157 
placed at the base of the antennae. Most of the Acari have only 
one pair of stigmata, the Oribatidae which bear four pairs proceding 
from the ,,acetabula"' of the legs, (Michael 10 pag. 171), excepted. 
The stigmata are placed in a curve in such a manner that the first 
pair is placed near the lateral margin. The second pair is placed 
more inwardly where the lateral longitudinal band is divided into 
two branches; the third pair is placed. still more inwardly and the 
fourth a little more laterally. 
The stigmata seem to have the following structure. They are 
more or less oblong and limited by a strong chitinous bar. The 
surrounding skin is provided with more or less circular folds. The 
stigma itself is. covered by a more or less complete chitinous roof, 
arising from the wall; it is a short tube the bottom of whieh is a 
thin plate with many small pores. This tube is continued into a 
terminal narrow part of the tracheae. 
The tracheae are extremely fine and difficult to follow in their 
whole length; I have not been able to do so in the material at 
my disposal. Especially the cephalothorax and the muscular sac 
of the intestine are interwoven with a great number of tracheae. 
1 have always found a spiral strueture, fine but distinct. The lumen 
deereases very slowly, they never ånastomose but are provided with 
dichotomous. or other furcations. Two trunks proceed from the first 
pair of stigmata; the one bifurcating near its origin, is directed 
forwards; the other lateral trunk emits a bifurcated branch back- 
wards. Besides these two there is a much stronger bifurcated branch 
directed in- and backwards. The inner branch is directed towards 
the stigma III, the outer one emits two branches. The trunk arising 
from the second pair of stigmata is directed forwards and is in the 
second thoracic segment divided into two branches each of which 
is: bifurcated.. From the wider base a thinner branch is directed 
backwards. The tracheae starting from the third pair of stigmata, 
placed near the inner margin of the outer longitudinal museular 
band is almost as strong. as' the inner branch arising from the first 
pair of. stigmata; it advances without any branches forwards to the 
