530 MEDICAL ENTOMOLOGY 



elephantis it is greatly elongated. In Pediculus and Haematopinus the 

 anterior half is produced to a blunt point, while the 

 posterior portion, behind the antennae, is roughly 

 cylindrical. The upper surface is strongly chitinized, and is marked by 

 several sutures, which divide the integument into named portions. A 

 median longitudinal suture runs forwards from the neck for a short 

 distance, and then bifurcates, thus forming a Y with the two limbs 

 directed forwards and outwards to the bases of the antennae. The 

 triangular lateral portions thus marked out make up the vertex (' scheitel ' 

 of Enderlein), and the median triangle between the two is the frons 

 ('stirn'). The dorsal area in front of the frons is divided, in Haemat- 

 opinus and others, by several transverse sutures, not very distinctly 

 marked, which are taken to represent, from before backwards, the labrum, 

 first and second clypeolus, and the clypeus. In Pediculus the dorso- 

 lateral area at the distal end of the head is specially thickened into a 

 curved plate of chitin, which is considered by Enderlein to be a rudiment 

 of the mandible. The under surface of the head is membraneous, 

 and is often marked by many transverse ridges and furrows. At the 

 distal end the integument is soft and loosely arranged, and shows one 

 or two circular furrows ; it thus permits of a certain amount of evagina- 

 tion of the part, which, as will be seen later, takes place when the insect 

 feeds. The extreme distal end is marked by the presence of a circlet 

 of small chitinous teeth arranged around the mouth orifice. In Pediculus 

 there are five such teeth on each side in the adult. As pointed out by 

 Enderlein, the number of teeth differs in different species and in the 

 several instars of the same species. They will be referred to again in 

 connection with the mouth parts and sucking apparatus. 



At the posterior border of the head there is a thickened collar of chitin, 

 united to the anterior end of the thorax by a short membraneous neck. 

 Two narrow elongate strips of chitin arise from this in the dorsal middle 

 line, and pass backwards into the thorax. These pieces are well marked 

 in Pediculus ; at first they lie side by side with their flattened surfaces 

 dorsal and ventral, but as they pass backwards they turn inwards, and 

 eventually fuse together. Their posterior portions dip below the level of 

 the integument of the dorsal surface, and appear in sections as the two 

 sides of a canal with a narrow and vertical lumen. They are apodemes, 

 serving for the surface of origin of large muscles. Enderlein terms them 

 ' hinterhauptfortsatz '. 



The antennae are inserted about the middle of the lateral borders. 

 They are simple, consisting of from three to five joints, of which the 



