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 pro thorax 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, T4 inch long, found clinging to the thorax 
of queen and drone honey-bees and known as the “bee-louse,” Braula 
cceca (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. 
