3o8 NATURAL HISTORY OF AQUATIC INSECTS CH. 



Ephemera there, though we should expect to find ' 



them heaped together by the thousand. ' 



" The larva of Pol}'mitarcys rarely swims free in the '< 



water ; it has a burrow excavated in the earthy bank I 



of the river. From the level of the water for two or j 



three feet downwards the bank is everywhere drilled ' 



with holes a quarter of an inch or less in diameter, i 



The holes take for the most part a horizontal direc- ! 



tion. The passage returns upon itself to a second i 



outlet close to the first, as may be seen by slicing a ,] 



clod of the earth horizontally, when a U-shaped j! 



burrow is exposed, the two limbs being separated I 



only by a slight partition of earth, which is often tl 



broken down. The larva can therefore enter or leave i 



its burrow without being obliged to creep backwards. '| 



Earth of the consistence of clay or marl is best suited ;| 



to the purpose ; gravel cannot be made to answer, ii 



The lining of the burrows is much finer than the ;{ 



'i 

 surrounding earth, and appears to be laid down in an 'i 



even laj-er by the larva. The burrow is proportioned \\ 



to the size of its inhabitant ; a full-sized one is more ! 



than a quarter of an inch in diameter, and over two i 



inches long, measured from one end to the other | 



along the curve. In such a burrow the larva is freely j 



exposed to the water, while protected from the j 



attacks of Fishes. Its food consists of eaith, from I 



which it removes by digestion all nutritive matter. ; 



" I suppose that the larval stage occupies two years, j 



and that in the last few months of the second }'ear I 



signs of wings appear ; at this time half-grown larvae, I 



destined to macure in a subsequent season, arc 



abundant. 



I 



