The Two-winged Flies 353 



upper were the under surface. The head is narrow and lies back on the 

 dorsum of the thorax, and the prothorax rises from the upper instead of 

 anterior aspect of the mesothorax. They are found only on bats and are not 

 common. 



The strange minute insect, -jV inch long, found clinging to the thorax 

 of queen and drone honey-bees and known as the "bee-louse," Braula 

 caeca (Fig. 504), is the only species known of the family Braulidae. Its legs 

 are rather short and stout, and each ends in a pair of comb-like brushes. 



ORDER SIPHONAPTERA. 



The fleas are blood-sucking parasites of mammals and birds which were 

 long classified as a family (Pulicidae) of the Diptera, being looked on as 

 wingless and otherwise degenerate flies. But they are now given by ento- 

 mologists the rank of an order, called Siphonaptera, subdivided into three 

 families of its own. Nearly one hundred and fifty species of fleas are known 

 in the world, of which about fifty are recorded from this country. They have 

 been taken from the domestic dog, cat, rat, and fowls, and from various wild 

 animals, such as several rabbit and squirrel species, the lynx, weasel, mole, 

 mountain-rat, shrews and mice, prairie-dog, woodchuck, opossum, etc. 

 Rothschild has recently described a new flea species from the grizzly bear 

 (British Columbia). But from the great majority of our wild mammals fleas 

 have not yet been recorded, although undoubtedly most of them are infested. 

 Baker, who has recently published a monograph * of the known North 

 American species, suggests that particularly interesting forms will probably 

 be found on bats. One flea species, Pulex avium, has been taken from several 

 kinds of birds, and two or three other fleas are recorded from bird hosts. 



The peculiar structural characteristics of fleas are their winglessness, 

 the extraordinary lateral compression of the body, and the curious modifica- 

 tion of their mouth-parts for effective piercing and blood-sucking. The an- 

 tennae lie in little half-covered grooves, extending down and back behind 

 the eyes; they can be lifted or stretched up whenever needed. Each antenna 

 is composed of three segments, the terminal one, however, being spirally or 

 transversely lined or grooved and variously shaped, so that it appears to be 

 composed of several segments. The mouth-parts consist of a pair of needle- 

 like mandibles, a pair of slender grooved labial processes, probably the 

 palpi, a pair of short, broad, flattened maxillae, each with a short antenna- 

 like palpus at its tip, and an unpaired needle-like hypopharynx. The needle- 

 like parts serve for piercing and the grooved labial processes for sucking. 

 Regularly arranged over the body are (in most fleas) many series of stiff, 

 spine-like hairs, often unusually conspicuous and strong on the head and 



* Baker, C. F. A Revision of American Siphonaptera. Proc. U. S. Nat. Mus., vol. 

 xxvii, 1904, pp. 365-469. 



