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



