$64 SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 
The possibility of creeping in this manner depends, not upon any 
‘repulsion between the water and the dry surface of the body,” to quote 
an explanation which is often given, but upon the tenacity of the sur- 
face film, which serves as a kind of ceiling to the water chamber below. 
The body of the leech is distinetly of higher specific gravity than the 
water, and falls quickly to the bottom, if the animal loses its hold of 
the surface film. The pond snails however actually float at the sur- 
face, and if disturbed, or made to retract their foot, they merely turn 
over in the water. 
What is the result of all the expedients which have enabled air- 
breathing insects to overcome the difficulties of living in water? They 
have been successful, we might almost say too successful, in gaining 
access to anew and ample store of food. Aquatic plants, minute ani- 
mals, and dead organic matter of all kinds abound in our fresh waters. 
Accordingly the species of aquatic insects have multiplied exceedingly, 
and the number of individuals in a species is sometimes surprisingly 
high. The supply of food thus opened out is not only ample, but in 
many cases very easy to appropriate. Accordingly the head of the 
larva degenerates, becomes small and of simple structure, and may be 
in extreme cases reduced toa mere shell, not inclosing the brain, and 
devoid of eyes, antenne, and jaws. The organs of locomotion also 
commonly afford: some indications of degeneration. Where the insect 
has to find a mate, and discover suitable sites for egg-laying, the fly at 
least must possess some degree of intelligence, keen sense organs, and 
means of rapid locomotion. But some few aquatic insects, as well as 
some non-aquatic species which have found out an unlimited store of 
food, manage to produce offspring from unfertilized eggs, and to have 
these eggs laid by wingless pup or hatched within the bodies of wing- 
less larvie. The development of the winged fly, the whole business of 
mating, and even the development of the embryo within the egg, have 
thus, in particular insects, been abbreviated to the point of suppres- 
sion. This is what [ mean by saying that the pursuit of a new supply 
of food has in the case of certain aquatic insects proved even too sue- 
cessful. Abundant food, needing no exertion to discover or appropriate 
it, has led in a few instances to the almost complete atrophy of those 
higher organs and funetions which alone make life interesting. 
