340 
Ornithodorus moubata 
The dimensions of individuals of maximum size in any one stage exceed 
those of individuals of minimum size in the succeeding stages. As would be 
expected, the adults emerging from one nymphal stage are smaller in size 
(mean measurements) than the adults emerging from the succeeding nymphal 
stages, presumably owing to the fact that the latter have secured additional 
food and have had longer growth periods. In addition, considering the fourth 
to the seventh nymphal stages, the males of any one stage are only slightly 
larger, whereas the females are considerably larger than the nymphs of the 
equivalent stage. Partially gorged individuals may require a greater number 
of meals than normal individuals before completing their metamorphoses, 
that is to say, the individuals of minimum size have the minimum vitality, 
are comparatively poor feeders and may require to undergo more moults 
before reaching maturity. How far, if at all, the extent of engorgement 
determines the number of nymphal stages still requires elucidation. 
Considering the larval and nymphal stages in order, the percentage in¬ 
creases in length are 9*1, 33-3, 68-8, 37-1, 24-3, 15-2 and 22-6 respectively. 
The 9 per cent, increase from larval to first nymphal stage is brought about 
by the change in the body contour and not by growth. The highly convex 
body of the larval stage is flattened dorso-ventrally in the first nymphal stage. 
The greatest growth in these experiments occurred in the second nymphal 
stage, the 68*8 per cent, increase in length being accompanied by a 53-9 per 
cent, increase in breadth, neither of these figures being reached in any of the 
other nymphal stages. The temperature at which the individuals were main¬ 
tained between times of feeding had no influence on their growth, the mean 
figures obtained from material bred at 22° C. and 30° C. varying only by small 
fractions of a millimetre at each nymphal and equivalent adult stage. 
Changes in external anatomy undergone during development. 
(a) The Hypostome. Fig. 2. 
The changes in the dentition of the hypostome during the development 
from larva to adult are indicated in Fig. 2, (a) to (h) representing the larval 
and nymphal organs arranged in order of development, (i) and (j), the livpo- 
stomes of females emerging from fourth and sixth nymphs and (k) and (/), 
of males from third and fifth nymphs respectively. 
The teeth are arranged in transverse rows and longitudinal files 1 . The 
number of teeth borne by the hypostome increases each time the tick under¬ 
goes ecdysis, both by the addition of teeth in the rows and by the formation 
of additional files. In the larva there are only two rounded teeth on each 
half of the distal extremity of the hypostome (1/1) their form indicating that 
they could scarcely serve for attaching the tick to a host. It is well known that 
the larva moults to the first nymphal stage without previous engorgement. 
The first-stage numph is, however, an active feeder and the dentition of its 
1 The signs 2/2, 3/3 used hereafter indicate the number of files on each side of the median line. 
