54 ILLINOIS BIOLOGICAL MONOGRAPHS [224 



parts show a greater reduction than in Gastrophilus, while in species 

 of Bulonchus (Fig. 364a) they are well developed. In Oncodes a chi- 

 tiuized ring is present in the membrane which covers the oral cavity, 

 and a broad plate extends dorsad from its caudal margin, giving rise 

 to a small membranous tube, the oesophagus, which has no opening to 

 the exterior as far as could be determined. It is impossible to homolo- 

 gize the parts within the oral cavity. The ental plate which gives rise 

 to the oesophagus, may be homologous with the basal portion of the 

 epipharynx and the hypopharynx. 



A general survey of the epipharynx and hypopharynx shows that 

 the relationship between these parts and the head-capsule corresponds 

 with the relationship between the mouth-parts and the head. Since the 

 epipharynx and the hypopharynx are always connected with the labrum 

 and the proximal part of the labium, they are projected ventrad when 

 the labrum and labium are extruded. The interrelation of the mouth- 

 parts and the epipharynx and hypopharynx is fixed, never changing thru- 

 out the order, no matter what specialization may take place. The espe- 

 cially striking feature of the epipharynx and the hpopharynx in various 

 genera which have functional mouth-parts, is the decided similarity of 

 the two thruout the order, as shown by the various figures. The parts 

 undergo secondary changes in their size and shape, but in no case where 

 the mouth-parts are functional is there an entire loss of a part, which, 

 however, happens in many cases with the mouth-appendages. The epi- 

 pharynx and hpopharynx of the Calyptratae in particular show a devel- 

 opment of joints, secondary sclerites, and membranous areas, which 

 permit a considerable amount of flexibility. 



SUMMAEY 



This investigation deals with the homology of all the sclerites of 

 the fixed and movable parts of the head of one or more representatives 

 of fifty-three of the fifty-nine families of the Diptera of North America 

 as listed by Aldrich. With this large series it has been possible to 

 make clear a number of little-understood relationships and structural 

 modifications in the head and mouth-parts, and also to point out their 

 homology with the corresj^onding parts and areas in insects of other 

 orders. The six hundred and more figures show the form and structure 

 of all the parts for each of the families studied. 



Modifications of the fixed and movable parts usually take the form 

 of reduction, change of shape, loss of chitinization, or expansion of the 

 membranous areas. The different parts have been discussed separately, 

 and a hypothetical or typical form has been constructed for each part. 



