350 SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 
they breathe by occasionally taking in atmospheric air. In yet more 
specialized members of the same order we find that the larva inhabits 
the mud at the bottom of the stream, and depends for its respiration 
entirely upon oxygen dissolved in the water. The motive is usually 
that the larva may get access to the decaying vegetable matter found 
in slow streams, but some of these larve have carniverous propensi- 
ties. 
Other insects merely dive into the water, coming up from time to 
time to breathe, or skate upon the surface. 
Nearly every order of insects contains aquatic forms, and the total 
number of such forms is very large. I believe that all are modifications 
of terrestrial types, and it is probable that members of different fami- 
lies have often betaken themselves to the water independently of one 
another. 
The difficulties which aquatic insects have to encounter begin with 
the egg. It is in most cases convenient that the egg should be laid in 
water, though this is not indispensable, and the winged, air-breathing 
fly is as a rule ill fitted for entering water. Some insect eggs hatch if 
they are merely scattered, like grains of sand, over the bottom of a 
stream, but others must be laid at thé surface of the water, where they 
can gain a sufficient supply of oxygen. If the water is stagnant it will 
suffice if the eggs are buoyant, like those which compose the egg raft 
of the gnat, but this plan would hardly answer in running streams, 
which would carry light, floating eggs to great distances, or even sweep 
them out to sea. Moreover, floating eggs are exposed to the attacks 
of hungry creatures of various kinds, such as birds or predatory insect 
larve. ‘These difficulties have been met in the cases of a number of 
insects by laying the eggs in chains or strings, and mooring them at 
the surface of the water. The eggs are invested by a gelatinous envel- 
ope, which swells out the moment it reaches the water into an abun- 
dant, transparent mucilage. This mucilage answers more than one 
purpose. In the first place it makes the eggs so slippery that birds or 
insects can not grasp them. It also spaces the eggs, and enables each 
to get its fair share of air and sunlight. The gelatinous substance 
appears to possess some antiseptic property, which prevents water 
moles from attacking the eggs; for, long after the eggs have hatched 
out, the transparent envelope remains unchanged. The eggs of the 
frog, which are laid in the stagnant water of ditches or ponds, float free 
at the surface, and do not require to be moored. The eggs of many 
snails are laid in the form of an adhesive band, which holds firmly to 
the stem or leaf of an aquatic plant. Some insects, too, lay their eggs 
in the form of an adhesive band. In other cases the egg chain is moored 
to the bank by a slender cord. 
The common two-winged fly, Chironomus, lays its eggs in trans- 
parent cylindrical ropes, which float on the surface of the water. 
During the summer months these egg ropes, which are nearly an 
