193] HEAD OF DIPTERA— PETERSON 23 



tion by a transverse constriction (ch), where the ommatidia are wanting. 

 This constriction is also present in Bibio, but in this form it is confined 

 to the caudo-ventral portion of the eye. 



The drawings of the lateral aspects of some heads show a line of 

 dashes or a solid line around the margins of the compound eyes. This 

 line indicates the extent of the infolding of the head-capsule adjacent 

 to the compound eye. This infolding, or ocular sclerite (o. s), is figured 

 only for those species in which it is closely related to the external mark- 

 ings found on the caudal aspect dorsad of the occipital foramen. The 

 influence of this invaginated edge will be more fully discussed later. 



The three ocelli (oc) of the hypothetical head-capsule (Fig. 1) are 

 arranged in the form of a triangle and located on the cephalo-dorsal 

 aspect of the vertex. The median ocellus is in the epicranial suture, 

 somewhat ventrad of the lateral ocelli. In Leia it is in this suture 

 somewhat dorsad of the bifurcation, and the other two ocelli are some- 

 what laterad of it. This location of the ocelli in the Diptera agrees 

 with Comstock's idea concerning the caudal migration of the ocelli in 

 specialized insects. In generalized insects all three ocelli may be on the 

 front or two on the vertex while the median ocellus is on the front. 

 The ocelli in the Hymenoptera and Hemiptera are similar in location to 

 those of the Diptera. 



Leia is the only form studied which has ocelli and a well-marked 

 stem of the epicranial suture. The chitinized, secondary, Y-shaped 

 thickenings on the ocellar triangle of Rhyphus (Fig. 9) and Mycetobia 

 (Fig. 7) should not be confused with the epicranial suture. Three ocelli 

 are present in all other genera of Diptera examined except Oncodes (Fig. 

 53) and Mycetophila, in which there are only two. The median ocellus 

 is wanting in Mycetophila, while the lateral ocelli are small inconspicuous 

 bodies, adjacent to the dorso-mesal margin of the compound eyes (not 

 shown in the figure). The figures show such variations as occur in the 

 various ocellar groups. 



Occiput and Postgenae. — No sutures occur on the caudal aspect of 

 the hypothetical head-capsule (Fig. 73) except the epicranial suture 

 (e. s). This absence of sutures makes it impossible to locate definitely 

 the boundaries of the occiput and the postgenae. The following in- 

 terpretation is based upon a study of the occiput and postgenae of 

 generalized insects, such as the Orthoptera. The occiput comprises all 

 the area dorsad of an imaginary transverse line drawn thru the middle 

 of the centrally located occipital foramen. The areas ventrad of this 

 line and laterad of the mesal membranous areas are the postgenae. The 

 occiput (occ) undergoes a secondary development about the margin of 



