SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 359 
But if the solid is denser than the water, the surface film around the 
solid will be pulled downwards, and will pull the solid upwards. Sup- 
pose that a solid of the same density as water floats with part of its 
surface in contact with air, and that weights are gradually added to it. 
The result will be that the surface of the water around the upper edge 
of the solid will become more and more depressed. The sides of the 
depression will take a more vertical position, until at last the upward 
pull of the film becomes unable to withstand further increase of weight. 
If this pointis passed, the solid will sink. Before this point is attained, 
we shall have the solid, though denser than water, kept at the surface 
by the pull of the surface film. 
This state of things may be illustrated by a model. [Float with glass 
tube attached to its upper surface.| You will readily see that the float 
has to be weighted appreciably in order to break the connection of the 
tube with the surface film. Now the pupaof the gnat has a pair of tubes 
which are in like manner attached to the surface of the water. When it 
requires to descend, the pull of the surface film would undoubtedly be 
considerable. Adding weight to the body is, of course, impossible, and a 
great exertion of muscular force would be wasteful of energy, even if it 
could be put forth. The gnat deals with its difficulty in a neater way 
than this, and saves its muscular power for other occasions. Let me show 
you a method of freeing the float from the surface, which was suggested 
by observation of what was seen in the pupa of the gnat. <A thread 
wetted with water is drawn over the mouth of each tube. It cuts the 
connection with the surface, and the float, loaded so as to be denser 
than water, goes down at once. Meinert has described a pencil of hairs 
which appear to perform the same office for the pupa of the gnat. The 
hairs draw a film of water over the open mouth of each respiratory tube, 
and muscular contraction, used moderately and economically, does the 
rest. When the pupa again comes to the surface the tubes are over- 
spread by a glistening film of water. This is partially withdrawn by a 
movement of the hairs, so that a chink appears by which air can be 
slowly renewed. When the insect is completely tranquil, the hairs 
appear to withdraw more completely, and the tube suddenly becomes 
free of all film. The act of opening or closing the film is so rapid—like 
the wink of an eye—that I can not pretend to have observed more than 
the closed tube, the slightly open tube, and then the sudden change to 
a completely open condition. [Living pup: shown by the lantern. | 
Another Dipterous larva described and admirably figured by Swam- 
merdam is the larva of Stratiomys, a larva which, as the structure of 
the fly shows, belongs to an altogether different group from Chirono- 
mus, Simulium, or the gnat. Though only remotely connected with the 
enat in the systems of zodlogists, the Stratiomys larva has learned the 
same lesson, and is equally well fitted to take advantage of the peculiar 
properties of the surface film. The tail end of the Stratiomys larva is 
provided with a beautiful coronet of branched filaments. When the 
