2 BULLETIN 248, TJ. S. DEPARTMENT OF AGRICULTURE. 



connection with the disease of warm climates known as infantile 

 kala-azar. There is also reason to believe that fleas play some part 

 in transmitting leprosy. It has been found that the tapeworm of 

 the dog, which has been known to attack man, is dependent upon 

 some insect in which to develop one of its stages, and the dog flea 

 often serves in this capacity. Another interesting role which a 

 certain species of flea has been found to fill, though of no known 

 direct importance to man, is the transmission of a blood parasite of 

 the rat known scientifically as Trypanosoma lewisi. One kind of 

 flea which is becoming widely distributed in the Tropics has the 

 peculiar habit of burrowing into the flesh of man, especially around 

 the toes. This species causes severe sores and often permanent 

 crippling. In the United States, aside from their connection with 

 plague transmission, we are concerned most with insects of this group 

 as annoyers of man and animals. In the latter case the pests often 

 become so numerous as to cause more or less loss. This is particularly 

 true of the chicken flea, or " sticktight," which will be discussed in the 

 following pages. 



Fleas, as is generally known, are true insects. They have been 

 thought by many entomologists to be closely related to the Diptera, 

 or two-winged flies, but now they are usually considered to constitute 

 a separate order of insects. Their peculiar shape, 1 flattened from side 

 to side, and armature of spines and bristles are closely correlated with 

 their parasitic habits, enabling them to move rapidly between the 

 hairs or feathers of their hosts. 



HOSTS OF FLEAS. 



Fleas in the adult stage may be said to be parasitic exclusively on 

 warm-blooded animals. A single exception has been recorded — that 

 of one flea which was found attached to a land snake in Australia. 



A great many species of birds and most mammals have been found 

 to be infested by these parasites. The group of animals of which the 

 horse, ox, and sheep are representative are probably least subject to 

 attack. It is not the purpose to convey the idea that there are as 

 many kinds of fleas as there are birds and animals. In fact, the num- 

 ber of distinct species of fleas now known is probably not greatly in 

 excess of 400. In general, there are certain birds or animals, spoken 

 of as hosts, upon which these insects prefer to feed. 



Some species of fleas appear to have much more restricted host re- 

 lationships than others; that is, they are found on comparatively 

 fewer animals. In other instances fleas may not be found uncom- 



1 Some knowledge of the structure of a few of our common kinds of fleas may be derived 

 from an examination of the illustrations in the following pages, which, with the exception 

 of figure 1, were drawn by Mr. Harry B. Bradford. 



