N. CUNLIFFE AND Gr. H. F. NlJTTALL 
345 
NOTE ON THE EXTERNAL CHARACTERS WHICH SERVE 
TO DIFFERENTIATE THE SEXES. 
By G. H. F. Nuttall, Sc.D., F.R.S. 
The structures surrounding the genital orifice constitute the only essential 
external difference between the sexes in 0. moubata. It is true that the 
average male is smaller than the average female but some males exceed 
certain females in size. The smallest males measure 4-5 x 3 mm. 1 , the largest 
8*6 x 6-1 mm. The females likewise vary in size, measuring 6 x 3-8 to 
11*4 x 9*3 mm. according to our observations, although Dutton and Todd 
(1905) record a gorged female which attained 12 x 10mm. We find that 
A 
B 
Fig. 5. Ornithodorus moubata, genital apertures, (A) of male, and (B) of female individuals. 
(E. W. et G. H. F. N. del. 1913.) 
the size of the average male is 6-1 x 4-3 mm. and that of the average female 
8 x 6*5 mm., these measurements being based on the examination of some 
400 specimens of both sexes. A variation in size is observable in moubata 
raised under apparently identical experimental conditions. The measurements 
of the capitulum correspond to those of the body; in small males the capitulum 
measures 0-95 x 0-58 1 , in large females it measures 1-4 x 0-9 mm. 
The genital aperture is usually smaller in the male, but in large males it 
may exceed the size of the aperture in small females. In the male (Fig. 5 A) 
1 The figures give the length x the width. The length of the capitulum was measured from the 
posterior ventral margin of the basis capituli to the distal extremities of the extended palps: 
the width given is that of the basis capituli at the palpal articulation. 
23—2 
