31 G A BOOK OF INSECTS 



decomposition. All these larvae dwell in mud at the 

 bottom of water, sometimes at a very great depth, and 

 there can be no doubt that their red blood enables them 

 to profit to the utmost by the very scanty supply of 

 oxygen which their environment provides. It is interest- 

 ing to note that there is another type of larva in this 

 midge family which lives at the surface of the water. Its 

 blood is colourless, while the tracheal system, although 

 closed, is more perfectly developed than in the " blood- 

 worm." Moreover, while the pupa of the latter, which 

 only comes up through the water just before the perfect 

 insect emerges, is provided with long gill-filaments, that 

 of the other type has a pair of breathing trumpets (as in 

 the gnat), and floats at the surface. 



The larvae of the pretty river-side beetles which con- 

 stitute the genus Donatio, (family Chrysomelidce) live on 

 the submerged roots of aquatic plants. They are pro- 

 vided with two sharp, tubular processes near the extremity 

 of the body, which they drive into air-spaces in the plant 

 tissue, and extract sufficient oxygen for their needs. They 

 thus derive both food and air from the plants with which 

 they are associated. 



It must not be supposed that the foregoing examples 

 exhaust the list of devices by means of which submerged 

 insects breathe. The fact is that while something is 

 known of this subject, much remains to be discovered. 

 As Professor F. W. Gamble has said, "It is probable 

 that the larval histories of insects will yield many interest- 

 ing additional facts to the known means of respiration, 

 for many flies which require an ample supply of atmos- 

 phere when winged, pass their larval life in surroundings 

 that are almost without oxygen — for example, as parasites 

 in the stomach of the horse (Gastrophilus equi), in wood 

 of trees, and the fleshy substance of nuts, galls, &c." 



