with considerably more whitish pollen laterally and 
ventrally in certain lights than in male. Otherwise similar to 
male except for normal sexual differences. 
Length. 6.0-9.5 mm. 
The difference in the number of postacrostichal bristles is one 
of the major characters distinguishing the sericata-pallescens 
group from a larger group composed of caeruleiviridis, mexicana, 
cluvia, and others. Consideration of this bristle series alone will 
permit placement of most specimens into their proper grouping. 
But several previous authors have questioned the value of the 
difference because of variation noted in the number of bristles 
on each side. For instance, the majority of specimens of sericata 
and pallescens have three postacrostichal bristles on each side, 
but some have three on one side and four on the other, and a few 
have four on each side. Because of such observations reared 
series of several of the species were obtained from different 
parts of the United States and these were studied and compared 
with each other. 
While the typical number of postacrostichal bristles in. both 
sericata and pallescens is three on each side, some specimens 
have four on one or both sides. The additional bristle is usually 
a weak one in advance of the regular series but it is sometimes 
as large as the normal bristle following it. If the following nor- 
mal bristle is absent, (it is missing in six out of 100 specimens) 
this causes the row to have the normal number of three bristles 
but the row is spaced for four. Of a total of 608 specimens reared 
from egg masses obtained from isolated females, 259 males and 
239 females of sericata (82 percent) had the typical number of 
three postacrostichal bristles on each side, 28 males and 62 
females (14 percent) had three postacrostichal bristles on one 
side and four on the other, while 3 males and 16 females (3 
percent) had four postacrostichal bristles on each side. One male 
had two postachrostichal bristles on one side but three on the 
other. The series of reared pallescens showed nearly identical 
percentages of variation in the postacrostichal series. An inbred 
strain of sericata showed wider variation than F, generation 
series from Menard and Dallas, Texas. 
Two postacrostichal bristles normally occur on each side in 
caeruleviridis and mexicana. Specimens of the former species 
in the F, generation, specimens of the latter species in the Fy 
generation, and numerous specimens of both species collected 
in nature exhibited no variation in this number. 
