NATURAL HISTORY. 19 
that of a single lens of a quarter of an inch 
focus (viz. forty times linear measure). If the 
instrument is a compound one, or engiscope, 
and a sprig of moss is employed, it must be 
fixed in an inverted position, by a little cement 
or sealing-wax in the aquatic slider, in order to 
appear erect through that instrument. 
I am not aware of any account of the trans- 
formation of this insect in print; indeed some 
consider it as a species of Nais, which does not 
undergo any metamorphosis. From my own 
observations, I find the larve change to pup 
in the month of May. In this state it is shorter 
than in the former, and has some resemblance 
to the pupa of the Tipulidan gnat (Corethra 
plumicornis), figured in the “ Microscopic Ilus- 
trations.” It is, however, devoid of its mem- 
braneous tail, and differs also about the head. 
When about to change, they fix themselves in 
the crevices of some floating body, to assist 
them in casting their exuvie. The pupe are 
rather inactive, and float near the surface of the 
water, with their head erect. Immediately after 
their transformation they are of a pale colour, 
but in a few days they become of a deep brown. 
In this state they may be preserved between 
glass and talc, as also the larve, and the skins 
