90 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTURE 
In 1923 two series of tests were made. In the first of these 192 
eggs deposited April 10 on cow hair were divided into four lots, 
each of which was kept in an incubator at about 90° F. The check 
lot contained 99 eggs. ‘These began hatching on April 13, and 
45 per cent of them hatched. The other three lots were submerged 
for one minute after they had been incubated for 1 hour, 25 hours, | 
and 49 hours, respectively. The respective percentages of larve | 
which hatched in these three lots were 42, 45, and 57.5. All of 
these lots began hatching on the same date as the check. 
The second series contained 74 eggs deposited on April 11, 1928; 
18 of these were submerged for three minutes in a 2 per cent solution 
of a coal-tar creosote dip on April 18. These began hatching in | 
the incubator on April 14, and 44 per cent of the larve emerged. 
The other eggs were kept in an incubator except for the time re- 
quired to submerge them for one minute in crank-case oil, drained | 
from an automobile. Of the 22 eggs dipped immediately after ovi- — 
position none hatched, of the 23 eggs dipped 24 hours after being 
laid 26 per cent hatched, and of the 11 submerged 65 hours after 
oviposition none hatched. 
ATTACK DIRECTED AGAINST HYPODERMA LARVZ IN THE BACKS OF CATTLE 
A number of facts point to the practicability of combating this 
pest by destroying the larval stages while in the subcutaneous tis- 
sues of the back. Control by this procedure is favored (1) by the 
fact that the species confine their attack almost entirely to cattle, 
(2) by the fact that there is a rather definite and somewhat re-— 
stricted seasonal occurrence of the larvee in that situation, (3) by 
the presence of an aperture through the hide immediately over the | 
grub during its entire existence in the subdermal tissue, and (4) — 
by the fact that the larve are limited in their distribution to the | 
back of the host between the withers and tail. In order to proceed 
intelligently against the species in this situation it is necessary to 
have very definite information regarding its seasonal occurrence in 
the subdermal tissues in each section where control work is under- 
taken, as also a knowledge of the developmental period in the sub- 
cutaneous tissues, particularly as to the minimum time required. 
The possibilities of attacking the species in this situation appear to 
have been recognized in a general way many years ago. Work along 
this line. however, has been of a very superficial and sporadic na- 
ture, with the exception, perhaps, of that carried on in certain dis- 
tricts in Denmark (6), Holland, and Germany (67 and 77.) 
EFFECT OF MATERIALS ON LARVA REMOVED FROM HOSTS 
In order to determine the toxicity of various insecticides and 
other materials to the larvee of Hypoderma, a series of tests was 
carried out in which larve in different stages of development were 
removed from the backs of cattle and submerged in these materials. 
This method of testing the insecticidal value of various chemicals 
is open to objection, owing to the fact that the conditions are ab- 
normal. Another objection to the use of larvee extracted from hosts 
is that practically none of them will produce adults even though 
