45 



cation, a chitinous expansion is given off which runs 

 forward and embraces the finer piercing organs (Fig. 17, 

 A and B, CK). The two together form a sheath round the 

 basal portions of the piercing organs. 



The relation of the three elements to one another and how 

 the sucking tube is formed may be seen from Fig. 17, C, 

 which represents a transverse section in the neighbourhood 

 of the line x-y in Fig. 17, A. Ch. is the chitinous sheath, 

 a the dorsal element which we may call the stylet, b the 

 median, and o the ventral element. The interlocking of 

 the lateral chitinous expansion of b and c forms the canal 

 (s.t.} up which blood is sucked. 



FIG. 16. 



Diagram of the alimentary canal and mouth parts of the 

 louse (Pediculusvestimenti), constructed from dissections by 

 Mr. Bacot. The precise termination of the ducts of the 

 two pairs of salivary glands is at present uncertain, 

 but they can be traced into the base of the sac. 

 PH, Pharyngeal pump. SG, Salivary gland. SD, Salivary 

 duct. P, Pricker. ST, Stomach, i, Intestine. R, Rectum. 

 MT, Malpighian tubes. 



Fig. 17, A, is a diagrammatic representation of the head 

 of a louse with the pricker retracted, B with the pricker 

 extruded ; p. is the " pricker " made up by the apposition of 

 the three parts mentioned above. Anteriorly these pass 

 through a tunnel in the substantial chitinous piece shaped 

 like an inverted Y, which apparently also forms a skeletal 

 framework for the attachment of the muscles (m. , Fig. 17, A 

 and B) which pull the whole organ forward. Ch. is the sheath 

 covering the basal portion of the piercing parts. At the base 

 the bifurcation of the three elements is seen, and also the 

 four salivary ducts (s.d.) which enter there, but the precise 

 connexion of which has not yet been made out. When the 

 whole organ is pulled forwards it not only drives out 

 the "pricker" but evaginates the lining membrane of the 

 sac and everts the ring of booklets (A.), whereby the head is 

 anchored to the epidermis. 



