POSITION AND TERMINOLOGY OF THE CULICID^] 9 



The head has a hinder plane, opposite the thorax, 

 called the occiput {occiput) ; that region of it lying over the 

 junction of the head is the nape {cervix). The part of the 

 head which reaches from the antennae as far as the occiput, 

 and is limited laterally by the compound eyes, is the front 

 {frons), the upper part of which is the crown {vertex), the 

 limit between the front and the occiput having the name of 

 the vertical margin {margo verficalis). The middle of the 

 front, being often of a more membranous substance, and 

 sometimes differing in colour from its borders, is called the 

 frontal stripe {vitta frontalis). On the crown are the 

 simple eyes {ocelli), which, however, are absent in the 

 Gulicidcv, but in the majority of Diptera are usually three 

 in number, sometimes on a sharply defined triangular space, 

 the ocellar triangle {trianguluni ocellcc). Most of those 

 Diptera which undergo their metamorphosis within the 

 larval skin possess, immediately above antennae, an im- 

 pressed arcuate line, which seems to separate from the 

 front a small, usually crescentic piece, the frontal crescent 

 {lunula frontalis). The impressed line itself, which con- 

 tinues over the face nearly as far as the border of the 

 mouth, is called the frontal fissure (fissura frontalis). It 

 owes its origin to a large bladder- like expansion, which 

 exists at this place in immature imagines, and which helps 

 them in bursting the pupa case. The frontal fissure is, 

 of course, the true anterior limit of the front, and the 

 frontal crescent in fact belongs to the face ; but from its 

 usual situation it is commonly considered as part of the 

 front. In many genera the eyes of the males meet on 

 the front, so as to divide it into two triangles, the upper of 

 which is called the vertical triangle {trianguluni verticale), 

 the inferior, the anterior frontal triangle {triangulmn 

 frontale anterium), or simply the frontal triangle. The 

 anterior portion of the head, reaching from the antennae 

 to the aural margin {'peristomiiLm) is called the face {fades). 

 In most Diptera it is divided into three parts adjoining 

 each other, the limits of which depend on the situation 

 which the frontal fissure, continued to the oral margin, 

 occupies in the developed imago. The form and mutual 



