SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS.* 
By brore lb; ©. MIALL. 
We understand insects to be animals of small size, furnished with a 
hard skin and six legs, breathing by branched air tubes, and commonly 
provided in the adult condition with wings. The animals thus organ- 
ized are pre-eminently a dominant group, as is shown by the vast num- 
ber of the species and individuals, their universal distribution, and their 
various habitat. 
The insect type, like some fruitful inventions of man—paper or lith- 
ography, for instance—has proved so successful that it has been found 
profitable to adapt it to countless distinct purposes. I propose to con- 
sider one only of its infinitely varied adaptations, viz, its adaptation to 
aquatic life. 
There are insects which run upon the earth, insects which fly in the 
air, and insects which swim in the water. The same might be said of 
three other classes of animals, the three highest, viz, mammals, birds, 
and reptiles. But insects surpass all other classes of animals in the 
variety of their modes of existence. Owing to their small size and hard 
skin they can burrow into the earth, into the wood of trees, or into the 
bodies of other animals. There are some insects which can live in the 
water, not as the mammal, bird, or reptile does, coming up from time 
to time to breathe, but constantly immersed, like a fish. This is the 
more remarkable because insects are, as a class, air-breathers. Air 
tubes or tracheze, branching tubes, whose walls are stiffened by spiral 
threads, supply all the tissues of the body with air. That such an ani- 
mal should be hatched in water and live almost the whole of its life 
immersed, a thing which actually happens to many insects, is a matter 
for surprise, and implies many modifications of structure, affecting all 
parts of the body. 
The adaptation of insects to aquatic conditions seems to have been 
brought about at different times, and for a variety of distinct purposes. 
Many dipterous larve burrow in the earth. Some of these frequent 
the damp earth in the neighborhood of streams. Others are found in 
earth so soaked with water that it might almost be called mud, though 
*Evening discourse, delivered before the British Association, Cardiff, 1891,— 
(From Nature, September 10, 1891: vol. xLiv, pp. 457-461. ) 
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