10 IXODOTDEA^ OR TICKS, OF THE UNITED STATES. 



to endure long fasts, and many finally secure an attachment to some 

 animal. In a few days the young tick is rapidly distended by blood, 

 and drops to the ground. Here it seeks a hiding place and rests, 

 during which time there are many changes in the internal anatomy. 

 In three or four days the skin splits and from the six-legged larva 

 there issues the octopod nymph. Climbing a plant it awaits the pass- 

 ing of some suitable animal, and, when attached, feeds and falls olf 

 again to moult, this time to the adult condition. It again waits for 

 a host, and, finding one, feeds and starts the life cycle anew. The 

 Texas fever tick and its allies do not drop off for moulting, but cast 

 the skin while on the host. This enables a more rapid increase in 

 the species. They drop to the ground, however, for the purpose of 

 depositing eggs. Several other ticks pass the first moult wdiile on 

 the host, and some Argasidae deposit eggs on posts above ground. 



In the true ticks there is a considerable difference between the 

 abdomen of the male and that of the female in the development of 

 the dorsal shield. In several genera the male has plates near the 

 anus, and in some cases the hind pair of legs is enlarged in the male ; 

 the porose areas are found only in the female; the sexes are, there- 

 fore, very easily distinguished. 



The males and females feed side by side; but the younger stages 

 are often restricted to a different part of the host; thus the larvjc of 

 several species occur in the ears, while the adults are on the body of 

 the host. 



Ticks are able to live for long periods w^ithout food or moisture. 

 Four to six months are common periods, and cases are recorded of 

 female ticks fasting for eleven or twelve months. The argasids can 

 live for a much longer time; specimens of Argas have remained alive 

 in pill boxes for two years and three months without food, and Riley 

 records one specimen living for five years in a corked vial without 

 food. Even the young may live several months without food. This 

 amazing vitality largely offsets the difficulty the tick may have in 

 finding a host. 



Ticks are also well fitted to Avithstand immersion in water; even the 

 eggs and young stages live through long periods of submergence, so 

 that rains have little effect upon tick life. 



GEOGRAPHICAL DISTRIBUTION. 



Africa is the home of ticks, all the known genera occurring there, 

 and more species than on any other continent. Of the genera found 

 in the United States, Rhipicephalus^ Margaropus^ and Amhlyomma 

 are tropical, and several of our species in these genera occur in Central 

 and South America. Dermacentor is our most characteristic genus, 

 and we have more species than any other country. Ixodes and 11 cv- 

 maj^hysalis are northern genera, both as well represented in Europe 



