152 THE NORTHERN MICROSCOPIST. 
on the muslin. ‘They are more difficult to obtain with the bottle. 
The first peculiarity one notices is that mentioned by Cooke. He 
says, “ They differ from all larva with which I am acquainted by 
the habit of remaining suspended in mid-water, seldom seeking 
the surface for purposes of respiration, as in the case of other 
diptera.” Another peculiarity is the rapid half turn they make— 
“Tt is familiar to every pond collector,’ Cooke says, “and has 
been for generations, and yet its life history is not yet fully traced 
through all its stages.” The larva butt at one another when they 
approach pretty closely, but I have not observed them do so to 
the extent he says. In the third segment of the body of the larva, 
which is the broadest part, are two kidney-shaped air sacs. These 
are beautifully marked with parallel lines when examined under 
the microscope, and dotted with a dark pigment. In the ninth 
segment are two similar air sacs, only smaller. The four air sacs 
-undoubtedly act as floats to keep it suspended in the water, but 
are also connected with the breathing system all through. These 
air sacs are the only traces of tracheee found. Dr. Cooke says 
this larva is a most beautiful object with polarized light and sele- 
nites, the muscles and other parts being wonderfully distinct. 
Down the body the ganglionic nervous system is to be seen, the 
nerve fibres vibrating backwards and forwards, and all the details 
of its structure may be made out. 
For further particulars as to the structural arrangements in this 
larva, I refer you to “Ponds and Ditches.” Dr. Cooke does not 
give us any particulars as to its pupa condition. This, however, is 
readily observed. Several of the larva obtained in Feb. changed 
into the pupa state about the middle of March, and continued to 
do so, one or more every day or two, hence I am able to bring 
for your inspection the larva, the pupa, and the perfect insect, male 
and female. 
At our Conversazione I gave our Secretary a trough containing 
two specimens of the larva to throw on the screen. While in the 
trough one of them assumed the pupa state, and we had therefore 
enlarged before us on the screen, larva, pupa, and larval skin. 
The larva appears to get rather darker before changing, and when 
the transformation has taken place it no longer floats horizontally 
in the water but in a perfectly upright position; the body is 
shortened ; the two air sacs in the third segment are now external, 
and stand upright, being connected with the tracheal system, now 
much developed. ; 
The two air sacs in the 9th segment have disappeared, and are 
represented by two external terminal plates connected with the 
tracheal system. These also appear to act as paddles, and enable 
the pupa by bending the body and straightening it again to strike 
the water and force itself along in jerks. 


