Jan., 1892. 
THE BRITISH ASSOCIATION AT CARDIFF. 
15 
a corresponding expenditure of power. In the corresponding 
case of a wetted body trying to emerge from a liquid, 
it is perhaps not so well realized that the same resist¬ 
ance is experienced as in the case of the body resting 
on its surface. Now, to apply this to the study of aquatic 
insects, let us take the case of the common gnat. The 
larvte of this insect are commonly to be found in large 
numbers in rain-water tubs in the country. When undis¬ 
turbed, they may be seen generally hanging head downwards 
at the surface of the water. Now the reason of this is that 
the animal is an air breather, and it obtains its supply by a 
tube from near its tail, whilst its head is busy below gather¬ 
ing its food out of the water. If alarmed, however, it dives 
with a few vigorous jerks below the surface, it having, as 
will be seen, a higher specific gravity than that of water. 
When it has regained confidence a greater expenditure of 
power is required to bring it to the surface, but the moment 
that its breathing tube has penetrated into the air its efforts 
cease, and it hangs serenely from the little cup which has 
formed itself at the extremity of that tube. Now, how does 
the little animal succeed in pushing this tube against the 
resistance of this surface tension ? Simply by converting the 
end of the tube, for the time being, into a point. There are 
three triangular plates surrounding the end of the tube, 
which, when folded together, form an acute pyramid, which 
penetrates the surface and then instantaneously opens out, 
so as to form the cup before spoken of. In the course of time 
the larva changes to a pupa, and now the conditions are 
altered. No food is taken by the pupa, but air is still required; 
whilst to facilitate the emergence of the perfect winged insect 
when the final transformation takes place, its specific gravity 
becomes less, so that, when at rest, it floats at the surface of 
the water. Moreover, as the perfect insect requires to 
emerge at the thickest part of the pupa, it is necessary that 
this portion of it shall float the highest, and hence two 
breathing tubes are developed from the back of the 
thorax. Now the animal is, of course, subject at times to 
the attacks of enemies from above, but with its decreased 
specific gravity it would be almost impossible for it to loose 
the surface film without some special contrivance, and this, 
according to a recent observer, appears to consist of a pencil 
of hairs, which draw a film of water over the end of each tube. 
A further application of the principle of surface tension is 
seen in the case of water snails. According to Professor 
Miall, it is only with those without shells, the others being 
specifically lighter than water ; but I have noticed it occur¬ 
ring with both kinds. If these animals be kept in an aqua- 
