SUBFAMILY CHRYSOMYINAE 135 
surround the three more or less oval slitlike apertures; greatest 
diameter of spiracular plate from 0.39-0.46 mm. (average of 
10, 0.43 mm.); the two main tracheal trunks darkly pigmented, 
the pigmented portion extending anteriorly to tenth or ninth 
segment. Upper border of posterior cavity (pl. 34, G, pc) 
bearing three pairs of low broadly rounded tubercles; inner (7) 
and outer (0) pairs approximately equal; median (m) pair 
greatly reduced and closely approximated to outer tubercle; 
lower border of cavity provided with similar tubercles, median 
pair largest and outer pair larger than inner pair; an additional 
pair of small tubercles situated medially near lower margin of 
posterior cavity; anal protuberances (ap) comparatively small, 
with two prominent conical anal tubercles; rather prominent 
spines located anteriorly and posteriorly on anal protuberance; 
short rows of three to four minute spines situated on lower 
portion on depressed area and on lower margin of posterior 
cavity. Cephaloskeleton (pl. 34, H) essentially as in second 
instar but the parts larger and rather different in appearance in 
mature larva; a narrow pigmented strip arising at anterodorsal 
margin of pharyngeal sclerites and extending posteriorly (this 
is not visible in younger third-instar larva but appears as the 
larva matures); entire cephaloskeleton from 1.40-1.56 mm. 
(average of 10, 1.49 mm.). 
When the larvae penetrate tissues, they assume a characteristic 
head-downward position in the wound, with the posterior 
spiracles usually exposed to the outside air. Larvae of this 
species are obviously gregarious, rarely move about in the 
wound (pl. 34, F) when once embedded in the flesh, and cause 
a typical pocketlike (fig. VIII) injury. They may feed from 
99.5 to 198 hours according to experiments conducted by Cush- 
ing, in Laake et al. (1936, p. 16), at Sonora, Tex. He found that 
_ the majority of mature larvae leave the wound between the hours 
of 9 am. to 2 p.m. When fully developed they drop to the 
sround and pupate in the soil. Travis, Knipling, and Brody 
(1940) showed that larvae of americana begin to migrate from 
a carcass immediately after the death of the animal, practically 
all of them leaving within 48 hours. They found that 69 percent 
of such migrating larvae pupated under the careass and that 
none migrated further than 2 feet. 
Puparium. Of typical calliphorid shape and appearance, 
averaging approximately 10.2 mm. in length and 4.3 mm. in 
diameter, dark brown in color, otherwise with the external 
characters of the mature larva. 
The pupal stage of americana was found by Laake et al. (1936, 
