82 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTURE 
emerge at approximately the same time, regardless of the time the 
pupe may have been formed, rainy weather immediately after 
emergence of one of these large groups of flies may affect materially 
the infestation of the following year. 
Clouds have been mentioned as being a factor in repressing ovipo- 
sition. Their influence is felt only by the adult insect, as cloudy 
weather in the absence of unusual precipitation does not seem to 
affect the development of the pups, except possibly to prolong 
slightly the pupal stage. i 
Winds apparently have a double influence: In the first place 
they cause a more rapid drying of the surface soil, which would 
benefit the pupz; and secondly they have an adverse effect upon 
oviposition, since the flies, especially those of H. lineatum, will not 
oviposit freely in a strong wind. ‘This influence is less potent than 
the others, however, since it is not so generally operative. Cattle § 
congregated in places protected from the wind are frequently at- 
tacked by heel flies. 
Temperature conditions probably rank next to precipitation in 
their effect on these species. Periods of unusually warm weather 
in the winter or early spring often tend to cause the emergence of 
flies which are destroyed by succeeding days of cold or rainy weather. 
As is generally known, warm bright days are most favorable for 
the activity of H. lineatum. It is probable, however, that during 
periods of high temperature, that 1s, when the daily maxima run 
above 95° F., the total number of eggs laid by a fly is reduced. 
When such higher temperatures are reached, oviposition occurs 
mostly before the hottest part of the day. The destructive effect of 
the direct rays of the sun on both H. lineatum and H. bovis is appar- 
ent when caged flies are placed in the sun during moderately warm 
weather. They become very active and fly most of the time, soon 
weakening and falling to the bottom of the cage where they die, 
sometimes very suddenly. 
Oviposition of “7. lineatum takes place at surprisingly low tem- 
peratures if the sun in shining. Often adults are active during the 
day when the minimum temperature in the morning ranges between 
19 and 22° F. Flies have been observed to oviposit when the tem- 
perature was as low as 45° F., the maximum temperature for the 
day being 46° F. There seems to be no doubt that heat is an im- 
portant factor in restricting the southern spread of ZH. bovis. 
Mature larve and pupe can withstand rather low temperatures. 
The pups appear to be more resistant to cold than the larvae. A 
considerable number of observations have been made by the writers 
on the effect of cold on these stages of H/. lineatum at Dallas, Tex., 
but the minimum fatal temperature has not been determined with 
accuracy. In 1918, when a minimum temperature of 3.5° F. was -_ 
reached, the larval and pupal transformations of a number of speci- 
mens was under observation. Considerable protection was aftorded 
these and a good percentage of adults emerged. Among a number of 
larvee and pup exposed to 9.5° F. an adult emergence of 54.5 per 
cent was secured. In other series of specimens which experienced 
minimum temperatures of about 19° FE. approximately 68 per cent 
produced adults. This is not much below the percentage of emer- 
gence under outdoor conditions where the temperatures do not drop 
