THE ROCKY MOUNTAIN SPOTTED-FEVER TICK. 173 
drop fully engorged on the third day after application. The longest 
period required for engorgement, of these reattached nymphs, was 6 
days. 
TaBLE LXXIX.—Engorgement of nymphs of Dermacentor venustus. 
Date nymphs applied. Host. 
do 
| 
5 A ee ee Rabbit........ | 1 | “Spelt 5 cig te Sees ae 9 
8 “ge Guinea pig... 6....... WY Pi a bee ope on 2 
EL ee ae tae SS ae _ fee er 5 eee eee eee Pe 7 
Sh ee eee | Bovine... ..... yi Bee ome, Fe peice lege tee Y, Saree 9 ee 3 
| | 
1910. | | | | | | | 
a SS ee eee Guinea pig _ ees aaa 2 | 5 ep ae ee 5 
oe ee re Squirrel....... J} eee gee 2 yr eee by. 22 7 
hy jo eee eee ren Guinea pig <i EB! | oes Tie a Eee ae ae, 1 See | 1 3 
ee Sa eee nee | Bovine........ i a eens Pre i Seen pe) sew |------ 4 
Pa CRS = meee ei: Se ii. Lees ie dates) 7 ty Coleuel weer eer 8 
1S eee, See oe Oe =P cs: TW ee ne 5 | ree) aes ERS ISS eS. Lea. @! S 
Ss eee bates ee Sieh 9 sions | 8 3 ae cae Base il 
Te i eo Guinea pig 30 | ie yee gett Peete S837 22 
NO | eee ee eee ele Rabbit... ... - aes U4 es 5 cH) Ae a Ve ie ee 1 7 
JN TOP ah i ee Guinea pig... -| Mt Sergey (ere see 4 ed Bes Oe | ee | 8 
Total st ls. +198 |... wag. aR oe | Meta) bastel. Lose: (ee 154 
< These individuals were collected on hosts when unengorged or slightly engorged and reattached on this 
te. 
In our observations engorged nymphs molted in two instances as 
soon as the eleventh day after dropping. The first instance occurred 
in a lot of ticks which dropped August 6, 1908. The mean tempera- 
ture during this period was 87° F., 485° F. of effective temperature 
having been required for this transformation. Most of the ticks 
from this same lot, which dropped on the same day, molted on the 
fourteenth, fifteenth, and sixteenth days. The second case, when a 
molting period of 11 days was observed, occurred with a nymph 
which dropped July 21, 1910. The mean temperature during this 
period was 89.5° F. and the total effective temperature was 511° F. 
There is a great variation in the periods required by this species 
for molting. Thus of 3 engorged nymphs which dropped May 20, 
1908, 2 molted to females on the seventeenth and eighteenth days, 
while the third did not molt to adult until the forty-ninth day after 
dropping. A still more marked variation in the molting periods of 
a group of ticks was observed in a lot of 3 nymphs which dropped 
October 3, 1909. Two of these molted on the eighteenth and nine- 
teenth days after dropping, while the third did not transform until 
March 22, 1910, or 170 days after dropping. Both of these lots were 
kept under identically the same conditions and all appeared to be fully 
engorged. In the second instance the molting of the last nymph 
