G. H. F. Nuttall 
181 
the black) when the tick ceases feeding, i.e. during metamorphosis from 
larva to nymph ; disregarding this, there is but one break in the chain 
of parasitism between the larval and adult stages. 
Type 5, represented only by species belonging to the genus 
Boophilus. Here the tick runs through its whole life-cycle, from larva 
to adult, upon one host. The feeding upon this host is twice interrupted 
(represented in the diagram by two fine white lines traversing the black) 
for a brief period during the metamorphosis from the larval to the 
nymphal and from the nymphal to the adult stages. There is no break 
in the chain of parasitism between the larval and adult stages. 
In types 1 and 2 we have ticks which feed in succession on an 
indefinite number of hosts. In types 3 and 4 the ticks require to find 
three or two successive hosts respectively, whereas in type 5 but one 
host is required. 
The longevity of a species differs according to the type of parasitism 
it possesses. In species rvhich attack numerous hosts in succession 
{Argasidae: types 1 and 2) the longevity is greatest and it is least in 
species (Boophilus: type 5) requiring but one host. Ticks belonging 
to types 3 and 4 presumably occupy an intermediate position in this 
respect. It is obvious that in a species I’equiring a succession of hosts 
the tick will fast repeatedly for variable intervals of time and that it 
must be greatly to its advantage to be able to withstand prolonged 
starvation. Longevity may be measured in two ways: (a) upon the 
unfed tick at different stages, noting how long it survives as a larva, 
nymph and adult respectively, reckoning from the time of its emergence ; 
(b) upon the time it takes for the tick to complete the cycle of develop¬ 
ment, starting at any stage and ending with the reappearance of that 
stage. We have, unfortunately, too few data at present at our disposal 
to permit of any generalization regarding the longevity of ticks 
measured by either of these methods. We know of Argasidae which 
may survive 2-6 years as unfed adults, this longevity being unequalled 
in the Ixodidae, unfed adults of which usually survive but 6 or 8 months. 
Argasidae may run through their life-cycle within a year or two under 
the most favourable conditions, but in nature this period must frequently 
be very greatly prolonged. On the other hand, in Ixodidae we find 
considerable differences in longevity, as measured by the life-cycle, even 
under what appear to be the most favourable conditions: thus the 
shortest period reckoned for the cycle of I. ricinus is 178 days, of 
R. appendiculatus it is 77 days, both species belonging to type 3 in the 
diagram. Boophilus may run through its cycle in from 21 to 23 days. 
