352 SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 
away by the rush of water. All need at least a moderate supply of 
oxygen, which has either to be drawn from the air at the surface, or 
extracted from the water by special organs. The difficulty of breathing 
is of course greatly increased when the larva seeks its food at the 
bottom of foul streams, as is the case with certain Diptera. The larva 
of Chironomus, for example, feeds upon vegetable matter, often in a 
state of decay, which is obtained from the mud at the bottom of slow 
streams, and in this mud the larva makes burrows for itself, cementing 
together all sorts of materials by the secretion of its salivary glands 
drawn out into fine silken threads. The burrows in which the larva 
lives furnish an important defense against fishes and other enemies, 
but they still further increase the difficulty of procuring a supply of 
air. Hence, the iarva frequently quits its burrow, especially by night, 
and swims towards the surface. At these timesit loops its body to and 
fro with a kind of lashing movement, and is thus enabled to advance 
and rise in the water. From the well-aérated water at the surface of 
the stream it procures a free supply of oxygen, which becomes dis- 
solved in the abundant blood of the larva. Four delicate tubes filled 
with blood, which are carried upon the last segment of the body, are 
believed to be especially intended for the taking up of dissolved oxy- 
gen. The tracheal system is rudimentary and completely closed, and 
hence gaseous air can not be taken into the body. The dissolved oxy- 
gen, procured with much exertion and some risk, must be stored up 
within the body of the larva, and used with the greatest economy. It 
is apparently for this reason that the larva of Chironomus contains a 
blood-red pigment, which is identical with the hemoglobin of verte- 
brate animals. The hemoglobin acts in the Chironomus larva, as it 
does in our own bodies, as oxygen-carrier, readily taking up dis- 
solved oxygen, and parting with it gradually to the tissues of the body. 
It is instructive to notice that only such Chironomus larve as live 
at the bottom and burrow in the mud possess the red hemoglobin. 
Those which live at or near the surface have colorless blood, and a 
more complete—though still closed—tracheal system. The larva of the 
varnivorous Tanypus, which is found in the same streams, but does 
not burrow, has a much more complete tracheal system, and only 
enough hemoglobin to give a pale red tint to the body. The larva of 
the gnat, again, which has a large and open tracheal system, and in all 
stages of growth inhales gaseous air, has no hemoglobin atall. <A list 
of the many animals of all kinds which coutain hemoglobin shows that 
for some reason or another each of them requires to use oxygen econom- 
ically. Hither the skin is thick, and the respiratory surface limited, or 
they are inclosed in a shell or they burrow in earth ormud. We might 
expect to find that haemoglobin would always be developed in the blood 
of animals whose respiration is rendered difficult in any of these ways, 
but any such expectation would prove to be unfounded, and there are 
many animals whose mode of life renders it necessary that oxygen 
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