﻿82 BULLETIN 1369, tJ. S. DEPAETMENT OF AGEICtlLTURE 



emerge at approximately the same time, regardless of the time the 

 pupse 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 jjupse, except possibly to prolong 

 slightly the pupal stage. 



Winds apparently have a double influence : In the first place 

 they cause a more rapid drying of the surface soil, which would 

 benefit the pupse; ancl 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 wincl 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 is, 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. hovis 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 ancl falling to the bottom of the cage where they die, 

 sometimes very suddenly. 



Oviposition of H, 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 H. hovis. 



Mature larvae and pupae can withstand rather low temperatures. 

 The pupse 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 11. 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 afforded 

 these and a good percentage of adults emerged. Among a number of 

 larvae and pupae 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° F. 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 



