168 
Larvae of Fleas 
as they are broad ; the other two are longer, and are slightly curved 
(Figs. 1 a and 2 b, l.p.). The relations of the parts of the labium 
are most easily made out in Ceratophyllus fasciatus. In Xenopsylla 
cheopis the two maxillary brushes come closer together than in Cerato¬ 
phyllus fasciatus, and the labium is greatly reduced, although the labial 
palps are clearly recognisable. We have been unable to detect in the 
larvae of any species a hypopharynx as a distinct structure from the 
labium ; the duct of the salivary gland has the appearance of opening 
as a groove or gutter on the median part of the labium itself. 
Body. 
There is no very obvious separation between the thorax and abdo¬ 
men, except in larvae about to pupate, in which the three thoracic 
somites show as distinct in form from the abdominal segments. The 
general somitic divisions of both thorax and abdomen are clear, the 
constrictions being sharp, though not deep (see Fig. 6). There is a 
steady increase in the size of the segments backwards from the head to 
the sixth, seventh or eighth abdominal, after which the segments decrease 
rapidly to the end. The tenth or anal somite is always small. In 
Xenopsylla cheopis the broadest segments are abdominal somites 
7 and 8, and abdominal 9 is only slightly smaller; but in Ceratophyllus 
fasciatus the largest segments are abdominals 6 and 7, and there is a 
tapering backward and forward from this region ; i.e. the ninth ab¬ 
dominal somite is relatively smaller in Ceratophyllus fasciatus than in 
Xenopsylla cheopis. The larva of a flea is more cylindrical at its 
ends than in the middle third of its length, where the muscular 
development produces a lateral flange in each segment similar to that 
commonly present in Lepidopterous larvae. 
In a fully extended living larva, and in a dead one which has been 
treated with caustic potash solution, there is no appreciable overlapping 
of the cuticle of the various segments, but in a contracted larva, and 
particularly in a larva about to pupate, the cuticle of each segment 
overlaps that of the segment behind. The head is overlapped by the 
cuticle of the first thoracic segment in a contracted larva. 
The chitinous cuticle of the body is unevenly thickened. As in 
most insect larvae there are definite plates (dorsal plates, etc.) where 
the chitin is thicker, browner and smoother than elsewhere. These 
plates are most clearly distinguishable just before the larva moults, 
and are difficult to make out in a recently moulted larva. In the figures 
