358 SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 
the air within the siphon is unable to escape. At the time when the 
larva rises to the surface the pointed tips of the flaps first meet the 
surface film and adhere to it. The attached muscles then separate the 
flaps, and in a moment the basin is expanded and filled with air. The 
surface film is now pulling at the edges of the basin, and the pull is 
more than sufficient to counterbalance the greater density of the body 
of the larva, which accordingly hangs from the surface without effort. 
When the larva is alarmed and wishes to descend the valves close, 
their tips are brought to a point, and the resisting pull of the surface 
film is reduced to an unimportant amount. [Living larve shown by 
the lantern. | 
Swammerdam found it necessary in explaining the flotation of the 
larva of the gnat to suppose that the extremity of its siphon was sup- 
plied with an oily secretion which repelled the water. No oil gland 
can be discovered here or elsewhere in the body of the larva, and, 
indeed, no oil gland is necessary. The peculiar properties of the sur- 
face film explain all the phenomena. The surface film is unable to 
penetrate the fine spaces between the flaps for precisely the same rea- 
son that it is unable to pass through the meshes in a piece of gauze. 
After three or four moults the larva is ready for pupation. By this 
time the organs of the future fly are almost completely formed, and the 
pupa assumes a Strange Shape, very unlike that of the larva. 
At the head end is a great rounded mass, which incloses the wings 
and legs of the fly, besides the compound eyes, the mouth parts, and 
other organs of the head. At the tail end is a pair of flaps, which form an 
efficient swimming fan. The body of the pupa, like that of the larva, 
is abundantly supplied with air tubes, and a communication with the 
outer air is still mamtained, though in an entirely different way. The 
air tubes no longer open toward the tail as in the larva, but toward 
the head. Just behind the head of the future fly is a pair of trumpets, 
so placed that in a position of rest the margins of the trumpets come 
flush with the surface of the water. Floating in this position the pupa 
remains still so long as it is undisturbed, but if attacked by any of the 
predatory animals which abound in fresh waters it is able to descend by 
the powerful swimming movements of its tail fin. 
Not that the descent is without its difficulties. The pupa is not like 
the larva, denser than water, but buoyant. There are two respiratory 
tubes in the pupa, whereas there is only one in the larva, and to these 
two tubes the surface film clings with a tenacity of which only experi- 
ment can give an adequate idea. Will youallow me to give you a little 
more borrowed physics? 4 
If we take a solid body, capable of being wetted by water, and place 
it in water, the surface film will adhere to the solid. If the solid is 
less dense than the water it will float with part of its surface out of the 
water. Under such circmnstances the surface film will be drawn up- 
wards around the solid, and will therefore pull the solid downwards. 
