Gr. H. F. Nuttall 
201 
and allowed to feed fully as O’s (Lot D) they almost attained a normal 
size. Apart from differences in size, the ticks which had been subjected 
to the picking process showed differences in structure. 
The normal nymph (Fig. 2) varies somewhat in structure, as follows: 
Scutum : the depressed areas between the cervical grooves and lateral 
ridges are at times well marked, sometimes they are not. In the small 
ill-nourished specimens the scutum is considerably smaller with straighter 
antero-lateral and sinuous postero-lateral margins; the basis capituli is 
less protruding laterally, and the protuberance is situate more anteriorly. 
The other differences between normal and ill-nourished O’s are trivial. 
There are considerable differences between normal and small ill- 
nourished males (Fig. 3). The latter appear less chitinized and paler ; 
the scutum is much smoother, shows but faint lateral grooves and 
is much less punctate; the palps do not show a concavity distally upon 
the dorsal surface, the characteristic transverse row of hairs upon the 
basis capituli is absent, and the base appears shorter; the anal plates 
are rounded posteriorly and almost touch behind the anus, they scarcely 
protrude above the surface of the abdomen and the accessory adanal 
plates are absent; the legs are markedly different, this being especially 
evident in pair IV, where the tarsus and protarsus are nearly of equal 
width instead of the latter being large and swollen. 
In ill-nourished females (Fig. 4) the porose areas are minute, the 
scutum appears less punctate, less depressed outside the cervical grooves, 
which are short; the latei'al ridges are absent. The tarsi are unarmed 
as in the nymph, to which it approximates more closely in general 
appearance than does the well-developed female. 
To summarize: Our experiments show that there is a considerable 
individual variation in the size of adult R. appencliculatus whose de¬ 
velopment has not been interfered with. The structural variations in 
such forms are slight. The great variation in size and structure 
observable in the genus is, however, merely due to the imperfect 
nutrition of smaller individuals in their immature stages. It is only by 
determining the range of variability under experimental conditions 
that the limits of a species in this respect can be determined and the 
making of bad species prevented. 
The variability in size and structure observed in R. appencliculatus, 
raised under experimental conditions, agrees with what we have found 
in this species of tick collected from different hosts under natural 
conditions. Arranging the specimens in series from the largest to the 
smallest, all grades of modification in the various structures between the 
Parasitology vi 14 
