134 THe BLOWFLIES oF NortH AMERICA 
spiracles (pl. 34, C) paired, small, each with two slits partly 
surrounded by a ring or peritreme which is incomplete ventrad 
of slitlike openings, narrowed, and lightly pigmented dorsally; 
greatest diameter of spiracular plates 0.1384-0.168 mm. (aver- 
age about 0.15); main tracheal trunks provided with dark pig- 
ment, the pigmented portion extending approximately one-half 
the length of segment 12. Posterior cavity more depressed than 
in first instar; upper border provided with three pairs of rather 
low, broadly rounded, fleshy processes; inner and outer pairs 
poorly defined; on lower border of cavity the median pair some- 
what the largest and the outer pair larger than the inner pair 
which are poorly defined; relative positions similar to those of 
third instar; anal protuberances rather small, with two promin- 
ent anal tubercles; spines on anal protuberance similar to those 
in third instar. Cephaloskeleton (pl. 34, B) with two prominent 
labial sclerites, each with a more or less rectangular basal por- 
tion; and with two long broadly curved labial sclerites; pharyn- 
geal sclerites produced anterodorsally and connected by the 
dorsopharyngeal sclerites; length of cephaloskeleton from 0.67- 
0.688 mm. (average of 10, 0.67 mm.). 
Third instar: Rather robust, tapering toward cephalic segment 
from sixth segment and slightly tapering toward posterior end on 
last three segments; length from 6.4-17.0 mm., and width from 
1.6-3.5 mm.; fully matured larva (pl. 34, E) usually 15-16 mm. in 
length; newly molted larva creamy white in color; mature larva 
with slightly reddish tinge. Heavily armed with large one- to 
three-pointed, more often one- or two-pointed, spines (sp) ; ante- 
rior margins of each of segments 2 to 9 completely encircled with 
a band of spines arranged in irregular rows, those in anterior 
rows largest; segment 10 with anterior band of spines somewhat 
narrowed and usually interrupted on dorsum as in second instar; 
segment 11 with anterior band never completely encircling seg- 
ment and spines laterally smaller and greatly reduced in num- 
bers; segment 12 with spines restricted to ventral and ventro- 
lateral surfaces; posterior margin of segment 11 provided with 
a loose band of two or three rows of anteriorly curved spines; 
segment 10 with spines ventrally and ventrolaterally, a few 
scattered ones laterally and dorsolaterally; segments 7 to 9 
each with spines at posterior margin reduced to one or two rows 
and confined to ventral surfaces. Anterior spiracles (asp) each 
provided with from 6 to 11, usually 7 to 9, fingerlike branches 
(average of 120 spiracles, 8.3 branches per spiracle) ; posterior 
spiracles (pl. 84, D) large, each with a prominent dark- 
pigmented peritreme (y~) which apparently does not completely 
