SOME HUMAN PARASITES 275 



easier by the fact that some closely allied animals have 

 absolutely distinct fleas. A statement that is always 

 received with incredulity by the unscientific, though 

 nevertheless true, is that monkeys have no fleas. Their 

 habit of diligently searching in one another's fur has given 

 rise to the belief that they are particularly susceptible to 

 these parasites ; but as a matter of fact, when in captivity, 

 if they harbour these insects at all, they have most certainly 

 caught them from some neighbouring animal or mayhap 

 from some human being. 



Another group of animals singularly free from fleas is 

 the order of Ungulates, including oxen, sheep, goats, etc. 

 To which species of flea numerical pride of place must be 

 awarded is a moot point, but " a German naturalist collected 

 2,036 fleas from theatres, concert halls, ballrooms, schools, 

 and barracks in the Grand Duchy of Baden, and found that 

 more than fifty per cent, were dog fleas. ... In Zoological 

 Gardens cat fleas are generally numerous in most of the 

 cages." 



The fleas are usually divided into three families, (a) the 

 Chigoes, truly parasitic fleas, (6) the typical fleas, of which 

 the human flea is an example, and (c) the bat fleas. The 

 general structure of a typical flea is too well known to 

 everyone to necessitate any description, and, as we are not 

 concerned here with minute anatomical details, we will rest 

 content with noting a few points of interest. The whole 

 flea is entirely covered with plates of chitin, a hard resistant 

 substance, as may be easily learned by trying to squash 

 one of these insects in the fingers; below this armour is 

 the true skin. Varying in colour from pale yellow to dark 

 brown, all fleas are more or less covered with bristles, 

 pointing backwards in every case and aiding the insect 

 materially in its progress. One anatomical feature, which 

 is almost peculiar to these insects, is their lateral flatten- 

 ing; it is quite usual to find insects flattened dorsi- 

 ventrally, the bed bugs and cockroaches are examples, but 



