SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 355 
flow of well-aérated water has apparently become a necessity to it. If 
the larvee are taken out of a stream and placed in a vessel of clear water, 
they soon become sluggish, and in warm weather do not survive very 
long. It matters little however to the larvee whether the water in 
which they live is pure or impure, and streams which are contaminated 
with sewage often contain them in great abundance. There are no 
externally visible organs of respiration, but the skin is supplied by an 
abundant network of fine tracheal branches, which, no doubt, take up 
oxygen from the well-aréated water in which the animal lives. From 
this network at the surface branches pass to supply all the internal 
organs. The Simulium larva is found upon aquatic weeds, and the 
pair of hindfeet, which in Chironomus were shaped so as to enable the 
larva to hold on to its burrow, here become altered, so as to furnish a 
new means of attachment. The two feet are completely united into 
one. The two clusters of hooks found in the Chironomus larva form 
now a circular coronet, and the center of the inclosed space becomes 
capable of being retracted by means of muscles which are inserted into 
it from within. ‘The larva is thus enabled to adhere to the smooth sur- 
face of a leaf, holding on by its sucker, which is, no doubt, aided by the 
cirele of sharp hooks. Efficient as this adhesive organ undoubtedly 
is, it must be liable to derangement by occasional accidents, as for 
instance if there should be a sudden rush of water of unusual violence, 
or if the larva should be obliged to quit its hold in order to avoid some 
dangerous enemy. In the case of such an accident it is not easy to see 
how it will ever recover its footing. Swept along in a rapid current, 
we might suppo.ce that there would be but a slender probability of its 
ever finding itself favorably placed for the application of its sucker 
and hooks. But such emergencies have been carefully provided for. 
The salivary glands, or silk organs, which the Chironomus larva uses 
in weaving the wall of its burrow, furnish to the Simulium larva long 
mooring threads, by means of which it is anchored to the leaf upon 
which it lives. Even if the larva is dislodged, it is not swept far by 
the stream, and can haul itself in along the mooring thread in the same 
way that a spider or a Geometer larva climbs up the thread by which, 
when alarmed, it descended to the ground. 
When the time for pupation comes special provision has to be made 
for the peculiar circumstances in which the whole of the aquatic life of 
the Simulium is passed. An inactive and exposed pupa, like that of 
Chironomus, may fare well enough on the soft muddy bottom of a slow 
stream, but such a pupa would be swept away in a moment by the cutr- 
rents in which Simulium is most at home. When the time of pupation 
draws near the insect constructs for itself a kind of nest, not unlike in 
shape the nest of some swallows. This nest is glued fast to the surface 
of a water-weed. The salivary glands, which furnished the mooring 
threads, supply the material of which the nest iscomposed. Sheltered 
within this smooth and tapering case, whose pointed tip is directed up- 
