144 
Sarcoptes 
Key to the Stages of Sarcoptes scabiei var. equi. 
The following key will probably be found useful; the egg stage is not 
mentioned. 
I (Legs six. 150/* long.Larva 
(Legs eight.’ ... ... ... ... ... ... ... ... ... B 
f Fourth legs with ambulacra. Posterior epimeres united. Three plastrons. 
B Length 225/* ... ... ... . . Male 
[Fourth leg terminating in long seta ... ... ... ... ... C 
j'Tocostome and its setae present. Plastron shagreened. Length 390/* 
C Adult female 
l Tocostome absent. Plastron not shagreened ... ... ... ... D 
I Bare area indefinite posteriorly. One lateral seta. Length 175/* Xymph 
(Bare area closed behind. Two lateral setae. Length 240/* Immature female 
TECHNIQUE. 
I have not found it necessary to develop a complicated technique. The 
following notes may be of service. 
Collection. Scrapings should be made deeply to include the scurf and 
scabs next the skin. If the scabs are thick and only the superficial layer is 
collected it will be found to contain many larvae and very few of other stages. 
The scrapings may be folded up, as a doctor folds a medicinal powder, in a 
piece of paper. The mites migrate on to the paper in a very few hours; black 
paper therefore should be used so as to make them conspicuous. If the 
material cannot be examined fresh it may of course be preserved, or fixed, 
en masse. 
Examination. It is essential to choose a fluid with the lowest possible 
refractive index, in order to render the sculpturing of the integument, and 
the fine setae transparent. Water is on the whole the most useful medium in 
which to examine specimens. Permanent mounts should be made in gum 
arabic (below). These mounts are as permanent as those made in balsam. The 
mite is placed in the gum alive, and dies rapidly in an extended position. 
Spirit specimens may be mounted in gum arabic, provided they are carefully 
washed in water. Levulose and lactophenol are also useful media, with lower 
refractive indices than gum arabic. They are unsatisfactory because they 
never harden, but at any rate they make useful semi-permanent specimens. 
Balsam is useless because it “clears” the specimen to such an extent that the 
chaetotaxy can only be made out with the utmost difficulty if at all. 
The formulae of the mounting media are as follows. Gum arabic medium: 
distilled water 50 c.c., glycerin 20 c.c., gum arabic 40 gm., chloral hydrate 
50 gm., cocain hydrochlor, 0-5 gm.; dissolve gum in water, add chloral hydrate 
and cocain, when dissolved add glycerin; filter if necessary. Levulose: satur¬ 
ated aqueous solution, evaporated to syrup on water bath, a trace of thymol 
added as preservative. Lactophenol: phenol crystals (puriss.) 1 gm., lactic acid 
1 gm., glycerin 2 gm., water 1 c.c. 
