352 SALMON AND TROUT. 



of all, but, beyond this, their life-history is, as far as known, 

 iiA-rrical, whether in the immature larval stages when under 

 tht- uater ; or the subirnago rising from the surface ,of the 

 stuum and flying to the shore; or the perfect insect, the 

 ima^o, under which form the reproductive functions are 

 exercised. 



The eggs, when deposited on the surface by the female 

 imago, sink to the bottom of the river, and after the lapse of a 

 certain time the young May-flies are hatched out in the form of 

 tiny active larvae. It is probable that the comparative tem- 

 perature and depth of the water, and the surrounding circum- 

 stances tending to increase or diminish the quantity of light 

 and sunshine, may have some effect in retarding or advancing 

 the period intervening between the deposition of the eggs and 

 the birth of the larvae. Having succeeded in hatching the 

 eggs of Fphemera danica in captivity, I am in a position to give 

 some precise data, which, however, must be taken as showing 

 the result only with this particular species under particular con- 

 ditions of light and temperature. A number of eggs taken on 

 June 9, 1887, hatched on August 15 in the same year, a-period 

 of sixty-seven days. 



The new-born larvae at once commence digging their way 

 into the mud by means of their tusk-shaped mandibles and 

 forelegs, and form tubular horizontal galleries of a diameter 

 only slightly greater than that of their bodies, but, according to 

 the observation of Pictet, not sufficiently large for them to be 

 able to remain in these retreats while growing. As they increase 

 in size they desert the galleries previously dug and form fresh 

 ones ; as, however, they always affect water of a certain depth, 

 in rivers subject to sudden floods they are obliged to change 

 their quarters from time to time in order to preserve these con- 

 ditions. 



As the larva grows it sheds its outer skin many times, some 

 further development of the various organs taking place with 

 each moult. The entire larval existence is passed in compara- 

 tively still portions of the stream, buried in the mud at a depth 



