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THE CATTLE GRUBS OR OX WARBLES S81 
614 to 19 hours failed to transform. In 1924 six larvee which had 
emerged normally from a host were submerged in lots of two for 
94, 30, and 40 hours. Adults emerged from all of these in normal 
time. Two pup were also submerged for 30 hours and both trans- 
formed to adults. 
Six mature larve of H. bovis 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. bovis throughout the United States. He says, 
One explanation of this possible barrier will be found in the fact that H. 
bovis is generally later in emerging from the backs of cattle than H. lineatum. 
The grubs 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 part at least for the failure of the species to es- 
tablish itself in the warmer portions of the country. 
No facts appear to have come to light since the publication of the 
foregoing statement which would tend to disprove this conclusion. 
H. tineatum 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 when the insects 
are separated from their hosts; that is, while they are in the pupal 
and adult stages. 3 
Among the several meteorological factors which affect Hypo- 
derma, rainfall and humidity during the pupal and adult stages are 
probably the most important. It has been found that where the soil 
beneath pupze is kept very moist the mortality is extremely high. 
Under such conditions some of the insects appear to die before 
development has proceeded very far, and others transform to adults 
but are unable to escape from the puparia. This appears to be due 
to the large size of the abdomens, which are not reduced sufficiently 
through evaporation to allow them to be withdrawn from the pu- 
paria. Ileavy 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 weather conditions 
of comparatively short duration will prevent individuals from ovi- 
positing. Owing to the fact that large numbers of flies usually 
58252 °—26—_6 
