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of the slits, both as to arrangement and number, as well as the variation 
of the position of the “ button ” (vide infra). 
In addition to these two main classes, there are several other forms, 
which occur particularly in the Oestridae. These forms differ from one 
another so remarkably that it will be sufficient to describe them 
individually, as each is quite characteristic of the genus to which it 
belongs. 
The Variability of the Form of the Stigmata. 
In the same genus and species there is apparently remarkably little 
variation in the position and structure of the posterior stigmata in the 
larvae (once these have reached the second instar), but the variation 
in the different genera and species that have been available for study 
is so marked that the most casual observer would have no difficulty 
whatever in recognising them. 
Under the two headings already mentioned, into which the types of 
the stigmata may be classed, a brief description of the most salient 
characters of each will now be given. 
The Scliizotreme-type of Stigmata. 
The vast majority of the stigmata examined belong to this class. 
Essentially they constitute two almost circular chitinous plates, with 
thickened edges, forming distinct rings that enclose the whole organ. 
The plates may be arranged at varying distances from one another 
at the posterior end of the larva, and as nearly as possible on each side 
of the median sagittal plane ; while they may be placed also 
(1) above the median horizontal plane, 
(2) in this plane, and 
(3) below this plane. 
Whatever the actual position in this regard, the slits of the schizo- 
treme-type however seem to be invariably three in number but the angle 
which they make with each other, and with the median sagittal plane, 
is very often, in the different genera and species, quite specific. 
These slits are spanned by delicate chitinous bars, which may pass 
from one side of the slit to the other, either as simple straight rods, or 
as rods having one or more branching processes that anastomose with 
the processes from other rods, and thereby form a network. 
In many of the more complicated examples, examined under the 
microscope, there appear what at first sight might be taken to be nuclei 
