A. Bacot and Tj. Linzell 
389 
Most of the dressings in common use in veterinary practice for the treat¬ 
ment of lousiness in horses will, if carefully applied, suffice to kill all living 
parasites, although the latter are capable of surviving treatment with many 
supposed lethal substances, and in particular seem to enjoy a truly remarkable 
immunity to arsenic, mild doses of which exert only a stupefying effect. The 
nits or eggs, however, are extremely resistant, and no dressing at present in 
use can be relied on to devitalise these with certainty, even the most efficient 
agent tested allowing 2 per cent, to hatch after five minutes complete im¬ 
mersion. 
The experiments herein detailed were undertaken to determine the in¬ 
cubation period of the eggs. It was hoped by this means to fix some definite 
period, during which dressings are necessary to prevent these apparent 
reinfestations which, as subsequently confirmed, may not infrequently be due 
to the late hatching out of eggs. 
PRELIMINARY TRIAL. 
In order to ascertain if the nits of H. asini could be incubated successfully 
when removed from the horse, some nits were collected on 20. iv. 1918, at 
No. 1 Reserve Veterinary Hospital, Woolwich. The nits, 79 of which appeared 
sound, were placed in a tube and carried away in a vest pocket (at ca. 75°- 
80° F.). Subsequently they were left in a discarded coat for 24 hours at 
ca. 55°-60° F., before being placed in a small entomological box suspended 
from a man's neck in a thin cotton bag between the skin and shirt. The 
temperature in this situation when indoors ranged between ca. 90° F. by day, 
and ca. 95° F. at night; when out of doors the temperature fell to a variable 
extent. On the fourth day, through an oversight, the bag with the nits was 
left for 10-11 hours at ca. 63° F., but afterwards it remained next the skin 
except when removed for examination. 
The age of the eggs was of course not known. They hatched as follows: 
Record relating to 79 eggs incubated on man. 
On day . 2 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 
There hatched ...1355237 4 1 1 2 2 4 - 2 - 1 1 eggs. 
Total hatched ... 46 (58 %). 
EXPERIMENTS WITH DATED EGGS. 
(a) Collection of Eggs. Attempts were made in various ways to obtain 
eggs within 24 hours of their being laid. Cages composed of felt rings with 
gauze tops failed because the horses usually succeeded in moving the pads 
by twitching their skin, and, when this difficulty was overcome, it was found 
that the lice persistently forced their way out underneath the felt. Various 
types of cage were tried with both tops and bottoms of gauze, but these failed, 
owing to the temperature inside the box being too low for oviposition. 
