438 MEDICAL ENTOMOLOGY 



and the second maxillae, these latter being only partly fused together, 

 yet fulfilling the same function of forming a sheath for the piercing 



parts. The essential difference between the conditions 

 The Mouth Parts . _. . . 



in the Siphonaptera and those in the Diptera is that in 



the former the hypopharynx has failed to develop in the same degree, and 

 does not extend beyond the base of the proboscis. It can consequently 

 take no part in the formation of the food canal, nor can it convey the 

 saliva into the wound. Its place in both these functions is taken by the 

 mandibles, which, by apposition with one another and with the epi- 

 pharynx, form a channel for the conveyance of the food, and which have 

 grooves on their inner sides to conduct the saliva downwards from the 

 salivary receptacle to the wound. There are thus, as in the Diptera, 

 two canals, one afferent and one efferent. 



The mouth parts project downwards, and, in the Pulicidae, usually a 

 little backwards, from the distal portion of the ventral surface of the 

 head ; their proximal ends, in the cat flea and its allies, are concealed 

 by the genal comb. The aperture through which they emerge from 

 the wall of the head cavity is surrounded by thinner chitin, and may be 

 termed the peri-oral ring. There is no membraneous area such as is 

 found in this situation in the Diptera. When seen in cleared and 

 mounted preparations the parts are usually a little displaced from 

 their natural positions. The maxillary palps lie in front, and are the 

 most conspicuous parts of the apparatus, projecting from the front of 

 the head like antennae ; the maxillae are frequently displaced from 

 the labium, which normally ensheaths them, and lie in front of it. 



The labrum-epipharynx (Plate LIV, rigs. 11 and 9), which has been 

 described as the labrum and also as the epipharynx, is a long and slen- 

 der organ, presenting a rounded contour on section, but 

 e i har nx flattened on its ventral surface where it is in apposition 

 with the mandibles. Its dorsal wall, corresponding to 

 the labrum, and its ventral wall, corresponding to the epipharynx, are 

 composed of rigid yellow chitin, while the lateral walls which connect 

 these parts are more membraneous. Within these walls there is a space 

 (Plate LVIII, fig. 10) continuous with the haematocoele of the head, and 

 limited distally by the fusion of the two parts, as in the Diptera. The 

 labrum extends to the distal extremity, the two parts taking about an 

 equal share in the formation of the compound organ. The lateral edge 

 of the labrum bears fourteen pairs of forvvardly directed tubercles of 

 extremely small size. They are not cutting teeth, but are probably 

 sensory organs, as under a high magnification a central canal can be 



