172 LIVERPOOL BIOLOGICAL SOCIETY. 
whole larva, with its hairs and jaws, together with the 
lining (‘‘intima’’) of the main tracheal trunks—this last 
is drawn out of the escaping pupa in a series of fragments, 
one pair being drawn out through the hinder thoracic 
stigmata, one pair through the stigmata of each of the first 
seven segments of the abdomen, and the hindermost 
through the larval respiratory siphon, the soft parts of 
which are withdrawn and introverted ito the eighth seg- 
ment of the pupal abdomen. The pupa is about 9 mm. 
long, and consists of head, thorax, and abdomen. 'The 
head and thorax together form a rounded mass somewhat 
compressed from side to side, the head with its appendages 
bent backwards below the thorax, the large wings extend- 
ing downwards and backwards and making the thorax 
appear larger than it really is. The abdomen is slender 
and dorsoventrally compressed. It alone is obviously 
segmented* and is highly flexible in the dorsoventral 
plane, but not from side to side. Its eighth segment bears 
a pair of large oval plates which serve as a propeller. 
The pupa does not feed. It breathes air directly by 
means of its two prothoracic siphons. It has no tracheal- 
gills. The position of its appendages will be easily under- 
stood from figures 1 and 2 (Pl. V.).. The internal struc- 
ture of the young pupa is of course essentially the same 
as that of the advanced larva. 
The pupa floats quietly at the surface of the water in 
the position shown in figure 1, except when disturbed, but 
a gentle tap on the side of the aquarium, a sudden loud 
noise, or other disturbance of the surface of the water, or 
the sudden fall of a shadow or of a bright light upon the 
pupa itself, causes it to dart downwards and backwards 
by powerful flexion of the abdomen. 
* In speaking of the pupa I shall use the names first, second, etc., segments 
to signify first, second, etc., segment of the abdomen. 
