M. E. MacGregor 
181 
domestica), and the stable-fly ( Stomoxys calcilrans). Nevertheless some 
care has to be taken in recognising one or two stigmata of the ptycho¬ 
treme-type, for although the convoluted chain is always present its 
recognition may be rendered difficult by the chitin of the “ ring ” 
having become so extended that it lies as a plate below, leaving spaces 
only at intervals where the course of the chain can be seen. A case of 
this sort is met with in the stigmata of the horn-fly ( Haematobia serrala). 
However, by careful focussing there is actually no difficulty in recognising 
the chain even in this case, especially when the attention has once been 
drawn to this not very commonly occurring condition. 
In the ptychotreme-type of stigmata the “ button ” is always present, 
and usually very distinct, and in some cases where the stigma is bounded 
by a very delicate chitinous ring, chitinous processes often run out to 
it like irregularly arranged spokes in a wheel, and no doubt serve as 
supporting pieces. 
The two classes, schizotreme-type and ptychotreme-type, are no 
doubt homologous with one another if the latter be regarded as a simple 
difference leading from a radiant to a convoluted arrangement; and 
this idea is to a degree supported by the fact that there are usually 
three breaks in the course of the convoluted chain, as there are also 
three slits in the other type of stigmata. 
Descriptions of the Stigmata illustrated. 
I will now give a brief description of the individual stigmata here 
illustrated, together with one or two others, beginning with the schizo- 
treme class first. As the typical example of this class the stigmata of 
Liicilia caesar may be selected. 
Lucilia caesar Linn. (Plate XIV, figs. 1 and 2.) 
These stigmata in the fully formed larvae occur as two fan-shaped 
stigmal plates, from 0 - 44 mm.-0'46 mm. in breadth, and lying from 
0 - 40 mm.-0‘48 mm. apart, when the distance measured is from the centre 
of one “ button ” to the centre of the other. The “ button ” in this 
genus is well developed, and the “ ring ” is delicate and sharply delimited 
from the rest of the plate. The slits are three in number, and are crossed 
by from 11-14 separate relatively broad bars, which run from the 
extreme edge of the one side of the slit to the extreme edge of the other. 
Between these bars the gaps are large, fairly equal in size, and clearly 
outlined : their longitudinal measurement being usually greater than 
the breadth of the bars. 
