L. Harrison 
103 
moulted individuals of darker species, good results may be obtained 
by examining the living insect under the pressure of a small cover- 
slip, without any mounting medium. Menacantlius pallidulus of the 
domestic fowl is a favourable species for examination in this way. 
In addition to these methods for examination of the whole insect, 
I have made use of both dissections and serial sections in studying 
the stigmata, and the latter have also been used to verify certain other 
points, and for minute structure. 
Tracheal System of MriiSjDJCA cucullaris. 
As a basis for the comparisons which will follow, I give a detailed 
account of the distribution of tracheae in Myrsidea cucullaris. Figure 1 
shows their general disposition in an immature individual of the second 
instar. The only difference between this stage and the adult is that 
in the latter the minor branches are more numerous, and their rami¬ 
fications wider. The alimentary canal is indicated in light stippling, 
which has been varied sufficiently to differentiate the several regions. 
It will be seen that the tracheae are disposed in two main longi¬ 
tudinal trunks; and that they open upon the surface by seven pairs 
of stigmata, one in the prothorax, and six in the abdomen. The 
prothoracic stigmata are ventral, behind the middle of the segment, 
at the junction of the lateral flange with the thicker median portion. 
The six pairs of abdominal spiracles pierce the pleura of segments 
3-8, the first five pairs being dorsal, the sixth ventro-lateral. 
The main trunks are not straight, but are bent outwards to form 
a sharp angle opposite each stigma, the two being connected by an 
irregularly sinuous stigmatic branch. Similar branches occur in the 
first and second abdominal segments, but end blindly. They evidently 
represent the stigmatic branches of these segments, the spiracles 
themselves having disappeared. On entering the thorax the main 
trunks converge on either side of the crop, then diverge sharply to the 
prothoracic stigmata. The trunks coming from the head meet those 
of the thorax and abdomen at a little distance from these stigmata, 
and the two sets are connected by short cross-pieces so as to form a 
complete triangle on either side. The tracheae forming these triangles 
are the largest in the body. The main trunks on entering the head 
diverge about the sides of the brain, coming together a little at the sides 
of the tentorium which is here present, and are continued forwards 
into the dorsal part of the frons, where they break up into many fine 
