380 THE ANIMAL PARASITES OF MAN 



approach the adult ; but so far no sexual organs occur. Nymphs are of 

 two sizes the smaller being future males, the larger females. 



(3) The next stage in the female is the age of puberty ; the female 

 now being provided with a vulvo-anal slit ; this so-called pubescent female 

 is fertilised by the male. The male then dies. But the female again 

 casts her skin and enters another stage, the 



(4) Ovigerous female the egg-laying female which has differently modi- 

 fied legs. 



The rate at which these acari breed is very great. Gerlach has found 

 that, roughly, in each Sarcopt gallery a female produces fifteen individuals 

 ten females and five males and that the progeny reproduce again in fifteen 

 days. The table given below thus shows that one pair may produce the 

 enormous number of 1,500,000 descendants in three months : 

 First generation after 15 days : ..-. . .-'. 10 females .. 5 males. 



Second ,, ,, 30 ,, .;.,- . v ; . - - 100 ,, ",'-.' 50 ,, 



Third 45 ,, .^V : '; ;: 1,000 -., -,,, ./; . . . , 500 / 



Fourth ,, 60 ,, .', - '- '. .-'-. 10,000 ,, : ' .. 5,ooo 



Fifth ..75 .. ~-V, N ."./ 100,000 .','' ' ,". 50,000 



Sixth ,, ,, 90 ,, v^r/; ^Y^V 1,000,000 ;;^;. ; '. ... 500,000 :'^.- 



1,300,000 individuals. 



These Acarinae are divided into three distinct genera : 



(1) Sarcoptes, Latreille. 



(2) Psoroptes, Gerv. ; Dermatodectes, Gerlach ; Dermatocoptes, Fiirstenberg. 



(3) Symbiotes, Gerlach ; Chorioptes, Gerv. ; Dermatophagus, Fiirst. ; Sarco- 

 dermatodectes , Del. 



The following are the main characters of these three genera : 



Sarcoptes round or slightly oval ; the two posterior pairs of legs being 

 nearly or quite concealed beneath the body ; the tarsi end in simple long 

 pedicles, with ambulatory suckers. 



Psoroptes oval ; the legs are all visible outside the margin of the 

 body ; the ambulatory suckers are carried on long triangulated stalks ; 

 the male has copulatory suckers and abdominal prolongations. 



Symbiotes oval ; legs long, thick, all visible ; ambulatory suckers very 

 wide, carried at the end of simple, short pedicles. 



Sarcoptes make channels or furrows beneath the epidermis, and in these 

 the female lays her eggs. This form of acariasis is thus difficult to cure. 

 It is the cause of human itch (vide Sarcoptes scdbiei). 



Psoroptes do not make sub-epidermic galleries ; they live and breed in 

 colonies beneath crusts or scabs formed by the changes they produce in 

 their host's skin. Sheep scab is a common type of disease produced by 

 Psoroptes. This genus is of little importance as a parasite to man. 



Symbiotes live as Psoroptes ; they also do not affect man. F. V. T.J 



Sarcoptes scabiei (L.), 1748. 



Syn. : Acarus scabiei, "L., 1748; A. psoricus, Pallas, 1760; A. siro, L., 

 1761; Sarc. exulcerans, Nitsch, 1818 ; 5. hominis, Raspail, 1834; S. galei, 

 Owen, 1853; 5. communis, Delaf. et Bourg., 1862. 



The body is oval or nearly circular and whitish in colour. 



