N. ClJNLIFFE 
205 
individuals (the species having a restricted distribution and being 
somewhat rare) have rendered the results of the investigation less 
instructive than they would otherwise have been. 
We are only concerned with the variability of those structural 
characters which are usually employed in determining a species of this 
genus. It should be pointed out that there is a regular gradation of 
all the characters, described and figured for the largest and smallest 
specimens respectively (the characters of any individual being correlated 
with its size) unless the contrary is stated. 
Size of Scutum. 
The variation in the size of the scutum is the most striking feature. 
The measurements were made by means of verniers on a moveable-stage 
microscope, the ocular of which contained a pointer, a method which 
has been found very useful by Prof. Nuttall. A series of ticks, suitably 
orientated on a ridge of plasticine fixed to a glass slide, could be moved 
rapidly in the desired direction and measurements accurate to 0T mm.' 
were easily obtainable. The limits of the ornamentation, which in both 
male and female practically covers the scutum, were used in place of 
the limits of the scutum itself, because in the majority of cases the 
exact margin of the scutum is vague. In the male the length was 
considered to be from the anterior emargination to the posterior border, 
the breadth across the posterior third, the scutum being broadest at 
that position ; in the female, the length was measured as in the case of 
the male and the breadth immediately posterior to the eyes. 
The results of the measurements of the scuta of 75 males and 
53 females are given in the form of correlation diagrams (Figs. 1 and 5). 
The male scutum increases more rapidly in width than in length, 
giving a large specimen a somewhat truncated appearance, which 
contrasts with the oval form of a dwarf. A glance at the correlation 
diagram (Fig. 1) for the male individuals shows that the correlation 
between length and breadth, as would be expected, is very close, the 
correlation coefficient being as high as 0 93. It. indicates the division, 
which is more clearly expressed in the frequency curve, of the in¬ 
dividuals into two groups, and also enables the dimensions of the 
average sized scutum to be determined easily. For the male scutum, 
the extreme and average measurements are as follows : 
Largest specimen 
Smallest specimen 
Average specimen 
Length in mm. 
4-4 
2- 4 
3- 2 
Breadth in mm. 
3-4 
1-6 
2-3 
