April, '13] BISHOPP AND KING: SPOTTED FEVER TICK 209 



host animals examined in 1910-11 in the Bitter Root Valley throws 

 much light on the question of the possibility of a cycle being com- 

 pleted in one year, from overwintered unengorged adults. During 

 1910, from March 26 to June 30, 393 animals which act as hosts for 

 nymphs were examined and a total of 907 nymphs found, or an average 

 of 2.3 nj^mphs per animal. From July 1 to August 17 of that year 

 180 nymph-bearing animals were examined, a total of 300 nymphs 

 being found, or an average of 1.6 per animal. From August 18 to 

 October 14, 127 nymph-bearing animals were examined on which ten 

 nymphs were found, or an average of .07 per animal. During 1911, 

 from April 1 to June 30, 149 nymph-bearing animals were examined 

 upon which 651 nymphs were found, or an average of 4.37 per animal. 

 From July 1 to August 17, fifty-four nymphs were found upon the 

 129 nymphal hosts examined, or .42 per animal. From August 17 

 to October 1, one nymph was found on the 33 nymphal hosts 

 examined, or an average of .03 per animal. Supposing these figures 

 to represent the true percentage, it will be seen that 74 per cent of 

 the nymphs which developed during the season of 1910 were to be 

 found on animals prior to July 1, and during 1911, 95 per cent of total 

 development took place prior to that date. 



The following will show the bearing of these figures on the life cycle 

 question. In 1910 the first larvae were taken on hosts on June 13. 

 This was an exceptionally warm season and the appearance of larvae 

 was probably from one to two weeks earlier than normal. In 1911, a 

 more nearly normal season, the first larvae were found on July 9. 

 Allowing the shortest possible time for engorgement and molting of 

 the larvae and the hardening of the resulting nymphs, the date of 

 appearance of the first nymphs of that generation on hosts would be 

 some time after July 1. From this time on, as has been stated, there 

 are very few nymphs on hosts: just at a time when one would expect 

 a heavy infestation, provided it was a normal habit for nymphs 

 developed from overwintered adults to attach the same year. It has 

 also been determined that unengorged nymphs emerging during the 

 fall of one year may live till August 18 of the following year. Hence it 

 is reasonable to suppose that the majority of the few nymphs found on 

 hosts after July 1 are straggling individuals from those which hiber- 

 nated as unengorged nymphs the previous winter. 



August 17 is used as the last date in the second period because of the 

 fact that by this date the principal hosts, the ground squirrel, Citellus 

 columhianus, and woodchuck, Marmota flaviventer, have gone into 

 hibernation. Then, too, as has been stated, a lot of sixty-seven 

 nymphs which had been placed on hosts on August 17-18, 1910, did 

 not transform to adults till the following summer, hence requiring 



