N"0. 64] DIPTERA OF CONNECTICUT: MORPHOLOGY 15 



actly to the primary sutures deiiiarkin<^ the principal vsclerites of the 

 head in the orthopteroid insects, and are for the most part second- 

 arily-formed lines, occurring only in the Diptera and their immediate 

 relatives. It is therefore inadvisable to use these secondarily-formed 

 lines for delimiting the primary sclerites of the head capsule in adult 

 Diptera, and other structures must be relied upon for this purpose. 



The principal landmarks of value for delimiting the sclerites and 

 areas of the head capsule are the frontal or pretentorial pits, the 

 points of attachment of the antennae, the median ocellus (when pres- 

 ent) and the compound eyes. 



The frontal or pretentorial pits {fp of Fig. 1, A, B, G and L, Fig. 

 3, D, etc.) which are the external apertures of the internal invagina- 

 tions forming the anterior arms of the tentorium, demark the pos- 

 terior limits of the clypeus and the anterior limits of the frons, while 

 the median ocellus, when present, demarks the posterior extent of the 

 frons. The median ocellus lies in the posterior angle of the frontal 

 suture (demarking the posterior limits of the frons) in lower insects, 

 and when the frontal suture is lost, as is the case in most Diptera, the 

 median ocellus may remain, and in such cases serves to mark the posi- 

 tion of the posterior angle of the frontal suture, which formerly 

 marked the posterior extent of the frons. 



Some clipterists mistake the ptilinal fissure {ptc of Fig. 4, J) for 

 the frontal suture of lower insects. The ptilinal fissure, however, is 

 formed by the infolding of the ptilinum, or eversible sac which is pro- 

 truded to push open the top of the puparium wdien the fly emerges 

 from its pupal case, and is therefore a wholly different structure from 

 the frontal suture, formed by the arms of the inverted Y-shaped epi- 

 cranial suture in lower insects (which have no ptilinum), so that the 

 ptilinal suture cannot be used to delimit the frons posteriorly, as 

 these dipterists maintain. 



The points of attachment of the antennae may serve in a general 

 way to divide the frons into a prefrons and a postfrons (^z-/ and fof 

 of Fig. 1, G and L) , and they likewise serve to demark the posterior 

 limits of the region called the face by AValton (1909), wdio defines the 

 face as "that area of the head bounded by the base of the antennae, 

 the oral margin eyes and cheeks." 



The compound eyes divide the vertex into an anterior and pos- 

 terior vertex {av and pov of Fig. 1, F) in the Tipulidae, and in certain 

 holoptic Diptera, the contiguous compound eyes may demark the pos- 

 terior boundary of the frontal triangle or the anterior boundary of the 

 vertical triangle, as will be discussed later. 



3. Sclerites of the head capsule 



The labrum, Ir, which probably represents a portion of the pro- 

 stomium of the annelid ancestor of the Arthropoda, is morphologically 

 a portion of the head capsule, while the mouth parts are modified limbs 

 of the gnathal segments of the head, but since the labruni is function- 

 ally one of the mouth parts in the Diptera, it ^vill be considered under 

 the discussion of the mouth parts. The epipharynx is a modified por- 



