n] EXTERNAL STRUCTURE OF A FLEA 25 



ancestor. Presumably all the fleas of long ago had 

 eyes and those that are now blind have lost their 

 organs of sight from disuse. In their simplest con- 

 dition, the eyes of the lower invertebrates only 

 enable the creature, so far as one can judge, to 

 distinguish light from darkness. Entomologists 

 believe that the power of vision of ocelli is probably 

 confined to very near objects and that this simple 

 form of eye is more useful in dark places than the 

 compound eyes. There is no reason for believing 

 that fleas can distinguish colours or can discern any 

 object which is more than a few inches away. It is 

 enough for their purpose to perceive from which 

 point light comes upon them and to make all despatch 

 to escape in the opposite direction. 



In blind fleas there is often a spine where the 

 eye should be. In one species the spine is rudi- 

 mentary and there is some black pigment beneath it. 

 It is not impossible that this is the vestige of a once 

 functional eye. In one genus, however, the eye and 

 the spine are both present. Of the fleas belonging 

 to this genus one species is South American and the 

 other European. The latter (TypMoceras poppei) 

 is confined to the long-tailed field-mouse. 



The organs by which fleas keep in touch with 

 the outward world, and with other fleas, are their 

 antennae. All fleas have antennae ; but unlike those 

 of a moth, a beetle, or a grasshopper, each fits neatly 



