658 
DIPTERA. 
figured on PL LXIX or than in certain parasitic Phoridce such as Termi- 
toxenia (fig. 400). Their active habits and the voracity with which they 
suck blood are known to most people, and these characteristics, together 
with the long strong hind legs and the curious shape of the body, which 
is excellently fitted for gliding easily among hair, fur or feathers, make 
them unmistakeable. The sexes are a good deal alike, but the female 
is larger, has a less abruptly tip-tilted tail, and has not the coiled internal 
horny attachments of the genital organs which are present in the male, 
and which are shown in all the figures on PI. LXXI. The eggs are laid 
on floors, dusty carpets, or dry earth, and hatch into slender larvse of a 
more or less dipterous type, with rather strong hairs and a biting mouth 
with which they consume what nutritive matter they can find among 
the dirt in which they live. When full grown they spin a cocoon which 
is usually coated with dust and dirt, and there transform to a pupa. 
The life-history may be easily observed by keeping females in glass 
bottles with a little loose dry dust and dirt, such as floor-sweepings, in 
which the larvae may live. Too much moisture is very distasteful to the 
young and adult stages, and they probably flourish best under certain very 
definite conditions of temperature and humidity, as is the case with many 
other insects. The larvae are killed almost immediately when wetted. 
The rat-flea (Pulex cheopis , Roth.) is an insect whose study has 
become of the first importance since the work of Lamb, Liston, and 
others has shown that its bite consti¬ 
tutes one of the chief ways, even if 
not the only one, whereby plague is 
spread. The rat-flea is essentially a 
parasite of the rat, but it does not 
confine its attacks to these animals, 
and it will bite man, especially (but 
not only) when there are no rats on 
which it can feed. It is well known 
that before plague attacks the men 
of a village, the rats of the place 
usually die of the disease. When the • 
rats die, it is presumed that the rat- 
fleas leave their bodies and are then 
particularly liable to bite men, and 
Fig. 434 —Pupa and Larva of Flea 
much enlarged. The figure 
is a bad one. 
