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Östen-Sacken: An essay of comparative ‘Chaetotaxy. 127 
II. The terminology of the bristles. 
I. In studying the chaetotaxy of the diptera it was natural for me to begin 
with those families, in which the arrangement of the bristles is the 
most simple, that is, with the Diptera Acalyptrata. After having ad- 
opted a terminology for these, the next step was to ascertain, how far 
it was applicable to the other great divisions of diptera. I found that 
the homologies were to striking that there was no great diffieulty in 
applying the same nomenclature of bristles to all the large divisions of 
the order. The terminology given below refers therefore, first of all, to 
the Acalyptrata (especially the Ortalidae and Trypetidae); but at the 
end of this paper a brief sketch of its application to the prineipal other 
divisions is given. 'This sketceh is necessarily approximative, and not ex- 
haustive; many other characteristie hairs and bristles remain, in each 
family, to be observed, described and, perhaps named; this is left to 
the specialist; my aim was, by indicating the homologies, to insure the 
uniformity of the nomenclature, 
II. It must be borne in mind that in enumerating the lateral bristles of 
LU: 
the head or thorax, one side only is always considered. Thus: two 
fronto-orbital bristles means’ that there are two on each side of the 
head, On the contrary the bristles on the central portions of the head 
or thorax are enumerated in pairs, Thus: vertical bristles, one inner 
and one outer pair, means that there are two vertical bristles on each 
side, one of which belongs to the inner, the other to the outer pair. 
The abdominal bristles do not require any especial terminology; their 
position is easily defined by naming the abdominal segment on which 
they are inserted. Only the marginal bristles may be distinguished 
from the discal, as Rondani has done it (Prodr, III, p. 244). 
In deseribing the bristles on the legs, the rule enuneiated by Prof. 
Mik (in his Dipterologische Untersuchungen, Vienna 1878, p. 3, note) 
should be followed: „On the legs, I distinguish a front and hind side, 
and upper änd under side. When a leg is stretehed out horizontally, 
and perpendieularly to the longitudinal axis of the body, the front side 
is that which is turned towards the head; the hind side that which 
looks towards the anus; the meaning of upper and underside follows 
as a matter of eourse, — I call praeapical bristles those which oceur 
prineipally on the front side of the hind femora of many Dolichopodi- 
dae; single, elongated, ereet bristles.“ The same term of praeapical 
bristle was used by Dr. Schiner for a characteristie bristle at the end of 
the tibiae in some Acalyptrata (especially Sciomyzidae). Thus the 
former would be the femoral, the latter the tibial praeapical bristle. 
