comparative Chceiotaxy. 511 



Hypopleural bristles.— On the hypopleura ; as far as I 

 have observed, occur only in some of the Diptera 

 Calyptrata, which have a row or a tuft of them. 



III. — Application of the Terminology of the Bristles 

 TO the principal large Divisions of Diptera.* 



piptera Acalyptrata. — To illustrate the chsetotaxy of 

 this division I select the genus Trypeta, as being pro- 

 vided with the most complete set of bristles among the 

 Acalyptrata. The chsetotaxy of the Trypetid Platyparea 

 poeciloptera may be expressed thus : — 



Head : Vertical bristles, outer pair but little shorter 

 than the inner ; post-vertical pair small ; ocellar pair of 

 moderate size; fronto-orbital (2) ; lower fronto-orbital (3). 



Thoracic dorsum : I. Humeral (1), post-humeral (2), 

 praesutural (1). II. Supra-alar (3). III. A praiscutellar 

 pair ; a second pair more in front, but farther apart. 



Pleura: Mesopleural (2), sternopleural (1), ptero- 

 pleural (1, very small). 



Scutellum (4). 



This is the usual type of chsetotaxy in the genus Try- 

 peta in the wider sense ; in describing, therefore, that of 

 any given species it will suffice to say chatotaxy normal, 

 and to indicate the differences. Instead of two fronto- 

 orbital bristles, there is sometimes only one ; instead of 

 three lower fronto-orbital bristles, there are often only 

 two : in some cases there are three dorso-central pairs 

 of bristles, the third being in front of the suture (Loew, 

 Die Europ. Bohrfliegen, p. 5) ; the scutellum has some- 

 times only two bristles, sometimes as many as six. 



If we compare the above formula with the chatotaxy 

 of the Asiatic Trypetid Ptilona, v. d. W., we perceive at 

 once important differences, which lead us to the con- 

 clusion that the genus, although a Trypetid, cannot be 



y An Arabic numeral, placed in brackets after the name of a 

 bristle or bristles, indicates the number of them; the Koman 

 numerals, whether placed before or in brackets behind, indicate 

 the region of the thoracic dorsum where the bristle is inserted : — 

 I., Dorso-humeral region; II., dorso-alar ; III., dorso-central. 

 Thus, one intra-alar bristle (II.) means one intra-alar bristle (dorso- 

 alar region). This adfhtion may seem superfluous, because the 

 term intra-alar bristle already implies that the bristle belongs to 

 the dorso-alar region ; still, I have occasionally used it because I 

 thought that, owing to the novelty of the subject, it would be easier 

 thus to recall the position of the different bristles. 



