536 MEDICAL ENTOMOLOGY 



than it normally is, on account of the shrinkage of the soft parts in 

 which it lies. It is for the most part homogenous and structureless, 

 though here and there a few hypodermal cells of the ordinary type can 

 be distinguished in the wall. The constituent parts of the proboscis 

 itself are very difficult to isolate in so small an insect as the body louse, 

 in which, moreover, they are more delicate than in Haematopinus. 

 Three stylets can be seen in dissections, corresponding to those already 

 mentioned as seen in sections of the head. The most ventral of these is 

 the labium, which has the shape of a long and narrow gutter, in which 

 the other parts rest. In section (Plate LXXI, fig. 2) it appears as a 

 V-shaped trough. At the posterior end it broadens out considerably, to 

 form a structure which recalls the ' bulb ' in the proboscis of the blood- 

 sucking Muscidae (see page 52). Its distal end is obliquely truncated 

 and ill-defined. Dorsal to it lie the two maxillae, long and narrow 

 blades of very delicate chitin, with numerous fine serrations on their 

 external borders. In sections the two blades appear as concave troughs, 

 open dorsally, and apparently united to one another by their internal 

 borders, forming two parallel channels ; probably in the living louse the 

 two external borders are bent inwards in contact with one another, so as 

 to form a closed tube up which the blood can flow. The inner edges of 

 the blades are certainly softer than the external in cleared preparations, 

 and would therefore undergo more shrinkage during fixation. Below the 

 two maxillae lies the hypopharynx, which is so delicate a structure that 

 it can only be recognized in dissections by the presence of the salivary 

 duct, which runs along its whole length. The duct appears in sections 

 (Plate LXXI, fig. 2) as a circle of delicate tissue, incomplete on the 

 ventral surface ; this gap in the wall is quite possibly due to shrinkage 

 of the soft parts. All the structures at the base of the proboscis are so 

 closely compressed and so firmly bound down to one another by fibrous 

 tissue that it is not possible to determine their relations without a series 

 of much thinner sections than can be prepared by ordinary methods. 

 The lumen of the salivary duct is so small that it cannot be followed 

 beyond the termination of the hypopharynx. 



It was shown by Schiodte, who cut off the head of a louse while in 

 the act of feeding, that the proboscis is actually thrust out of the head 



- Musculature of the ^ urm S t ^ ie act f feeding, as one would expect on 



c .-proboscis anatomical grounds, and as can be readily shown. in 



dissections. It is of some interest, therefore, to look 



for the muscles by which the acts of protraction and retraction are 



brought about. Such are to be found in the posterior part of the head, 



