22 THE FLEA [OH. 



tiles of a house. The segmented structure of a flea 

 is there most clearly seen ; this we may suppose is 

 an inheritance from the segmented worms. 



The chitin which forms the external skeleton of a 

 flea is secreted by an outer layer of cells on the 

 insect's body. The deposit being thin at the joints, 

 and thick on the plates, which serve for protection, 

 the flea is encased in a suit of flexible armour. It 

 is made of a fairly solid and dense substance, but, 

 owing to the absence of carbonate and phosphate of 

 lime, is much lighter than the familiar external 

 skeleton of the lobster. Chitin is a very peculiar 

 and durable substance which resists boiling in acids 

 or alkalies. It is a structureless substance, in the 

 sense that it does not consist of cells. Though horny 

 in appearance it is, of course, in no sense true horn 

 like that of the nails, hoofs, claws, and horns of 

 vertebrates. 



The different species of flea vary considerably in 

 size. Some are smaller than the familiar human 

 flea. Others are much larger. A very large flea 

 (Hystrichopsylla talpce) is that found on the mole. 

 The largest known flea (Dolichopsyllus stylosus) is 

 found on small rodents in the United States where, 

 as we know, all things are on a bigger scale than in 

 the Old World. It is seven millimetres long. 



The colour of the horny integument varies from 

 a pale or light yellow to a ruddy or dark brown. 



