THE FLEAS OF NORTH AMERICA 13 



Fleas like those of the genus Vermipsylla have many so-called seg- 

 ments in the labial palpus. Most of these evidently are not true seg- 

 ments, but pseudosegments, as is indicated by the fact that they are 

 very short and frequently only partly separated from one another 

 by sutures that are incomplete and not transverse. 



The present writers would derive the fleas and the Diptera from a 

 common ancestral stock. However, the fleas are to be regarded as 

 older and more primitive than the Diptera. There are two reasons 

 for this, one being that the flea larva is decidedly of a more gener- 

 alized type than the larvae of most of the Diptera, and the other, 

 that fleas lack reduction and modification of the metathorax such as 

 that found in flies. Fleas probably developed from ancestors that 

 yet had functional metathoracic wings and hence were not true 

 Diptera. 



The Order SIPHONAPTERA 



Small insects, with a complete metamorphosis ; parasitic in adult state on 

 warm-blooded vertebrates ; without wings or wing vestiges ; body greatly com- 

 pressed and frequently provided with 1 or more combs of spines. 



Head always divided into 2 regions by an oblique antennal groove and in 

 some genera also divided into an anterior and a posterior part by the presence 

 of a deep fronto-epicranial groove. Antennae short, stout, each contained in a 

 deep sulcus known as the antennal groove; segments beyond the second greatly 

 shortened and formed into a club. Eyes simple, consisting of a single pair ; 

 degenerate or absent in some genera. Mouth parts adapted for piercing and 

 bloodsucking. Maxillae each with a filiform, segmented palpus and a sub- 

 triangular, ventrally projecting, acute, maxillary lobe. Labrum long, flattened, 

 almost needlelike, together with the mandibles forming a piercing beak. 

 Mandibles long, piercing blades, with parallel, saw-toothed margins. Labium 

 with greatly reduced base, but with segmented palpi, grooved on inner surfaces, 

 the 2 fitting together to form a labial beak. 



Thorax composed of 3 freely moving segments. Prothorax with large prono- 

 tum, frequently bearing a transverse comb of spines ; sclerites of sternal and 

 pleural regions anchylosed, forming a saddlelike structure bearing the front 

 pair of legs. Mesothorax larger than prothorax. Mesonotum usually somewhat 

 expanded, never with comb of spines. Mesosternum, mesepisternum, and 

 mesepimeron anchylosed, but the boundaries of union usually indicated by 

 internal sclerotized ridges. Metathorax similar to mesothorax except that 

 the metasternum is greatly reduced, the metepisternum is divided into 2 parts 

 by a horizontal suture, and the metepimeron is greatly expanded posteriorly. 



Abdomen apparently composed of 10 segments, but in reality of 11. Segment 

 I greatly reduced ventrally, without sternum. Tergal plate VII usually with 

 a clump of 1 to 4 large or stout bristles (antepygidial bristles) on each side 

 at posterior margin. Dorsum IX with a specialized, saddlelike plate known as 

 the pygidium, which is studded wtih minute setae. In female, segment XI 

 with a pair of small, 1-segmented stylets (? cerci) situated dorsally. In 

 males of most genera sternal plate VIII modified, being either greatly reduced 

 in size or extended posteriorly into a long process of a variety of shapes. 

 Sternal plate VIII in the female occasionally somewhat rudimentary. Ninth 

 abdominal segment in male bearing a pair of claspers. Sternal plate IX in 

 male greatly modified, consisting usually of a pair of inner vertical processes 

 and a long, highly modified, posterior or horizontal process or lobe. Male flea 

 with a complicated protrusible penis and female with 1 or 2 receptacula seminis. 



Legs typically insectan. Each with a large, long, flattened coxa; a small, 

 ringlike trochanter; a large femur; a tibia enlarged distally; a 5-segmented 

 tarsus, and each tarsus terminated by a pair of subequal, curved claws. 



The order contains six families as follows: Dolichopsyllidae, 

 Hystrichopsyllidae, Ischnopsyllidae, Puliciclae, Stephanocircidae, 

 and Hectopsyllidae. 



