22 
BULLETIN OF THE BUREAU OF FISHERIES. 
The larva uses its mandibles to some extent for digging, biting off chunks of 
earth and thrusting them to one side and backward; but the burrowing is mainly- 
accomplished by thrusting its flattened head forward and then enlarging the hole 
by sidewise and up-and-down movements. After reaching a depth of 2 or 3 
inches in this manner the larva begins a rotary movement of its entire body, swinging 
both ends horizontally around the longitudinal center as an axis. During this 
movement the head and tail are raised until the body assumes the shape of a crescent 
or semicircle, the ventral surface convex, the dorsal concave (fig. 14). As a result 
the pupal chamber when completed is subspherical in shape, the upper portion 
flatter than the lower, and is usually about 40 mm. in diameter. The smoot hn ess 
of the walls depends in large measure on the consistency of the soil. When the 
soil contains the right amount of clay and moisture the walls are very smooth, 
but when the soil is sandy the walls are rough. 
It requires 36 to 48 hours to complete this chamber, but after it is apparently 
finished the larva still keeps up its rotary motion at intervals until it is ready to 
pupate. Pond 2E was drained July 15. From its shores were obtained two pupal 
chambers, apparently just formed and containing larvae. These were transferred 
to the laboratory and kept on moist sand. One larva pupated during the night of 
July 20 and the other during the night of July 23. Both kept up the rotary motion 
until the day before pupation ; then they remained quiet, the head and tail being 
withdrawn from the sides of the chamber and curled over the back. Finally, the 
skin split along the back of the thorax; the head was withdrawn, leaving the hard 
chitinous covering of the larval head, with the antennae, the cornea of the eyes, and 
the mouth parts intact; the abdomen was withdrawn, leaving the larval cerci in 
like manner intact; and the larva was transformed into a pupa. 
After pupation the larval skin, with the head and mouth parts, was pressed into 
the earth on one side of the pupal chamber, and the pupa assumed its characteristic 
position. It may be noted that Lyonet (1832) said of the larva of Hydrous piceus: 
I therefore put it upon freshly turned-up soil, on which I scattered some grass. It made a hole, 
which it lined with grass, and remained within it several days in a curved position, lying on its back. 
This lining of the pupal chamber with grass was probably accidental, some 
being carried in by the larva during its burrowing; but it is worthy of note that the 
larva remained in the chamber lying on its back and that the subsequent pupa 
rested also back downward. In our American species both larva and pupa rest upon 
the ventral surface. 
DESCRIPTION OF PUPA. 
The general form of the pupa is ovoid, narrowed to a rounded point posteriorly 
and strongly flattened dorsoventrally. In dorsal view the prothorax entirely con- 
ceals the head, which is folded down onto the breast, and the knees of the legs are 
just visible at the sides of the body, the first pair opposite the posterior margin of 
the prothorax, the second pair in contact with the first and opposite the groove be- 
tween the pro and meso thorax, and the third pair opposite the posterior margin of 
the first abdominal segment. Only the bases of the elytra and wings are visible, 
the rest of them being carried around the sides of the body onto the ventral surface. 
In ventral view (fig. 16) all the body regions and their appendages are visible. 
