184 
Dipterous Larvae 
Chrysomyia sp. incert. (Text-fig. 2 a.) 
The general arrangements of all parts of the stigmata are similar to 
the stigmata of Sarcophaga , but differ in the manner described, i.e. in 
the angle that the slits make with the median sagittal plane, and in the 
fact they are obliquely parallel with one another. 
The Ptychotreme-type of Stigmata. 
As the typical example of this class, the stigmata of the house-fly 
(Musca domestica) will be taken. In this class the stigmal plate is 
rather different from that found in the schizotreme-type, tending to 
be more circular, or in some cases triangular, than fan-shaped : while 
the position of the “ button ” is also different, since when this occurs 
instead of lying on the “ ring ” it is situated within it, and usually on 
a line with the horizontal diameter, either touching the inner side of 
the “ ring,” or removed some little distance from it. 
Musca domestica Linn. (Plate XV, figs. 7 and 8.) 
In the larva of this fly the “ ring ” is always very heavily chitinised, 
and may be as narrow as in the example shown in fig. 7, or it may be 
very much broader. The periphery of the “ ring ” slopes forward (i.e. 
with reference to the anterior end of the larva), so that the stigmata 
become saucer-shaped, with the concave surface applied to the animal. 
The exterior surface, which may be regarded as the bottom of the 
saucer, is flattened, and it is upon this that the convoluted or undulating 
chain occurs. 
This chain consists of a chitinous net-like band, due to a complicated 
anastomosis of the rods, and is somewhat similar to the arrangement of 
the rods spanning the slits of the schizotreme-type of stigmata. At 
first sight, under the low powers of the microscope, the chain appears 
to be continuous, but on closer observation it is seen to be made up of 
three sections which are very easily distinguished under the higher 
magnifications. Taken as a whole the chains of each stigmatic plate 
have nine complete convolutions in their course; when these are counted 
by the crest of each wave, but under the high magnifications again, it 
can be seen that they occur actually as three convolutions to each of 
the three sections. 
Breadth of plates, 0'24-0'28 mm.; distance between “ buttons ” 
0‘25-0’30 mm. 
