398 Prof. Miall and Mr. R. Shelford on the Structure and 
7. The development of the imaginal organs. 
As usual in Diptera Nemocera, the rudiments of the 
future legs and wings appear simultaneously as three 
dorsal and three ventral invaginations. ‘The ventral in- 
vaginations give rise to legs ; of the dorsal rudiments the 
prothoracic becomes the pupal respiratory organ, the 
mesothoracic the functicnal wing, the metathoracic one 
of the halteres. ‘he homology of the pupai respiratory 
organ, whether tube or bunch of filaments, with a pro- 
thoracic wing seems to be evident, thongh such a wing is 
unknown in any recent insect. The Carboniferous 
Hphemeridze seem to furnish the best example. 
The invaginations for the antennee of the fly extend 
from the larval antenna to the brain; the rudiments of 
the compound eyes form near their hinder ends. Paired 
labial invaginations (fig. 6) form beneath the cesophagus 
as good-sized oval projections, extending backwards into 
the larval head. 
8. The process of pupation. 
A larva kept in confinement pupated on Oct. 16th. 
Air-bubbles were seen to form upon the processes, and 
when these were detached by shaking, new ones appeared. 
The larva was thus made buoyant, andl floated in a hori- 
zontal position at the surface of the water. Shortly 
before the larval skin opened the spiracles were exposed 
to the air. ‘The skin split along the dorsal surface of the 
prothorax, and the white head-end of the pupa began to 
protrude. ‘The tips of the pupal respiratory tubes were 
brought to the surface of the water, and the body took a 
vertical position in consequence of changed hydrostatic 
conditions. ‘lhe abdomen was alternately flexed and 
extended until the larval skin was slipped off backwards. 
As soon as it fell off and sank, the pupa lay horizontally 
at the surface. ‘The work of extrication occupied a 
quarter of an hour. The pupa was at first pure white, 
except for a pair of bright-red spots on the fourth 
abdominal segment. These spots, which can sometimes 
be seen before pupation through the larval skin, are due 
toa red pigment which forms in the fat-body ; they 
appear in both sexes, but only show through the larval 
skin in male larva. T'wo hours after pupation the pupa 
