No. 555] SPECIES-FORMING OF ECTO-PARASITES 131 



habits, also, of the two groups add to the likelihood of 

 the derivation of the Mallophaga if not directly from the 

 Atropidae, at least from an ancestor that might well have 

 been the conmon progenitor of both types. 



The Mallophaga are all of small size, ranging from 1.5 

 to 10 mm. in length, with the great majority of them not 

 more than 4 or 5 mm. long and from 1 to 2 mm. in width. 

 The body is much flattened dorso-ventrally, offering an 

 interesting adaptive contrast, in this respect, to the later- 

 ally flattened fleas of similar habitat. The body is always 

 wholly wingless, usually strongly chitinized and smooth. 

 The mouth parts are of true biting type, the food being 

 bits of hair or feather or dermal scales, and dried or fresh 

 blood only incidentally and not by a direct breaking of the 

 skin of the host. There are no compound eyes, but a 

 single pair of ocelli placed on the lateral margins of the 

 much flattened head, and the antennae are from three- to 

 five-segmented, and in about half the genera are capable 

 of being concealed in a small protecting groove on the 

 ventral face of the cheeks. The legs are of normal num- 

 ber, rather long and strong, flattened and fitted for run- 

 ning and clinging. The body is whitish, pale brownish 

 or even dark-brown, often well patterned by blotches or 

 bands of light to black-brown, on the paler ground. 

 These markings almost always indicate specially heavily 

 chitinized portions of the body wall. 



The eggs are fastened to the hairs or feathers of the 

 post, and the hatching young are much like their parents 

 m appearance except for their smaller size and paler, 

 ^patterned softer skin. They acquire maturity without 

 any marked metamorphosis. They run freely about 

 from birth, and feed on the hairs or feathers just as their 

 Parents do. Neither they nor the adults leave the body 

 ot the host except under unusual circumstances. In hen- 

 houses the parasites of the fowls have been occasionally 

 iound on the perches or in the nests, but I have often 

 sought carefully, without finding a single parasite, on 

 seaside rocks from which I had just frightened scores of 



