56 THE BLOWFLIES OF NorTH AMERICA 
Clean healthy animals are rarely attacked by blowflies. Moore 
(1937, p. 227) suggested that some abnormal attrahent must be 
present on fleece of sheep to attract ovipositing females. He 
considered such attrahents to be possibly the remains of urine and 
feces on the wool, foot-rot, wool-rot, discharging wounds, or dips 
grossly contaminated with urine or feces. 
Temperature and humidity are important factors in the de- 
velopment and growth of ovaries. High humidity is apparently 
favorable and low humidity is not. Evans (1935b) considered 
humidity to exert little effect upon oviposition at temperatures 
of 10° to 35° C., but low humidity to be unfavorable at 40° C. 
LARVA. The larvae of blowfly species have the posterior spir- 
acles nearly flush with the rim of the anal. area and the dorsal 
cornua is entire. The larvae of Sarcophagidae have the posterior 
spiracles within a deep depression and the dorsal cornua is 
incised or divided. However, the larva of Sarothromyia femoralis 
(Schiner), a widespread Neotropical sarcophagid, does not have 
the typical deep posterior depression, and the larva of Stomor- 
hina lunata (Fabricius), a blowfly species occurring in Bermuda, 
misleadingly has the dorsal cornua with a longitudinal clari- 
fied area which sometimes appears as a division. 
The usual custom of describing the three obvious larval instars 
attained by two moults is followed in this paper. Apparently, 
however, there is another larval instar which is usually over- 
looked. Fraenkel (1938) stated ‘‘The cuticle of the third instar 
becomes the puparium by a process of contraction, darkening, 
and hardening, which is generally called pupation. The third 
moult is the prepupal moult, by which the puparium becomes 
separated from the hypodermis; the fourth moult is the pupal 
moult, which separates off the thin sheath—from the now-com- 
pleted pupa; and by the fifth moult the emerging adult frees it- 
self from the pupal cuticle.’’ 
The three obvious instars of blowfly larvae may be easily rec- 
ognized by characters found in the posterior spiracles. First-in- 
star larvae have little or no indication of peritreme surrounding 
the posterior spiracles and the two ovate apertures in each spir- 
acle are nearly or actually united basally. Such larvae are also 
metapneustic. Differentiating characters in this instar are dif- 
ficult to discern and formal descriptions of the stage are usually 
omitted. Second-instar larvae are amphipneustic as are third- 
instar larvae, the anterior spiracles found laterally at the base 
of the second segment, have circular openings radiating fanlike 
from main trunks. The peritreme surrounding each posterior 
spiracle is more definite than in the previous instar although it is 
