OF AQUATIC LAEV^. 
89 
It is highly probable that these assist in the breathing, as 
the large surface exposed to the surrounding fluid would naturally 
cause a good deal of oxygen to be absorbed through the walls, 
but I think it is materially assisted by the broad fan-shaped tail, 
which has a large tracheal tube ramifying through it. 
The manner in which these larvae and pupae maintain them- 
selves suspended in water is of great consequence to their 
preservation, for their near relation, the Chironomus, which is 
always either on the surface or at the bottom of the water, 
is constantly falling a prey to the Planarians, which are roving 
about in search of what they may devour ; being unable to swim 
freely through the water they fail to catch the Coreth^'a larvae or 
pupae, which may be kept in the same glass with Planarians 
without chance of injury, whereas the Chironomus, as I have 
frequently found to my discomfiture, immediately falls a prey to 
them ; this is owing to its habit of falling to the bottom of the 
water, or resting at the top against some weed or the sides of the 
vessel, according to whether he has his tubes full of air, or 
empty. In either of these positions it falls an easy prey, and I 
have no doubt that most other aquatic larvse are troubled with 
the same pest, unless they have some means of protecting 
themselves, such as the case of Caddis worms, a burrow like 
the Ephemera, a means of floating as in Corethra, &c., or 
continually swimming about like Daphne. 
Thus it will be seen that the breathing apparatus in Agrion, 
Libellula, Ephemera, and Corethra, are used not only as such, 
but also as the means of locomotion and of protection from 
their enemies. This beautiful adaptation of means to an end 
cannot fail to strike the most careless observer, and -appears to 
me a good illustration of the variations which circumstances may 
produce in any given organ. 
