6 BULLEBTIN 1369, U. 8. DEPARTMENT OF AGRICULTURE 
LIFE HISTORY IN BRIEF 
There are two distinct species of cattle grubs and these of course 
present differences in appearance and habits in each of their stages. 
In a general way, however, the life histories of the two are similar, 
and the following summary of that of Hypoderma lineatum will 
serve as an illustration. Beginning with the large grubs which are 
well known to practically everyone who handles livestock, the life 
may be traced as follows: | 
As the grubs reach maturity in the subdermal tissues of the back 
during the late fall, winter, spring, or early summer, the holes 
through the skin gradually become larger and finally the grubs 
crawl out and drop to the ground. They seek protection under any 
loose material at hand and the outside skin shrinks, becoming hard, 
and within this the flies develop, emerging from 30 to 60 days later. 
These flies mate soon after emerging and without partaking of food 
begin to deposit eggs on cattle. The eggs are laid for the most part 
on the legs, probably the majority of them being below the knee or 
hock joint. The eggs hatch in from 214 to 6 days, depending upon 
the temperature. The young larve crawl down the hair to the skin 
and immediately begin burrowing into it. At the point of entrance 
serum usually exudes and rather characteristic scabby and tender 
areas remain for a few days. After penetration little is known of 
the minute larves from the time they pass through the skin until 
they appear in the body eavity, especially in the submucous layers 
of the gullet of the host, having increased considerably in size and 
become more opaque, and evidently having passed through a molt. 
They spend several months in the host, mainly in the tissues between 
the mucous membranes and the muscular walls of the gullet, and ~ 
evidently, as shown by one of the writers (Laake, 57, 58), pass 
through a molt in that situation or en route to the back. Growth 
continues during the summer months, and in the fall or winter the 
grubs have attained a length of from 15 to 17 millimeters and are 
ready to start on their migration to the back. Here again the exact 
route followed is not absolutely known; but these larve, which are 
of sufficient size to be found easily, have been met with in various 
places in the chest and abdomen and on the diaphragm. Not infre- 
quently they are observed in the spinal canal and a little later are to 
be found in the connective tissues beneath the skin along the back. 
A hole is cut through the skin almost immediately after the larva 
reaches that situation and within 2 to 6 days the larva again molts” 
and a wall of tissue begins to form around it in the form of an 
encystment sac. The next molt takes place about 24 days later, and 
the insect is now in its final larval stage. The duration of this stage 
averages about 30 days. It is thus seen that the development requires 
approximately a year’s time, there being one generation during the 
year. The major part of this time, from 9 to 11 months, is spent — 
within the body of the host. 
DISTRIBUTION 
The distribution of the two species of Hypoderma affecting cattle 
has received comparatively little attention, certainly not as much 
as the subject deserves. It appears that the grubs reach their maxi- 
¥ WS 
