(t. H- F. Nuttam 
295 
dark point of what we shall call the dilator^ (Plate III) may, or may 
not, protrude in life or in preserved specimens. Basally is seen the basal 
plate (internal) upon which the dilator articulates, the same being 
visible by translucency through the body wall. 
The postero-ventral abdominal wall, which is very convex trans¬ 
versely, is traversed by a number of accordion-like folds which allow 
of considerable extension of its surface (Text-figs. 1, 3). The male is 
usually smaller than the female, its abdomen is slenderer and more 
clearly segmented. Although both sexes show variable degrees of 
pigmentation, the male usually shows the deepest coloration. In well 
pigmented males the abdominal tergites appear as pairs of dark trans¬ 
verse ban(^, an appearance never seen in the female whose integumentary 
structure differs in this region. In the male, the abdominal hairs are 
fairly regularly disposed in rows corresponding to the body segments, 
whereas, in the female, most of the hairs are scattered about irregularly 
except on the last segments. A very marked sexual difference is 
observable in the structure of the first pair of legs in the male, these being 
also larger and more powerful than the others. 
The longitudinal abdominal inter segmental muscles in the male (Text- 
fig. 3) are very powerful; dorsally, at the base of the abdomen, they fall 
into line with corresponding muscles arising in the thorax (M. levator 
abdominalis of Muller) and form a continuous chain of five pairs of 
muscles on each side extending backward to the last segment. The 
corresponding ventral muscles are confined to the basal part of the 
abdomen, extending from the thorax backward so as to include only 
the first four recognizable sternites. 
The Female. We have already indicated some of the characters 
of the female in the preceding paragraphs. The abdomen is broader 
than that of the male and posteriorly it presents a bilobed appearance 
(Text-fig. 7). The anal aperture lies dorsally between the lobes beneath 
which is situated the vaginal aperture whose entrance is seen when the 
insect is viewed from behind or in ventral aspect. When thus viewed, 
the aperture is seen to be guarded ventrally by tw'o incurved flattened 
processes, the gonopods, which are fringed with hairs and connected in 
the middle line by a fold of the integument surrounding a circular space. 
Anterior to the gonopods, which are more or less pigmented, a darkly 
chitinized and thickened area marks the region occupied beneath by 
^ We apply the term dilator to the structure described as tlie paramcres by Mjbberg 
and by Cummings. All writers on P. humauns falsely call it the penis (v. infra, p. 206). 
