EXTERNAL ANATOMY. 19 



the other pieces except the labrum, which covers them like a 

 sheath from above. At the tip of the labiura are two small oval 

 lamellae in the form of two spatulate jointed valves, and these 

 represent the labial palpi. 



"During the act of piercing the skin, all the mouth-parts but 

 the lower lip (labium) are inserted ; the labium bends and guides 

 the other mouth-parts into the skin. The blood is drawn up the 

 upper lip, the tube being formed by the upper lip, and closed 

 below by the hypopharynx." 



Meinert wrote, in 1881, a copious anatomical essay on the 

 mouth-parts of the Diptera, presumably in Danish,* but his views 

 on many points are very original and not corroborated by the 

 examinations of other writers. He employs also a terminology 

 entirely his own. 



The palpi. For purposes of classification the most important 

 organs appertaining to the mouth t are the maxillary palpi, usually 

 spoken of simply as the palpi, as the labial palpi are considered to 

 be represented by the labella; some authors do not agree with this 

 view and think that the labial palpi are entirely absent in this 

 order. Wesche, in fact, has in recent times contested that either 

 maxillary or labial palpi may be present and functional, but no 

 cases are known of both pairs being functionally present. 



The palpi in the vast majority of Diptera are either long and 

 composed of four joints, as is the case with the great bulk of the 

 NEMATOCERA, or quite short and composed of two joints, the first 

 being very small, as is the case in the bulk of the remaining 

 families. Occasionally a 5th joint is evidently present, though 

 its existence has been generally denied till quite recently ; in 

 these cases the conical protuberance of the proboscis, upon which 

 the palpi are in many instances inserted, is sufficiently differ- 

 entiated to form a distinct basal joint, but such instances are not 

 common. In two or three Oriental species of Phlebotomus this 

 basal joint is present. It seems hardly necessary to note that 

 the joints are counted from the base outwards, so that in a 

 2-, 4-, or 5-jointed palpus the last or terminal joint is the 2nd, 

 4th, or oth respectively. In size, shape, and relative length of 

 the joints they exhibit great diversity, a considerable amount of 

 variation being found in the same genus, frequently a single joint 

 being enormously enlarged, or peculiarly formed. 



Generally situated at or very near the base of the proboscis, 

 the palpi are in rare instances placed at its middle (Geranomyia), 

 or tip (Elepliantomyia) ; in these cases the proboscis is very con- 

 spicuously elongated. Williston says "the tendency in Diptera 



* I have not seen the work. It is called ' Fluernes munddele,' 91 pp., 6 pi. 

 Stockholm, 1881. 



t In the description of the mouth-parts I am much indebted to Prof. Wil- 

 liston's admirable manual on North American Diptera (3rd Ed.), and inoet of 

 the quoted passages are from that work. 



C2 



