170 
Biology of Dermacentor 
when engorged (PL XI, fig. 9). The size ranges from 10 mm. in 
length (including capitulum) by Off mm. wide, by 0'5 mm. thick to 
2 - 0 x TO x Off mm. The larvae, as well as all other stages of this 
species, are very easily detached from the host. This is particularly 
true of specimens in the quiescent period prior to moulting. In our 
experiments many engorged larvae were found detached, and moulting 
records were kept of a number of these which were placed on moist 
sand. Since many larvae must be rubbed or shaken off in nature, 
moult off the host and again become attached, we have given in Table 
IV some records on this point to show the range in the moulting period 
under such conditions. The temperature has much influence on the 
time required for this moult as also the amount of moisture present. 
It is thought, however, that the state of development prior to becoming 
detached from the host is of still greater importance in considering this 
period. 
The shortest period observed by us from the application of the 
larvae to the first moult was nine days. Some larvae moult as late as 
20 days after being applied to the host (see Table VI). 
There is a high mortality among the larvae of this species after they 
find hosts. Dead larvae have been observed in all stages of engorge¬ 
ment either attached to the skin or hanging in the hair of the host. 
The Nymph (PI. XI, figs. 8, 10): The longevity of nymphs, which 
moulted from individuals detached from hosts as engorged larvae, has 
been found to vary from two to 77 days. In these tests the nymphs were 
kept on moist sand in the laboratory at Dallas, Texas. 
When moulting takes place on the host in the normal manner, the 
nymph crawls backward or sideways out of the larval skin and attaches 
itself to the host near the point of larval attachment. The larval skin 
usually remains attached to the host for some time. Immediately after 
moulting, the legs, mouth-parts and anterior part of the scutum are pale 
yellow and translucent. The posterior part of the scutum is darker 
and the remainder of the body is pale yellowish-brown. The body is 
elongate. Nymphs have been observed to become detached, crawl 
about, and reattach themselves several times before they become 
noticeably engorged. While engorging, the nymphs retain the elongate 
form and become light bluish-gray in colour, although some are 
yellowish and translucent, due probably to the character of the food. 
The size of engorged nymphs has been found to vary from 5'4 mm. 
in length (including the capitulum), by 3'9 mm. in width, by P5 mm. in 
thickness, to 2'8 by T7 by 0'9 mm. Some individuals have been found 
