270 SarCoptic Scabies 
never been observed in the human Sarcoptes, but Munro found it on two 
occasions, and describes it as follows: 
“Length 160/x; rugose area indistinct; dorsal scales numerous, bare area 
well defined; notogastric spines twelve in number. Fourth pair of legs ending 
in bristles which are shorter than those of the third pair; only one pair of 
anal bristles present. Epimeres of the anterior pairs of legs forked.” 
Dubreuilh and Beille (p. 20) say that the nymphs are of two sizes (measure¬ 
ments not given) and that the smaller produce males and the larger females. 
The larva is hexapod, the last pair of legs ending in bristles. Its length 
is from 110 to 140/z and its breadth 90 to 110 g. Bugose area ill defined; 
dorsal scales indistinct except at the sides so that a large bare area is present; 
notogastral spines ten; only one pair of anal bristles. 
The characters which have generally been relied on to separate the various 
forms which have been regarded by some authors as distinct species and by 
others as varieties are size, shape, the rugose areas, the epimeres, and the 
number and particular pattern of the various spines, scales, etc., which go to 
make up the dorsal armature. It has to be noted that the limits of size in 
a single form like S. scabiei are very considerable, and that the shape is 
necessarily rather indefinite in animals so feebly chitinised, while the other 
characters are so minute and so difficult of observation that no two writers 
are absolutely in agreement in their description of any one variety. It would 
seem to be the first task of a new investigator to examine minutely the human 
sarcopt in its various stages and to determine authoritatively the details of its 
structure, verifying or rejecting the statements of previous writers. W hen 
this is once accomplished the ground will be cleared for a comparison between 
it and the forms which affect other mammalian groups. 
Life-History of the Sarcoptes of Man. 
Excavation of the gallery. Immediately after her last moult the female 
begins to burrow, showing a distinct preference for certain regions. If con¬ 
fined under a watchglass she may be caused to burrow anywhere, but only 
sufficiently to bury herself. If free to wander, she selects either the inter- 
digital spaces, the wrists and their ulnar margin, the elbows, the anterior 
folds of the axillae, the pubic area, the buttocks, the back of the knee, the 
ankle or the toes. 
Holding on by the suckers of the anterior legs she raises the hind end of 
the body on the bristles of the posterior legs till her position is almost per¬ 
pendicular, and commences to cut into the skin. She can completely bury 
herself in 2J minutes (Munro, p. 15). When concealed, she may either remain 
inert for a time or continue burrowing, and this depends somewhat on the 
temperature, which, according to Delafond and Bourguignon, must be between 
10° and 30° C. Normally the burrowing only occurs at night, when the patient 
is warm in bed, but it may be induced by artificially warming the place where 
the parasite has buried itself. Munro (p. 15) found that the average length of 
