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 modem mechanical 

 theories of reflex action, negative phototropism (repulsion by light), etc., 



Fig. 506. — The house-flea, Ph/ca; i>rito;is. y4, larva; B, pupa; C, adult. 

 (.■\fter Beneden; much enlarged.) 



allow us to give the elusive flea little credit for its ingenuity; we must look 

 on it as an unusually well-made and smoothly- working organic machine. 



