﻿6 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTUEE 



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 Hypodervia Uneatum 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 Avithout 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 2i/2 to 6 days, depending upon 

 the temperature. The young larvae 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 larvae from the time thej^ pass through the skin until 

 they appear in the body cavity, 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 seA^eral 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, 67^ 68), 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 larvae, 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- 



