178 
Dipterous Larvae 
The next procedure is to put the section in 10 % KOH solution, 
which is raised to the boiling-point and kept boiling gently until the 
specimen is quite free from all attached tissue, and only the chitinous 
structures remain. After removal from the KOH solution, it is washed 
in water thoroughly, dehydrated in the usual way, and cleared in clove- t 
oil. It is then transferred to a drop of balsam on a slide, and arranged 
with the exterior surfaces of the stigmata upwards. A coverglass is 
applied, and gentle pressure administered over the object by means of 
a clip, until the balsam has set, in order to get the object as flat as 
possible. 
The pressure must be most carefully adjusted however, as some 
specimens are extremely delicate. 
The Structure of the Stigmata. 
Owing to the great difficulty of obtaining material in the winter¬ 
time, my observations (which in this paper are given merely in pre¬ 
liminary form) are necessarily confined to comparatively few genera, 
and one is thus unable to predict what degree of variation in form and 
structure of the stigmata may occur in other genera and species, but 
there is no reason to suppose that these would not exhibit equally 
marked variation ; as is borne out to some extent by other examples 
besides those published here, and also by the rough illustrations that, 
here and there, are to be found scattered through the literature on the 
anatomy of dipterous larvae. 
Of these forms of stigmata there are two main types which may be 
classed under the following headings, namely the schizotreme-type and 
ptychotreme-type (see Plate XIV, fig. 1 and Plate XV, fig. 7), and 
these it would seem, from the material studied, coincide individually 
with the main taxonomic relationships of the insects : that is, it 
appears in many cases that related genera have the same main type 
of stigmata, while the one genus is distinguished from the other by 
variations in 
(1) the orientation of the stigmata with reference to the angle made 
to the longitudinal axis of the larva ; 
(2) in the distance between the stigmata themselves ; 
or, (3) by variations of the gross structure, and shape of these organs : 
such as a thickening or otherwise of the chitinous ring that borders them, 
etc. 
The specific differences are to be found in the quite often remarkable 
variations that occur in the finer structure, i.e. in the transverse bars 
