THE BIOLOGIES AND HABITS OF TICKS 



437 



from the host to molt and is a very general feeder in the immature stages, 

 practically every rodent of the region being attacked. The species is 

 widely distributed in the Rocky Mountain and intermountain region, 

 but the disease of man which it carries is somewhat more limited in range. 

 In the Bitter Root Valley in Western Montana occurs the most virulent 

 form of the disease. Investigations conducted by the Bureau there, 

 indicate that the adult ticks develop almost exclusively on the larger 

 domestic animals and this point has been utilized in control. In other 

 regions, however, the jack rabbit plays a considerable part in the engorg- 

 ing of adults. This species is commonly known as the "wood tick" and 

 in the region where spotted fever is not known it is considered of little 

 importance, although occasionally it becomes so abundant as to injure 

 live stock through irritation and blood loss. It also occasionally produces 

 a form of paralysis in man and animals. 



Fig. 86. — The Rocky Mountain Spotted Fever Tick, Dermacentor andersoni (Bishopp.) 



The larvae are comparatively short lived but the nymphs and adults 

 live for many months. In fact it is possible for individual ticks which 

 have access to hosts in the nymphal stage to live so long as to carry 

 the species over three years. The larvae develop on the animals in from 

 three to eight days and these molt their skins in from one to 

 three weeks. The nymphal engorgement is practically the same as in 

 the larval stage, but the molting requires from eleven days to two 

 months or even longer. The females become filled with blood in from one 

 to three weeks. From 4,000 to 7,000 eggs are deposited. The winter is 

 usually passed in the nymph and adult stages, and these stages, espe- 

 cially the adult, are markedly active in the spring months. Seldom are 

 any of the adults to be seen on hosts after the middle of July, and practi- 

 cally all cases of spotted fever occur in March, April and ]May. The 

 disease may pass from one generation to the next through the egg and 

 all stages are capable of transmitting the malady. However, the imma- 

 ture stages are seldom found on man and only occasionally on the large 

 domestic animals. 



