HEAD CAPSULE AND MOUTH-PAETS OF CHLOKOPS T^NIOPUS. 399 
On the Morphology o£ the Head Capsule and Mouth-parts o£ Chlorops 
tceniopus Meig. (Diptera). By J. Gr. H. Frew, M.Sc, Ministry of 
■ Agriculture Research Scholar. (From the Entomological Department, 
Rothamsted Experimental Station, Harpenden.) (Communicated by 
Dr. A. D. Imms, F.L.S.) 
(With 7 Text figures.) 
[Read 3rd May, 1923.] 
The following paper deals with a part of the anatomical work done in 
connection with an investigation into the life-history and bionomics of the 
Gout-fly of Barley {CJdorops tceniopus) which is being carried on at Rot- 
hamsted under the direction of Dr. A. D. Imms. Considerable attention has 
recently been directed to the Dipterous head capsule owing to Peterson's 
valuable monograph on the subject (3), and as the results of the present 
investigation seem to necessitate important modifications of some of the 
homologies put forward by Peterson, it has been thought advisable to deal 
with the morphology of the head capsule of Chlorops in a separate paper. 
Peterson's nomenclature is followed in all cases unless there is very strong 
evidence that it is incorrect. 
The Tentorium and the Epicranial Suture. 
The location of the epicranial suture is of fundamental importance in 
determining the homology of the regions of the head capsule, and the' 
position of the invaginations of the arms of the tentorium is^ in cases of 
doubt, the safest guide to the position of the epicranial suture. The epicranial 
suture is typically J^-shaped. The stem represents the line of junction of the 
paired sclerites of the head, while between the arms lie the unpaired sclerites, 
the frons, clypeus, and labrum. Except in a few forms (Peterson, p. 15) the 
stem of the epicranial suture is wanting in Diptera. The tentorium consists 
of an arrangement of chitinized rods and plate-like structures. These arise 
from three pairs of invaginations on the head — the openings of the posterior, 
anterior, and dorsal arms of the tentorium. The invaginations of the posterior 
arms are situated one on each side of the occipital foramen and are joined by 
a transverse chitinous bar, forming the body of the tentorium and dividing 
the occipital foramen into dorsal and ventral halves. " The invaginations of 
the anterior arms are usually associated with the lateral margins of the 
clypeus, with one of the points of articulaiion of the mandibles, and frequently 
with the ventral ends of the arms of the epicranial suture. The invaginations 
of the dorsal arms are associated with the points of attachment of the antenna; 
