October 1S92.J 



PS T CHE, 



331 



curved seam is close to the edge of the 

 mouth 011 each side, the long vibrissa on 

 the vibrissal angle is close over or on 

 the edge of the mouth (Phorocera) ; if, 

 however, the curved seam ends high 

 above the edge of the mouth (Macron- 

 ychiu, Phasia, Oestrus), the vibrissal 

 angle and the long vibrissa (or if the 

 latter is lacking, the angle is distinct) 

 are high above the mouth, and the 

 angle forms sometimes the lower edge 

 of the antennal groove. The vibrissal 

 ridges are therefore sometimes long, 

 sometimes short or absent (Oestridae). 

 These variations have their effect on 

 the clypeus. When the vibrissal angle 

 lies low down, the clypeus generally 

 readies far below, and runs out past it 

 or ends between the two ; if the vibris- 

 sal angle rises higher, it is apparent 

 that it is not correlated with the edge of 

 the mouth, as that was before between 

 it and the facial depression, but it may 

 stand much higher than the edge of the 

 mouth (the lower edge of the clypeus), 

 and the latter run through between the 

 angles, or separate them with a nose 

 like ridge, or the clypeus above the edge 

 may by their convergence (re-entrant 

 vibrissal angles in Macronychia, Dexia, 

 and others) be narrowed before its end, 

 or in the middle, or entirely above 

 (clypeus biscuit-shaped or half biscuit- 

 shaped). If the vibrissal angle ends 

 high up and bounds the antennal groove 

 below and on the outside, then the 

 downward-reaching clypeus is set oft' 

 from the cheek edges — extending for- 

 ward in this case— by the angle, and 

 forms, if narrow, a facial ridge, furrow 



or groove (Dexiosoma, Oestrus) ; or, 

 if it broadens below the antennal 

 grooves, it forms a smooth or convex 

 facial shield (Hypoderma). 



The region bounding the oral aper- 

 ture on the side is called the cheek, and 

 its so-called breadth is really its hight 

 in a profile view. Compared with the 

 vertical diameter of the eye, it is gener- 

 ally called broad, if it measures 1-3 or 

 more of the latter; and narrozu. if it 

 measures 1-4 or less. The breadth is 

 also the diameter of the cheek from the 

 lower edge of the eye to the lower edge 

 of the head, measured in profile while 

 the posterior margin of the eye is in a 

 vertical position. 



Bristles on the edges of the frontal 

 stripe from the vertical area to that of 

 the upper cheek (sides of face) are 

 called frontal bristles or stripe-bristles; 

 if they are in several rows, the super- 

 numerary rows are on the orbital region, 

 and we may speak of "several-rowed 

 frontal bristles." Those on the ocel- 

 lar triangle are ocellar bristles. Longer 

 ones, at the extreme top of the head 

 near the ocellar triangle, are vertical 

 bristles. Single or paired bristles, or 

 several in a group, on the periorbit 

 near the edge of the eye below the lat- 

 eral vertical bristles are called orbital 

 bristles. They occur mostly in the 

 female, but often in both sexes, are 

 rarely entirely lacking, or are repre- 

 sented by rows of finer inconspicuous 

 bristles (many Muscidae and Phasii- 

 dae). As a rule the orbital bristles 

 are strong and bent downward, rarely 

 upright or belt outward. Bristles sit- 



