360 
Transactions of the Society. 
The significance of this increase in pigment spots becomes 
obvious on recalling Miall’s hint concerning their possible mission. 
He says : “ The pigment upon the air sacs of Corethra larva may 
conceivably help in the respiratory process. Pigment is not 
unusual in the respiratory organs of insects. The large tracheal 
trunks of some dragon-fly larvse are tinged with purple, and the 
tracheal gills of some caddis worms show a pale violet colour. Gas 
may be secreted by a chemical process in which pigment plays its 
part.” 
With pupation well advanced, we find on dissection the forma- 
Fig. 30. — Showing one main pupal tracheal tube complete, the other 
partially developed, both arising from valve ends of posterior sacs. 
tion of the main pair of pupal tracheal tubes arising from the valve 
ends of the posterior pair of sacs. 
We have been successful in taking the larva when it is half 
larva and half pupa. With the eyes of the pupa towards the 
observer, the left-hand tracheal tube was complete, whilst the right- 
hand was only two-thirds completed (fig. 30). On another occasion 
the tracheal tubes were both completed, but the discharging gas 
from the right-hand sac had only reached half way up the tube 
(fig. 29). We also noticed a change in position of the pigment 
spots, which are now somewhat small, oval, and densely-black 
bodies. These are distributed from the posterior sacs along the 
EXPLANATION OF PLATE XIX. 
* Fig. 23. — Transverse section of sac, showing hoop complete, suggesting 
invagination — not part of spiral thread. Leitz § obj. NA. 40, No. “ O.” E.P. 
Wratten yellow screen, x 80. Exp. 2 sec. 
* Fig. 24. — Part of hoop, displaying overlapping spiral ribbon of tissue. 
Leitz ^ fluorite NA. 1-30, KI. screen, x 500. Exp. 1 sec. 
* Fig. 25. — Hoops of the air sac buckling under pressure into short rods. Zeiss 
D £ NA. 65 Ilford panchro., F. screen, x 240. Working aperture 0-50; exp. 
2£ min. 
* Fig. 26. — Portion of longitudinal section of air sac, showing connecting tissue 
with parts of embedded hoops. The large granular patches are pigment spots. 
Leitz fluorite NA. 1 • 30, H. screen, x 1000. Exp. 20 sec. 
t Fig. 31. — Longitudinal section, showing integument, outer layer darkly 
stained (iron hsematoxylin), indicating it is probably a covering of a fatty nature. 
Leitz ^ fluorite NA. 1‘30, H. screen, x 1000. Exp. 15 sec. 
| Fig. 34. — Collapsed air sacs. Swift 2-in. obj. Imperial orth., G. screen, x 24. 
Working aperture 0 - 13; exp. 1^ min. 
* Taken with arc lamp. 
f Taken with thorium disc. 
