REVISION OF THE SARCOPSYLLIDAE 37 
of the ventral surface of the segment, being placed in many Pulicidae in between the 
second pair. In other species this first bristle is lost on each side. In others again, 
the fourth is replaced by a fi'ie hair. These bristles are in several species of Sarcopsyllids 
much more reduced in size and number than in Pulicidae (PI. Ill, Fig. 26 ; PI. IV, 
Fig. 28.) In Hectopsylla psittaii, however, there are no less than eight bristles on each side, 
at least on the mid tarsus (PI. IV, Fig. 32). In this species there is apparently an 
increase in the original number of lateral bristles, the increase having taken place in 
the apical region of the segment. None of the other species of Sarcopsyllidae have 
more than five bristles, the fifth always being much thinner than the others. The 
number is reduced to four and three in some species, the bristles being all very thin 
in Dermatophilus. The claw of Sip/iouaptera bears usually a basal ventral projection. 
This process is occasionally lost in the Pulicidae, tor instance, in Lycopsylla. There is 
one species of Ecliidnophaga in which the claw has a large basal projection (PI. Ill 
Fig. 24) ; in other species of Sarcopsyllids the projection is very small, and in others 
again lost altogether (PI. IV, Fig. 28). The gradation exhibited by the various Sarcop- 
syllids in respect to the claws is complete, Dermatophilus possessing the greatest 
specialization, the claw being in this genus very slender, like the whole tarsus. 
The posterior abdominal segments which are partly modified for the purpose of 
copulation we now propose to discuss. It is well known that these segments are 
usually so diversely developed in the various species among insects that they afford 
very good diagnostic characters, more especially in the In Siphonaptera these 
characters are the more important for descriptive work as there are frequently but few 
other characters by which to distinguish the more closely allied species with any 
degree of certainty. Modification for sexual purposes is found in segments eight and 
nine of the In the however, it is the eighth segment and often also the 
seventh sternite which has assumed a peculiar shape. The eighth and ninth segments 
of the we propose to discuss first. 
The tergite of the eighth segment bears in both sexes the last stigma which is 
usually situated in a more or less large hairy cavity, the tergite being often divided in 
the mesial line. On account of the development of the sensory plate on the ninth 
tergite the dorsal portion of the eighth tergite is in both sexes usually much shorter 
than the lateral portion. The relative size of the eighth tergite and sternite is very 
variable in the Siphonaptera. Originally the two plates were doubtless of nearly equal 
size, as is the case in the preceding segments. From this original state of organization 
development in two opposite directions has taken place. In some Pulicidae the 
eighth tergite of the has become reduced and the sternite enlarged, while in other 
Pulicidae the tergite is enlarged and the sternite reduced, the latter being sometimes 
represented only by a narrow strip of chitin or being occasionally vestigial. This 
small sternite occurs in a number of species of Ceratophyllus, while all the other 
Siphonaptera have a large or comparatively large eighth sternite. In some species of 
