288 THE NORTHERN MICROSCOPIST. 
frequents the rivers of Germany and Holland, which made its 
appearance in great swarms regularly in June. Reéaumur, also, on 
the banks of the Marne, saw countless numbers of these insects in 
the middleof August. 
He describes them 
as filling the air more 
completely than the — 
thickest fall of snow. 
Dr. Hagen describes 
those of the genus 
Ccenis as being so 
plentiful sometimes 
in Prussia that they 
are used to feed the 
pigs. Other observers 
of undoubted vera- 
city have testified 
to having witnessed 
similar phenomena. 
Inthe British Islands, 
however, we have 
nothing analogous to 
this ; although these 
insects are occasion- 
ally found in great . 
eaters under pecu- tars 
liarly favourable circumstances. The Zphemerica danica, the May- 
fly of the angler (Fig. 58), has been seen on the river Dove in such 
numbers, in the subimago state, as almost to cover the water 
where it was from twenty to thirty feet broad. 
The swarms of these insects, when they occur, consist mostly of 
males. Being filled with gas, no food ever being found in the 
alimentary canal of an imago, the male insect is extremely light. 
The female, whose ovaries occupy about two-thirds of the abdo- 
men, and must necessarily add to her weight, doubtless finds her 
powers of prolonged flight more restricted than the male. Besides, 
she is much engaged in depositing her eggs, after fecundation, in 
clusters on the surface of the water; for, as they extrude from the 
oviduct, she is constantly skimming along the surface of the 
water in order to detach them, becoming at the same time an easy 
prey to the trout. Having deposited her eggs, she returns to the 
swarm, and then betakes herself to the water a second time, where 
she deposits more eggs. She repeats this process until all the eggs 
are disposed of, after which exhausted nature succumbs, and she 
dies. In some genera, however, she deposits all the eggs at once, and 
her life is then much shorter. The life of the male is equally short. 
Imago. x14 
