INSECT ENEMIES OF LIVE STOCK 163 



build that they are not easily removed from a host, when 

 once attached. A more striking protective adaptation is 

 seen in the fowl tick, Argas persicus. The larvae of this 

 tick feed upon domestic fowls, becoming attached in posi- 

 tions where they are with difficulty reached by their host. 

 During the greater part of the time they are attached, 

 their shape is globular (fig. 45), but, just before dropping 

 from the host, they become flattened, assuming the typical 

 adult shape. There is good reason for this abrupt change : 

 in their new form they are able to crawl into cracks and 

 crevices out of reach of the fowls, which eat them with 

 avidity whenever they catch them. 



Adaptations of habit, favourable to attachment and pro- 

 tection, form still more striking illustrations of the process 

 of natural selection. The habit of dropping from the host 

 to the ground, to moult, necessitates long periods of waiting 

 for a new host, and occasions considerable mortality. This 

 disadvantage is overcome, in certain species, by the acquired 

 habit of moulting on the host, or, as in the case of the 

 spinous ear tick, by passing the first moult on the host 

 and then becoming so engorged as a nymph that, after the 

 second moult, which takes place off the host, no further 

 engorgement is required before oviposition. 



The adaptation of tick habits to the habits of hosts is 

 a most interesting study. Rabbits, as is well known, pass 

 most of the day in their burrows and only become active 

 towards evening the rabbit tick, Hcvmaphysalis leporis 

 palustris, has adapted itself to this habit to such an extent 

 that it only drops from its host during the day-time, in 

 order to undergo a moult. By so doing, it is practically 

 assured of being able to find a host, when one again becomes 

 a necessity. Conversely, the fowl tick, Argas persicus, 

 drops from its host for moulting purposes at night, during 

 roosting time; departure from the host during the day, 

 when fowls are active, would give the ticks but a poor 

 chance of being able to re-attach to their host. The adults 



