56 ILLINOIS BIOLOGICAL MONOGRAPHS [226 



ture of the head. The relation between the invaginations of the ten- 

 torium and the movable appendages of the mouth, wliich is so important 

 a feature of all generalized insects, is also characteristic of the members 

 of this order. 



The development of the antennae from a generalized filiform type 

 to that found among the Cyclorrhapha can be traced on the figures. 



Only a few generalized Diptera have mandibles. These are only 

 present in the females except in Simulium, in which they are well 

 developed in both sexes. 



All Diptera having functional mouth-parts have maxillae. The 

 maxillae of generalized Diptera resemble the maxillae of generalized 

 insects except for the absence of palpifers and the fusion of the cardines 

 and stipites with the head-capsule. The maxillae undergo considerable 

 modification, and are reduced to a mere ental rod and a palpus in the 

 Calyptratae. 



The labium is the most characteristic and specialized appendage of 

 the mouth, and shows modifications due to reduction and membranous 

 development. The palpigers and labial palpi are always wanting. The 

 submentum and mentum are represented by a membranous area of the 

 caudal surface of the head. The ligula, or the movable portion of the 

 labium, has a basal part which usually gives rise to two large bulb-like 

 paraglossae and to glossae situated between them. The paraglossae are 

 specialized, and have chitiuized areas on their lateral and caudal sur- 

 faces and pseudotracheae on their mesal surface. 



The parts of the epipharynx and the hypopharynx can be homolo- 

 gized with the corresponding parts in generalized insects. There is a 

 great similarity in the form of the epipharynx and hypopharynx of all 

 Diptera, which is especially striking when considered in connection with 

 the modifications that have taken place in all other parts. 



The various mouth-parts show striking modifications thruout the 

 order, but all, including the epipharjrax and the hypopharjmx, retain 

 their relative positions, even tho they may be extruded from the head- 

 capsule for a considerable distance, as in some of the Calyptratae. The 

 proboscis of the Cyclorrhapha is composed of the labium, maxiUae, 

 hypopharynx, labrum-epipharynx, and tormae. The paraglossae of the 

 labium form the large lobes, or labeUae, at its distal end. 



The mouth-parts of Oncodes and Gastrophilus are not functional, 

 and are so greatly reduced that it is difficult to homologize their parts. 



