CHAETOTAXY 91 



on the peristomalia, which are ridges on the lower edges of the peristoma, 

 or the cheeks, regions which correspond to those described as the genae. 

 The facial bristles, often very conspicuous, are situa- 

 ted on the front of the head, external to the antennae, 



xix fi* 



and above the vibrissae. The lateral facial bristles, pj ate n^ ^ ^ 



when present, are external to them and on the lower 

 part of the head. The frontal bristles are situated on the upper part 

 of the head, external to the ptilinal suture, on each side of the inser- 

 tion of the antennae, and above and external to the facial bristles ; the 

 rows of the two sides converge towards one another near the vertex 

 of the head. There may be two pairs of vertical bristles, an inner and 

 an outer, at the upper and inner angle of the eye, and there may be 

 post -vertical bristles behind them. In front of the vertical bristles, and 

 behind the frontal bristles, there are three sets of fronto-orbital bristles, 

 upper, middle and lower, the last named being rarely present ; the first 

 are always reclinate, and may appear as a continuation of the frontal 

 rows posteriorly; the second are always proclinate and are generally 

 nearer the orbit. The ocellar bristles are situated in the ocellar triangle 

 at the vertex of the head ; post-ocellar and prae-ocellar bristles may be 

 present. The facio-orbital bristles are situated, when present, on the 

 narrow strip between the eye and the lower half of the ptilinial suture. 

 There may also be bristles on the cheeks. 



On the thorax : Humeral bristles are situated on the humeral callus, 

 and post-humeral ones on its inner margin. The notopleural bristles, 



usually two on each side, are situated on the edge of 



, . Thoracic Bristled 



the praescutum near its posterior borders, that is to say, (Plate XIX) 



between the humerus and the root of the wing. The 

 Prae-sutural bristles are placed near the lateral borders of the praescutum, 

 just in front of the suture between it and the scutum. On the scutum 

 there are the supra-alar bristles, immediately above the root of the wing, 

 usually three in number, and the intra-alar, internal to them. In the 

 middle portion of the thorax there are two rows of bristles on each side, 

 extending through the praescutum and the scutum. The inner two rows 

 are termed the acrostichal bristles, the outer two the dorsocentral. The 

 post-alar bristles lie behind the base of the wing, and behind and below the 

 supra-alar. On the margin of the scutellum there is a row of scutellar 

 bristles, and there may be also others removed from the margin. On 

 the side of the thorax there are the propleural, immediately above the 

 coxae of the forelegs ; the mesopleural, on the mesopleura, near the 

 angle between the dorsopleural and mesopleural sutures (or, using other 



