SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 363 
In the case of very small insects it becomes possible not only to run 
on the surface of the water but even to leap upon it,as upon a table 
This is particularly well seen in one of the smallest and simplest of all 
insects—the little black Podura, which abounds in sheets of still water. 
The minute and hairy body of the Podura is incapable of being wetted, 
and the insect frisks about on the silvery surface of a pond, just as a 
house fly might do on the surface of quicksilver. This is all very well 
so long as the Podura is anxious only to amuse itself, or move from 
place to place, but it has to seek its food in the water, and, indeed, the 
attractiveness of a sheet of water to the Podura lies mainly in the 
decaying vegetation far below the surface. But, if the insect is thus 
incapable of sinking below the surface, how does it ever get access to 
its submerged food? I have endeavored to arrive at the explanation 
of this difficulty by observation of Poduras in captivity. If you 
place a number of Poduras in a beaker half full of water, they are 
wholly unable to sink. They run about and leap upon the surface, as 
if trying to escape from their prison, but sink they can not. I have 
chased them about with a small rod until they became excited and 
much alarmed, but they were wholly unable to descend. Even when 
large quantities of alcohol were added to the water the dead bodies of 
the Podura are seen floating at the top, almost as dry as before. It is 
only when they are placed upon the surface of strong aleohol that the 
dead bodies become wetted, and after a considerable time are seen to 
sink. How, then, does the Podura ever descend to the depths where its 
food is found? 
I found it an easy matter to make a ladder, by which the Poduras 
could leave the upper air. A few plants of duck-weed introduced into 
the beaker enabled them at pleasure to pull themselves forcibly through 
the surface film, and climb down the long root hanging into the water 
like arope. Once below the surface, the Podura, though buoyant, is 
enabled, by muscular exertion, to swim downwards to any depth. 
Other aquatic insects, not quite so minute as the Podura, experience 
something of the same difficulty. A Gyrinus, or a small Hydrophilus, 
finds it no easy matter to quit the surface of the water, and is glad of 
a stem or root to descend by. 
To leave our aquatic insects for a moment, we may notice the habit 
of creeping on the under side of the surface film, which is so often prac- 
ticed by leeches, snails, eyclas, ete. I find this is often described as 
creeping on the air, and some naturalists of the greatest eminence 
speak of fresh-water snails as creeping ‘on the stratum of air in con- 
tact with the surface of the water.”* The body of the animal is, nev- 
ertheless, wholly immersed during this exercise, as may be shown by a 
simple experiment. If Locopodium powder is sprinkled over the water 
the light particles are not displaced by the animal as it travels beneath. 
*Semper’s ‘‘ Animal Life,” Eng. trans., p. 205, and note 97, 
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