184 



that while the body-louse is frequently referred to as Pedicidus vesti- 

 menti, Nitzsch, and occasionally as P. corporis, de G., the name which 

 has priority, is P. humanus, L. A bibliography of 29 references is 

 given. 



Shilston (A. W.). Sheep Scab. Observations on the life-history of 

 Psoroptes coiiDiuniis var. oris, and some points connected with the 

 epizootiology of the disease in South Africa. — Union S. Africa, 

 Dept. Agric, Pretoria, 3id & 4tli Repts. Director Veterinary 

 Research, November 1915, pp. 69-98. [Received 12th October 

 1916.] 



In the experiments described here the complete life-history of this 

 parasite in South Afi'ica has been worked out. The weather had very 

 little effect on the duration of the life-cycle, which never occupied 

 more than ten days and was usually complete in nine days. The 

 majority of the eggs hatch after an interval of two days. The larval 

 Btage is short, nymphs usually appearing forty-eight hours after hatch- 

 ing and always within three days. The nymphal stage lasts from three 

 to four days. At this time the sex could frequently be determined, 

 the smallest individuals usually giving rise to males. Females were 

 observed five and a half days after hatching from the egg, while males 

 were never seen earlier than the sixth day. The proportion of males 

 to females has been stated by Gerlach to be as one is to two ; whilst 

 this was noticed in some cases, the females were more often three, and 

 occasionally four times as numerous as the males. After feeding for a 

 short time, pairing takes place, and after moulting, the female begins 

 to oviposit nine days after hatching from the egg and may continue 

 to do so up to the 38th day. From numerous observations it was 

 found that a single female is capable of laying up to one hundred eggs, 

 the number being however dependent on weather conditions. The 

 life of the ovigerous female on the sheep is from thirty to forty days, 

 though the longest period over which an unfertilised female was kept 

 alive on a sheep was seven weeks, the majority of such individuals 

 however dying in three or four weeks. 



The question of the interval to be allowed between the two dippings 

 necessary to effect a cure in sheep scab has given rise to much discussion. 

 The present observations having shown the life-cycle to be complete 

 in nine days, dipping should be repeated in ten days at the most. An 

 extra day may be allowed because the fleece usually remains wet for a 

 day after the first dipping. This interval would ensure the hatching 

 of all eggs capable of doing so before the second dipping. Some dips, 

 although they fail to affect the vitality of the eggs, will effect a cure in 

 certain cases after one dipping. The explanation appears to be that 

 the eggs hatch, but owing to the persistence of certain ingredients of 

 the dip on the skin, the larvae are poisoned shortly after hatching ; 

 this has been actually observed in several instances. This delayed 

 lethal effect usually occurs in the case of dips containing an insoluble 

 constituent in suspension, usually finely powdered sulphur. Experi- 

 ment has shown that a fine deposit of this material alone over eggs 

 placed on the skin is sufficient to kill larvae shortly after hatching. 

 The value of such an ingredient is somewhat discounted by the difficulty 

 of maintaining it in suspension in the fluid, and the fact that i;b does not 



