50 PARASITOLOGY. 
are provided with ambulacrum and the third pair with 
one bristle at each terminal extremity. 
Female.—The first two pair are provided with am- 
bulacrum and the last two pair with one bristle at 
each terminal extremity. 
The genus Psoroptes, species Communis, is pre- 
vided with a head that is longer than broad; the 
rostrum is conical in shape and elongated, and not 
margined by cheeks. 
Male.—The first three pair of legs are provided 
with ambulacrum, the fourth pair with two bristles. 
Female.—The first two and fourth pair of legs are 
provided with ambulacrum. The third pair with two 
long bristles. 
The genus Symbiotes, species Communis and 
Auricularum,—rostrum as wide as long, and desti- 
tute of cheeks. 
Male.—All legs are provided with ambulacrum. 
Female.—The first, second and fourth pair are 
provided with ambulacrum and the third pair with 
two bristles. 
LIFE HISTORY OF THE FAMILY SARCOPTIDAE. 
The life history is divided into five stages, viz.: 
The oval, larval, pupal, adult and ovigerous female. 
First Stage.— Ova.—Under favorable conditions the 
eggs hatch in five to ten days. 
Second Stage.—Larval Stage.—Six-legged and asex- 
ual; it passes through two or three moults. 
Third Stage. Nympha.—The last moulting in the 
larval stage adds a fourth pair of legs; no sexual 
organs ; the males and females are distinguished by 
the difference in size, the females being larger. 
