213] HEAD OF DIPTERA — PETERSON 43 



membranous glossae (gl) which are located between the paraglossae. 

 The palpigers and labial palpi are wanting, but if in the future some 

 form is discovered wiiich shows these structures, they will undoubtedly 

 be found on the area here described as the ligula. The proximal portion 

 of the ligula has a decided furrow or thickening on its caudal aspect 

 along the meson. This thickening is characteristic of a number of 

 Diptera and resembles the proximal portion of the ligula of a number 

 of generalized insects. This mesal thickening marks the line of fusion 

 of the two parts of the labium during embryonic development. 



The above interpretation of the labium is on the whole very satis- 

 factory for the numerous modified types found in the various families 

 of the Diptera, and with this interpretation it is possible to formulate a 

 hypothetical labium. This has been done in this study; but there have 

 been added to this labium the early stages of development of the more 

 important secondary structures wiiich are characteristic of the labia of 

 Diptera. It will therefore be advisable to call such a hypothetical labium 

 a typical labium in order to distinguish it from the true hypothetical 

 type of other parts of this study. 



A typical labium of the Diptera (Fig. 1, 73, 140, 362, and 363) is 

 made up of a submentum, mentum, and ligula. The submentum 

 and mentum (su. me) are firmly united with the head and constitute 

 the greater portion of the mesal membranous area of the caudal aspect 

 of the head. The ligula (Ig) is the large swollen and movable portion 

 of the labium and consists of the mediproboscis (mpr) and the disti- 

 proboscis (dpr). The mediproboscis has a chitinized area on its caudal 

 aspect which is commonly called the theca (the). The distiproboscis is 

 composed of two large membranous bulb-like paraglossae (pgl) and two 

 small membranous glossae (gl) which are located between the proximal 

 parts of the paraglossae. The important and characteristic features of 

 a typical labium are the chitinized pieces on the caudal and lateral as- 

 pects of the paraglossae and the trachea-like structures on the mesal 

 aspects. The details of the various parts will be more fully discussed 

 as each part is considered and its modification traced thruout the order. 



The submentum and mentum (su. me) are present as a membranous 

 area in a majority of the Nematocera and in the females of Tabanus 

 (Fig. 74). This area undergoes considerable modification, as was seen 

 in the discussion of the maxillae and postgenae, and is illustrated by 

 the figures. Rhyphus (Fig. 80 and 374) is apparently the only genus 

 which has within this area a chitinization which can not be considered 

 as a modification of the maxillae or of the postgenae. This piece is a 

 more or less distinctly chitinized, inverted-flask-shaped area between 

 the maxillae. If this is a primary chitinization, it is probably a rem- 



