V 



I30 NATURAL HISTORY OF AQUATIC INSECTS CH. 



free oxygen. The burrows in which it lives furnish 

 an important defence against Fishes and other enemies, 

 but they still further increase the difficulty of securing 

 a sufficient supply of air. The larva can only freely 

 aerate its blood by leaving its burrow, and swimming 

 about near the surface. The thin-walled and trans- 

 parent appendages near the hinder end of the body 

 are probably of special service in taking up dissolved 

 oxygen. The tracheal system is rudimentary and 

 completely closed, and hence gaseous air cannot be 

 taken into the body. The dissolved oxygen, pro- 

 cured 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 haemoglobin of 

 vertebrate animals. The haemoglobin acts in the 

 Chironomus larva as it does in our own bodies, as an 

 oxygen carrier, readily taking up dissolved oxygen, 

 and parting with it gradually to the tissues of the 

 body. 



It is instructive to notice that only such Chirono- 

 mus larvae as live at the bottom and burrow in the 

 mud possess the red haemoglobin. Those which li\c 

 at or near the surface have colourless blood, and a moiC 

 complete, though still closed, tracheal system. 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 haemoglobin at all. A list of the 

 many animals of all kinds which contain haemoglobin, 

 shows that for some reason or other each of them 

 requires to use oxygen economically. Either the 



