636 MEDICAL ENTOMOLOGY 



has been found in all its stages on most of the common mammals in 

 Madras, but never on reptiles. In this class both the larvae and nymphs 

 always drop off to moult. 



CLASS 3. Ticks the larvae, nymphs and adults of which feed on dif- 

 ferent animals of the same or another species, but undergo the first moult 

 on the host, while the nymph always drops off when replete with blood. 

 The larval and nymphal stages are therefore passed on one individual, 

 the adult stage on another of the same species or a different one. As far 

 as is known at present, Oniithodorns megnini, Rhipicephalus bitrsa and 

 R. evertsi are the only ticks which belong to this class. Ornithodonts 

 megnini is of special interest, since the adult, like that of the Oestridae in 

 the Diptera, does not feed, reproduction being carried out on the food 

 material accumulated in the early stages. 



CLASS 4. Ticks which in all their stages feed on one particular 

 species of animal or its near allies ; the larvae and nymphs always moult 

 on the host. To this group belong the species Margaropns annulatiis 

 with its varieties, Dennacentor -nitens, D. nigrolineatits and D. albipictiis. 



The degree of association between ticks in their several stages and 

 the host presents an interesting and somewhat perplexing problem in 

 parasitology, all the more interesting because it is full of practical 

 importance. The simplest life history would appear to be one in which 

 a tick in any of its stages could adapt itself to any host, and thus ensure 

 the continuity of its species ; the most advanced is probably to be found 

 in the fourth class, in which the larvae and nymphs moult on the host. 

 In view of the fact that the Ixodini as a rule leave their hosts to moult in 

 their larval and nymphal stages, large numbers must necessarily fail to 

 find a host and die ; this great disadvantage is probably overcome by the 

 very large number of eggs laid by these ticks ; in the case of Hyal- 

 omina aegyptium, for instance, the number may run to many thousands. 

 The Argatini, as Nuttall has pointed out, are ticks of the habitat and the 

 difficulties of finding a host are very much minimized, and as a result 

 their progeny is proportionately smaller. The worker interested in this 

 subject should consult Nuttall's paper on the ' Adaption of Ticks to the 

 Habits of their Hosts '. 



The practical interest of the subject lies in the transmission of parasites 

 by means of ticks from one vertebrate to another. The chances of trans- 

 mission are evidently greatest in those cases in which each stage feeds on 

 a different individual, dropping off to moult, and least when the cycle 

 from larva to adult is passed on the same animal. Parasites ingested by 

 one stage may be transmitted to another host when the next stage becomes 



