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Osten-Sacken: An essay of comparative Chaetotaxy. 133 
none in Loew’s Platystomina. In some Anthomyiae, I per- 
ceive three (one anterior, two posterior), in some Dexiae and 
Tachinae also three (two anterior, one posterior); in Sarcophaga 
three (one anterior, one posterior and one between them), 
Pteropleural bristles inserted on the ptero-pleura are of rare 
oceurrence and generally diffieult to perceive; Trypeta, for in- 
stance, has a weak bristle, inserted on the pteropleura, under 
the root of the wing. 
Metapleural bristles on the metapleura: they are especially 
conspicuous in the Asilidae, where they form a fan-like row. 
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. 
Application of the terminology of the bristles to the princi- 
pal large divisions of diptera.*) 
Diptera acalyptrata. To illustrate the chaetotaxy of this 
division , I select the genus 7rypeta , as being provided with 
the most complete set of bristles among the Acalyptrata. The 
chaetotaxy of the Trypetid Platyparea poeciloptera may be ex- 
pressed thus. 
Head. Vertical bristles, outer pair but little shorter than the 
inner; postvertical pair small; ocellar pair of moderate size; 
Fronto-orbital (2); lower fronto-orbital (3). 
Ihoracic dorsum. I. Humeral (1), post humeral (2), prae- 
sutural (1). II. Supra-alar (3). 111. A praescutellar pair; 
a second pair more in front, but farther apart. 
*) A arabic numeral, placed in brackets after the name of a bristle 
or bristles, indicates the number of them; the roman numerals, whether pla- 
ced before, or, in brackets, behind, indicate the region of the thoracie dor- 
sum where the bristle is inserted: I. Dorso-humeral region, II. dorso-alar, 
IIl. dorso-central Thus one intra-alar bristle (II), means: one intra-alar 
bristle (dorso-alar region). This addition 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. 
