104 FLEAS, FLUKES AND CUCKOOS 



mites had a special affinity for the pupal case which they completely 

 destroyed. In the laboratory they can, in this way, eliminate whole 

 flea cultures. 



One of the most curious and interesting facts about these mites is 

 that in the hypopus stage (see p. i8) they use the adult flea as a means 

 of transport to new nests. They attach themselves to the outside and 

 even creep just beneath the chitinous plates or sclerites and hang on 

 firmly by their sucking disks which are special adaptations for "hitch- 

 hiking" purposes and are developed only at this stage. In some 

 mysterious manner the mites can evidently distinguish between the sexes 

 of the fleas, for they almost always attach themselves to females — a wise 

 precaution for a parasite preying on the larval stages of the flea. The 

 mites differ considerably in size. The species found on 0. rothschildi 

 (Plate V b) is one of the largest parasitising British bird fleas. Those 

 illustrated on Plate V a, which look like ghosts beneath the sclerites of 

 the host, are considerably smaller. These mites have not been identified 

 with certainty. Sometimes as many as 150 hypopus larvae have been 

 found attached to a single "transport." Such numbers greatly hinder 

 the movements of the flea and in some cases may even cause its 

 death. 



We have already seen that when the bird host dies and grows cold, 

 the fleas leave it and seek their fortunes elsewhere. Similarly, when the 

 flea dies the mites also leave it. They moult, shed their sucking disks 

 and change into octopod nymphs. How is it that the "hypopus" mites 

 are aware their transportation has broken down ? Perhaps the sudden 

 cessation of movement is the stimulus to which they respond. One of 

 the older writers observing them at such a moment wrote : "In 

 bestirring themselves from their inactive condition one would imagine 

 that a state of demoralization had seized them, for they were seen to 

 pry free the sucking disks, leave their perch and move away from the 

 dead host." 



Classification 



It is convenient to classify animals — that is to name them, describe 

 them accurately and then arrange them in groups — just as it is con- 

 venient to name and classify the goods for sale in a shop. It is reasonably 

 easy for a customer to locate cheese in Harrods' stores, because food, as 



