224 DIVERSIONS OF A NATURALIST 



creatures which live in impure water, where free oxygen 

 is very small in quantity, namely, it enables them to 

 absorb and hold by loose chemical combination the 

 small quantity of oxygen available. The minute midges 

 called "Hessian fly"and"Cecidomyia" injurious to cereal 

 crops should be mentioned here as among the allies of 

 crane-flies, as also the blood-sucking midges, Ceratopogon, 

 and the minute blood-sucking sand-flies or Buffalo-flies, 

 called " Simulium." Species of Ceratopogon, so minute as 

 to be barely visible, cause terrible annoyance by their bites 

 to the salmon-fisher in Scotland, where they often swarm 

 in countless numbers. The Buffalo-flies attack man, but 

 in some districts of North America alight in thousands 

 on cattle, and cause death in a few hours. A harmless 

 long-horned fly is " the plumed fly," Corethra, the large 

 aquatic larva of which is glass-like and quite trans- 

 parent, and offers splendid facilities for microscopic 

 research. I used to take it every year in a pond near 

 Hampstead Heath. 



The leather-jackets, or grubs of the common crane- 

 fly (Fig. 22, B), sometimes destroy hundreds of acres 

 even whole districts of grassland in England and 

 France by gnawing the young subterranean roots and 

 shoots of the grass. They also destroy young wheat 

 crops. The leather-jacket is regarded by agriculturists 

 as an intractable pest, since it gets too deep into the 

 turf to be destroyed by chemical poisons. Its thick 

 skin also makes it very resistant to such treatment. 

 When immersed in brine for twenty-four hours the 

 grubs are not killed ; prolonged immersion in water 

 is equally ineffective ; they may be frozen until they 

 are brittle, and will yet recover ; and when kept three 

 weeks without food, still remain alive. Birds are their 

 natural enemies, and rooks not only dig after the grubs, 



