354 
The Two-winged Flies 
thorax. The head is ridiculously small and malformed, so that a flea under 
the microscope always suggests an idiotic (microcephalous) creature. But 
if its insidious attack and brilliant tactics in retreat be due to wit, this 
Fig. 505.—Dog- and cat-flea, Ctenocephalus canis. (After Lugger; much enlarged.) 
small-headedness is truly deceptive. However, our modern mechanical 
theories of reflex action, negative phototropism (repulsion by light), etc., 
Fig. 506.—The house-flea, Pulex irritans. A, larva; B, pupa; C, adult. 
(After Beneden; much enlarged.) 
allow us to give the elusive flea little credit for its ingenuity; we must look 
on it us an unusually well-made and smoothly-working organic machine. 
