INSECT ENEMIES OF LIVE STOCK 161 



of eggs; in other species engorgement is repeated and 

 followed each time by further oviposition. In all the 

 genera of ticks, the females differ strikingly in appearance 

 in their gorged and unengorged states ; so marked are the 

 differences, in some cases, that individuals of the same 

 species have often been taken for females of different 

 species : in the males the differences are not nearly so 

 striking, because they engorge on serum rather than blood. 

 In the nymphal stage there is no genital pore, and this 

 serves to distinguish the nymphs from the adults; the 

 sexes, with few exceptions, can only be distinguished after 

 the final moult. 



The ticks fall naturally into four separate classes, accord- 

 ing to their moulting habits. (1) Both moults, that is larval 

 and nymphal, take place on the host: in this class are 

 certain ticks of the genus Maryaropus, and the tropical 

 horse tick, Dermacentor nitens. (2) Larval moult takes 

 place on the host, but nymphal state off: examples may be 

 found in the spinous ear tick, Ornithodorus meynini, in 

 Rhipicephalus bursa, and Rhipicepkalus evertsi. (3) Both 

 moults take place off the host, as in the castor-bean tick, 

 Ixodes ricinus, and the fever tick, Ornithodortis inoubata. 

 (4) Larval moult off the host, but nymphal moult during 

 attachment, as in the " bont " tick, Amblyomma hebrcvum. 

 At first glance this purely artificial classification of the 

 ticks may not appear of much importance, but, from the 

 point of view of disease transmission, it merits some atten- 

 tion. Ticks of the first class pass the whole parasitic 

 period of a generation on a single host, and, as a conse- 

 quence, they exhibit a rapid rate of reproduction and 

 assume importance as external parasites, as far as the 

 removal of blood is concerned, for they are continually 

 imbibing the blood of the same animal. Individuals of 

 classes two and four may attach to two hosts during a 

 generation, and those of class three to three hosts. When 

 it is essential for a tick to find three separate hosts in its 



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