186 



cannot be explained in the same way, since dipping and, in the case of 

 Africander sheep, shearing are not carried out as general practices at 

 definite seasons of the year. The presence of grease on sheep is 

 believed by several South African authorities to be the chief deter- 

 mining factor. In carrying out the observations on the life-history 

 of these mites it was frequently noticed that development occurred 

 just as rapidly on the washed and shaven patches on fat sheep as on 

 those in poor condition, but Avhen the covers were removed and the 

 parasites allowed to spread to the surrounding skin, multiplication 

 was much more rapid in the case of the thin sheep than on those with 

 excess of yolk in the fleeces, while in a few of the latter no development 

 occurred at all. In order to observe the effect upon the mites of the 

 presence of wool fat, two patches were prepared on the same sheep ; 

 one was washed and shaved in the usual way, while on the other the 

 wool was only cut short and in addition the patch was smeared with 

 yolk obtained from the wool of another sheep. On each patch six 

 ovigerous females were placed. On the ungreased patch development 

 occurred as usual and by the twenty-first day (from the time of hatch- 

 ing) the fourth generation was developing. By that time no living 

 acari were observed on the greased patch and in the intervening period 

 what development there was proceeded very slowly. In all cases 

 where there was sufficient grease to soak down under the crusts develop- 

 ment was very slow and the acari gradually died out, usually in about 

 twenty days. 



Bedford (G. A. H.), Experiments and Observations carried out with 



Psoroptes communis at Onderstepoort. — Union S. Africa, Dept. 

 Agric, Pretoria, 3rd & 4th Repts. Director Veterinary Research, 

 November 1915, pp. 99-107, 5 figs., 2 plates. [Received 

 12th October 1916.] 



A brief description of the different stages and life-history of Psoroptes 

 communis var. avis is given. In testing the vitality of adults under 

 abnormal conditions only one individual survived as long as twelve 

 days after removal from the sheep. The longest time nymphs were 

 kept alive was eight days and larvae three. In an incubator at 98° F. 

 the eggs hatched in two days when the atmosphere was moist, Init 

 failed to hatch in a dry atmosphere. When kept in an ice-chest for ten 

 days and afterwards transferred to an incubator at 98° F., they usually 

 hatched on the third day ; if kept in the ice-chest for longer periods, 

 they all failed to hatch. Eggs kept at room temperature all hatched 

 in five days in summer, but failed to hatch during the winter months. 

 In dry sheep faeces at room temperature they all failed to hatch 

 whether in summer or winter. Attempts to transmit the mites to 

 goats, horses or calves all failed. Experiments made to determine 

 the length of time that enclosures can remain infected after the removal 

 of scabby sheep showed that the infection does not last more than 

 nine days. It should therefore be safe to recommend farmers to avoid 

 placing clean sheep in previously infected enclosures until about 

 sixteen days have elapsed. 



P. communis var. cajmie is not such a serious pest as P. communis 

 var. ovis and is much easier to exterminate on account of its only 

 being able to live in the ears of the host. In September 1912 and 



