﻿THE CATTLE GEUBS OR OX WARBLES 81 



61/^ to 19 hours failed to transform. In 1924 six larvae which had 

 emerged normally from a host were submerged in lots of two for 

 24, 30, and 40 hours. Adults emerged from all of these in normal 

 time. Two pupa? were also submerged for 30 houre and both trans- 

 formed to adults. 



Six mature larvae of H. hovis were submerged in water for periods 

 ranging from 49 to 122 hours. All except one, which was submerged 

 for 121 hours, produced adults in about the normal length of time. 



From these tests it is clear that these stages are not easily drowned, 

 and this might enable them to survive carriage for considerable 

 distances by flood waters. 



CLIMATIC CHECKS 



Climatic conditions undoubtedly have a marked effect on the 

 distribution and abundance of Hypoderma. The relationship be- 

 tween climate and the distribution of the two species of Hypoderma 

 has been discussed in a general way under the heading "Distribution." 



One of the writers (4) has made the statement that he believes 

 that climatic barriers have prevented the general dissemination of 

 H. hovis throughout the United States. He says, 



One explanation of this possible barrier will be found in the fact that H. 

 tovis is generally later in emerging from the backs of cattle than H. lineatum. 

 Tl'.e gi'ubs emerging from the backs of cattle shipped to the Southern States 

 would, on account of their later emergence, encounter excessively hot weather, 

 and this may account in ijart at least for the failure of the species to es- 

 tablish itself in the warmer portions of the country. 



Xo facts aj^pear to have come to light since the publication of the 

 foregoing statement which would tend to disprove this conclusion. 

 //. Uncahim establishes itself in new sections with less difficulty, and 

 uninfested areas are less general with this species. It must be recog- 

 nized, however, that meteorological conditions are potent factors in 

 limiting its abundance as a parasite of cattle. 



Climatic conditions appear to have very little direct effect upon 

 the development of the grubs within the host. Climatic influences 

 are therefore restricted to the portion of the year Avhen the insects 

 are separated from their hosts; that is, while they are in the pupal 

 and adult stages. 



Among the several meteorological factors which affect Hypo- 

 derma, rainfall and humidity during the pupal and adult stages are 

 proljably the most important. It has been found that where the soil 

 beneath pupse is kept very moist the mortality is extremely high. 

 I'nder ;-uch conditions some of the insects appear to die before 

 develojmient has proceeded very far, and others transform to adults 

 but are unable to escape from the puparia. This a])pears to be due 

 to the large size of the abdomens, which are not reduced sufficiently 

 through evaporation to allow them to be withdrawn from tiie pu-- 

 2)aria. Heavy rains coming at the time the adults emerge may de- 

 stroy many of them, and continuous rainy or even very cloudy 

 weather after the emergence of the adults will prohibit oviposition. 

 Since the longevity of the flies is limited, adverse wcatlici- conditions 

 of c()nii)arativ<'ly short duration will pj-event individuals from ovi- 

 positing. Owing to the fact that large numbers of flies usually 



5S2:"32°— 20 



