MOUTH PARTS OF P. VESTIMENTI 531 



terminal one is provided with minute sense pits, and are directed out- 

 wards and forwards. They are moved freely during life. The number of 

 joints may vary with the stage of development ; in Phthirus, according to 

 Enderlein, there are only three joints in the larva, while the adult has five. 



The eyes are always small and inconspicuous, as is usually the case 

 in insects which have become confirmed in a parasitic mode of life, and 

 may be entirely wanting. They are small dome-like elevations, showing 

 a dense black pigment, and are situated on the lateral borders of the head, 

 behind the antennae, sometimes on small forwardly directed processes. 

 The facets can only be seen in sections. 



The mouth parts of the louse present an interesting problem in insect 

 morphology, and have been a favourite subject of research from the time 

 of Swammerdam. The obscure nature of the relations 



of the parts to those in other insects, together with the The uth part8 * nd 

 . . . sucking apparatus 



almost insurmountable technical difficulties which arise 



owing to the minute size and delicacy of the structures, has led to a con- 

 siderable amount of confusion in the literature on the subject, both as 

 regards the actual facts of the case and in the nomenclature adopted, on 

 the grounds of supposed homology, by the various workers. It would 

 be out of place here, and it would serve no useful purpose, to enter into a 

 discussion of the views set forth by different observers ; the reader who 

 is interested in the subject should consult the paper of Pawlowsky, in 

 which there is a good account of the history of the controversy, and the 

 papers of Enderlein. These two authorities have made the most recent 

 and perhaps the most valuable contributions to the subject, but unfortu- 

 nately they attacked it from different points and by different methods, 

 and do not adopt the same systems of nomenclature, thereby further 

 confusing the reader. The researches of Enderlein are founded on dis- 

 sections and on the comparative study of species of several genera, while 

 the paper of Pawlow r sky deals solely with the appearance of the parts 

 in sections of P. vestimenti alone. 



The following account of the mouth parts of Pediculus vestimenti has 

 been prepared, with the assistance of the above papers, from sections and 

 dissections. As a matter of convenience, and because the parts are used in 

 classification, the sucking apparatus, or the first part of the alimentary 

 canal, will also be dealt with here. 



In the resting position the mouth parts and sucking apparatus of the 

 louse are entirely concealed within the head.* The disposition of the 



* The mouth parts are usually either entirely concealed, or are so arranged as to present no 

 impediment to forward progress, in those insects which have adopted an entirely parasitic habit. 



