580 SUMMARY OF CUIUIENT RESEARCHES RELATING TO 



Head-capsule and Mouth-parts of Diptera.* — Alvab Peterson has 

 made an elaborate study of the sclerites of the fixed and movable 

 parts of the head in one or more representatives of fifty-three of the 

 fifty-nine families of North American Diptera. There are more than 

 600 figures. The chief diversities are due to reduction, change of 

 shape, loss of chitinization, or expansion of the' membranous areas. 

 One of the most important conclusions concerning the generalized head- 

 capsule relates to the position of the epicranial suture. The stem of 

 this suture along the dorsal median line represents the line of fusion of 

 the paired sclerites of the head, while the arms of the suture ventral to 

 the antennal fossae enclose the unpaired sclerites of the head. In all 

 but one or two genera the frons and clypeus form a continuous fronto- 

 clypeus. The labrum is distinct from and ventral to the fronto-clypeus ; 

 it is joined to the epipharynx. 



The tormse are chitinized lateral pieces of the epipharynx which 

 project anteriorly and unite with the fronto-clypeus in generahzed 

 Diptera. They are also present in such generalized insects as the 

 Orthoptera. In the more specialized Diptera the tormsfi are interpolated 

 between the fronto-clypeus and the labrum, and in all but a few genera 

 lose all connexion with the chitinized portions of the fronto-clypeus. 

 Their exposed surface is best seen from a cephalic view. 



The crescent-shaped frontal suture dorsal to the antennal fossa 

 marks the line of invagination of the ptiUnum, the origin of which has 

 not been determined. The vertex is the paired continuous area on the 

 cephalic aspect of the head, and the region of the vertex ventral and 

 median to each compound eye is a gena. 



The compound eyes show secondary sex-characters in a greater 

 number of species than do any other of the fixed and movable parts. 

 The three ocelli form a triangle. A description is given of the occiput, 

 the postgense, the tentorium and its invaginations. 



The evolution of antennae is traced from a generalized filiform type 

 to that found among the Cyclorrhapha. Only a few generalized Diptera 

 have mandibles. These are confined to the females, except in Simiilium, 

 where they are well-developed in both sexes. All Diptera having 

 functional mouth-parts have maxillse ; there are no palpigers, the 

 cardines and stipites are fused to the head capsule ; the maxillae show 

 considerable diversity, and are reduced to a mere ental rod and a palpus 

 in the Calyptratje. 



The labium shows considerable diversity, due to reduction and 

 membranous development. Palpigers and labial palps are always 

 wanting. The submentum and mentum are represented by a mem- 

 branous area of the posterior surface of the head. The basal part of 

 the ligula usually gives rise to two large bulb-like paraglossas and to 

 glossse situated between them. The paraglosste are specialized, and 

 have chitinized areas on their lateral and caudal surfaces and pseudo- 

 tracheaB on their mesial surface. Epipharynx and hypopharynx are 

 very uniform. The proboscis of the Cyclorrhapha is composed of the 

 labium, maxillae, hypopharynx, labrum-epipharynx, and tormte. The 

 paraglossEe of the labium form the distal labellie. The mouth-parts of 



• Illinois Biol. Monographs, iii. (1916) pp. 1-112 (25 pla.)- 



