298 Genus Ixodes 



proceeds to undergo its metamorphosis. This process is much influenced 

 by temperature, being retarded by cold and accelerated by warmth. 

 At first the replete larva is quite active, but when it has begun its 

 metamorphosis it lies still. Beneath the larval skin a series of profound 

 changes takes place. The nymphal exoskeleton developes beneath that 

 of the larva. The first three pairs of legs are formed within the larval 

 legs, and a fourth pair appear posterior to these, as can readily be seen 

 in mounted specimens of the ticks at a period shortly before they emerge 

 from the larval skin. 



The nymph possesses two spiracles, similar to those of the adults in 

 general structure, from which branching tracheae spread to all parts of 

 the body. When matured, the nymph withdraws its legs from the 

 larval skin which encloses the first three pairs, it ruptures the old skin, 

 emerges, and abandons the exuviae. The nymph differs from the female 

 in two essential points of external structure : it does not possess an 

 external sexual orifice, although there may be an indication (Anlage) of 

 where it will be situated in the adult, and the nymph does not possess 

 porose areas upon the dorsal surface of the basis capituli. The nymph 

 takes some ten days to harden and darken after casting the larval skin, 

 and it is now ready to attack a fresh host upon which it behaves 

 similarly to the larva, dropping off gorged after three to five days. 



The adult develops within the nymphal skin and emerges from it 

 after a variable interval of time (weeks or months). The sexual organs 

 in both sexes begin to appear in the nymph but they do not attain their 

 full development in the female until she has sucked blood. The adults, 

 having hardened, proceed to attack a fresh host, thus completing the 

 cycle. 



It will be noted that ricinus, in common with most Ixodidae 

 requires three hosts upon which to feed in the larval, nymphal and 

 adult stages. 



Duration of parasitism upon the host. 



Larvae and Nymphs. The following experiments may be cited to 

 show the behaviour of this tick when raised in the laboratory. On 

 July 5, 1905, some gorged % s, taken from cattle in Co. Cavan, Ireland, 

 were received in Cambridge. The ticks were placed at room tempera- 

 ture in a receptacle containing sand which was occasionally dampened. 

 Large masses of eggs had been laid by the end of July from which 

 innumerable larvae issued in September, 



