﻿THE CATTLE GRUBS OR OX WARBLES 83 



below freezing. A mature larva of U. lineatwm placed in a freezing 

 room for 24 hours at a temperature of about 7° F. failed to pupate, 

 whereas a fresh pupa exposed in the same way produced a normal 

 adult. Nine mature larva? of H. hovis subjected for from 24 to 

 26 hours to temperatures in a freezing room of 7 to 9° F. failed 

 to pupate. Five larvse of this species placed in the room at 25 

 to 29° F. for 26 hours gave an adult emergence of 80 per cent, and 

 five larvae kept at from 32 to 33° F. for 14 days pupated promptly 

 upon removal and 60 per cent emerged. 



SOIL CONDITIONS AND DRAINAGE 



Naturally soil character and drainage are linked with rainfall 

 in their relationship to Hypoderma. The writers' observations in- 

 dicate that porous, well-drained soils are more favorable for Hy- 

 poderma than heavy, flat-lying lands. 



In the valley of the Red River of the North, where Hypoderma 

 seldom occurs, there is probably a soil condition which prevents 

 fully developed larvee from producing adults. It is known that 

 animals infested with Hypoderma have been shipped into that sec- 

 tion, but the pest has never become well established. 



HOST RESISTANCE AND OTHER CONTROL FACTORS 



The great disparity in the number of grubs found in different 

 cattle in the same herd is well known. The writers' investigations 

 indicate that this difference in degree of infestation is brought 

 about by a combination of several causes. Probably the main rea- 

 son why one animal will show a heavy infestation of grubs while 

 another will be free from grubs or comparatively so, is that there was 

 a marked difference in the numl^er of eggs which were attached to 

 them. The reason why one animal may receive a much larger 

 number of eggs than another, however, is not altogether apparent. 

 Certainly the element of chance enters here strongly. The indi- 

 vidual idiosyncracies of the animals are also factors. Some ani- 

 mals seem more able to detect the presence of flies than others and 

 secure protection from them. 



A large series of observations made on individual animals year 

 after year shows that the extent of infestation may vary greatly. 

 The writers have found, however, that certain animals are uni- 

 formly resistant to the grul)s from the time the eggs hatch to the 

 time the larva? complete their development or die within the host. 

 This individual resistance is probably due in a large measure, as 

 pointed out by Hadwen (-5-5, 40), to eosinophilia. That there is 

 a very definite reaction against the larvae is apparent when they 

 penetrate the skin of such resistant hosts. The exudate at the point 

 of entrance is usually more profuse than ordinarily, and the infil- 

 tration of the connective tissue Ix'neath is pronounced. Apparently 

 this strong reaction against the invading parasite is present 

 throughout its course in tlie body of the iinimal. It is after the 

 larvai have reached the subcutaneous tissues of the back, however, 

 that the destruction of the larvie is most apparent. The reaction 

 against the larva; appears to differ considerably in dill'ereut hosts. 

 In some, large svvelliug.s occur; in others, tlie reaction seeujs to bo 



