186 
Dipterous Larvae 
at a plane so much higher than these structures that when they are in 
sharp focus there is not the slightest indication of the chain’s presence, 
and the microscope has to be racked up several revolutions of the fine 
adjustment before it in turn comes into focus. Here again the con¬ 
voluted chain is broken into three segments which have a similar 
position to those of Stomoxys calcitrans, but differ so markedly in the 
extraordinary arrangement of the convolutions that there is no 
possibility of mistaking the one for the other. The form of one seg¬ 
ment of each is shown in Text-figure 3 a, and the measurements of the 
stigmata are : distance between “ buttons,” 0 - 27-0'28 mm. ; breadth 
of plate, 0'23-0'25 mm. 
Text-fig. 3. Diagram showing the difference in the form of one segment of the convoluted 
chain in the stigmata of (a) Haematobia srrrata and ( b ) Stomoxys calcitrans 
respectively. 
Gastrophilus equi Clark. 
(Plate XV, figs. 9, 10 and Plate XVI, figs. 11, 12.) 
In Gastrophilus equi we meet with a very unusual condition, in that 
the stigma occurs as a single large plate, which may be regarded as 
formed by the fusion of the usual two stigmal plates found in those 
larvae that have been already considered. The fusion may be regarded 
as having taken place at the top and bottom of each of these two plates, 
while the parts between have become modified, and hollowed out, so 
as to leave a gap midway between, into which, one from above and 
one from below, two tongue-bars project (see fig. 9). On each plate, 
and running in sinuous parallel arrangement with one another, are three 
chitinous structures which remind one of the “ slits ” in the schizotreme- 
type of stigmata, and like these are crossed by bars. These bars, 
nevertheless, are more widely set apart, and rarely have a tendency to 
branch in the same way as those of the other types : moreover their 
finer structure is entirely unlike that found in the previously described 
