﻿24 BULLETIN 136&, "tJ. S. DEPARTMENT OP AGEICtfLTtTBE 



larvse were found four months later, but they were not seen subse- 

 quently. 



A few tests have been made to determine if H. lineatum. larvse will 

 develop in guinea pigs. Flies were induced to deposit nearly 100 

 eggs on 2 guinea pigs on April 10 and 11, 1923. Eggs from these 

 females were shown to^ be fertile by tests in an incubator, but both 

 guinea pigs apparently lost all the eggs before hatching began. 



Several larvse of H. lineatum- from cattle gullets 'were introduced 

 through an incision of the skin on the backs of two guinea pigs. 

 Both animals were somewhat stiff and inactive the following day 

 and the female soon became very sick. An edematous swelling 

 developed on the bell3\ This finally broke and the animal slowly 

 recovered, but there was no evidence of the presence of the larvse. 

 The male guinea pig recovered rapidly and a few weeks later 

 developed a small lump with a hole through the skin over it, just 

 behind the shoulders. This was a typical grub lesion, but the larvae 

 could not be seen. This disappeared in a few days and no further 

 evidence of the infestation was seen. 



As is shown in the discussion of " injury to man," infestations of 

 humans, especially children, by Hypoderma are not uncommon, 

 although it appears that man is not a favorable host. 



ACTIONS OF CATTLE WHEN ATTACKED BY ADULT HYPODERMA 



The reactions of cattle to the attack of these insects is so remark- 

 able and so much discussed that a brief statement on this subject 

 seems warranted. 



As with the reactions of a host to various other stimuli, we find 

 a marked variation in the effect of Hypoderma attack on different 

 individuals. This is, however, mainly a matter of degree of violence 

 of reaction rather than of kind. As has been mentioned, the fright 

 produced b}^ H. hovis is much more pronounced than that produced 

 by H. lineatum^ but the latter often causes a wild stampede. 



In the case of both of these species the female has a very flexible 

 telescopic ovipositor, in no way fitted for pricking the skin. In fact 

 it can scarcely be felt on one's hand when it is extended by the fly. 

 Convincing evidence of this is also afforded in the fact that flies 

 have frequently been seen to approach an animal unnoticed and 

 deposit many eggs without any annoyance whatever. 



Careful observations among cattle during heel-fly activity will 

 soon convince anyone that it is the attack of the fly which produces 

 the fright. The writers agree with Hadwen (-56') that this fear is 

 produced mainly by the persistent attack of the fly. This is par- 

 ticularly true in the case of H. hovis. A-N^en the fly first attacks 

 cattle at the beginning of the season the reaction against it is usuallj^ 

 not very violent. The animal kicks or shakes the foot, but the 

 immediate return of the enemy alarms the cow and she starts walk- 

 ing away; being pursued, the fright becomes more pronounced and 

 she may run a short distance and begin walking again. The fly 

 immediately resumes its attack and then the animal dashes aWay 

 in terror (fig. 5, 6), with the insect often at its heels, in the fashion 

 of a dog. ' There is reason to believe that the fear of this insect 

 is to some extent instinctive ; also that, as the areas of attack become 



