286 THE NORTHERN MICROSCOPIST. 
having an abortive mouth, in the smallness or absence of hind- 
wings, and the two or three long filaments that form the tail. As 
it lives in the imago state seldom longer than a day, it does not 
require food; and, therefore, has no jaws or mouth adapted for 
prehension or mastication. It leaves these instruments behind it 
in the water when it emerges from its last aquatic skin; for the 
larva lives a considerable time in the water previous to attaining 
the winged form, the time varying with the genus from a few 
months to about two years. 
The Ephemeridz possess both compound and simple eyes. 
The antenne consist of two rather large basal joints surmounted 
by a bristle, indistinctly jointed, with a somewhat bulbous base. 
The abdomen has ten segments. ‘The first segment is immove- 
ably attached to the metathorax, and is often of the same colour. 
This has led some entomologists to describe them as possessing 
only nine abdominal segments; but the fact that this segment 
belongs to the abdomen, and not to the thorax, is proved by its 
possessing, in some of the larvee, the first pair of aquatic gills, all 
of which are abdominal, and are thrown off when the insect quits 
the water; whereas the appendages of the thorax are retained by 
the imago. The ninth segment is furnished in the male with a 
pair of forceps or claspers, with which he embraces the female, 
and the oviducts in the female terminate in openings between the 
seventh and eighth ventral segments. The legs are short and 
slender, the front pair the longest; the male has this pair much 
longer than the female, and advanced more in front of the head. 
The tarsi have four or five joints, the fifth joint being sometimes 
nearly obsolete and immoveably attached to the tibia. The 
anterior wings are large, and never folded ; the posterior, if present, 
small or rudimentary only. In the large wing the costa is 
“united by a stout cross-veinlet to the radius near the base” ; 
“ subcosta uninterrupted at the nodus.” ‘The caudal setze, or tails, 
are composed of many joints. In some genera the middle tail is 
absent in the imago; the larvze, however, all possess three tails. 
The tails act as balancers during flight. After ascending by the 
action of the wings, the insect succumbs to the attraction of 
gravitation, and, during its descent, the tails assist in supporting 
the abdomen. The male has usually longer tails than the female. 
His compound eyes are also larger than hers ; and in some genera 
he is provided with a second pair, raised on large pillars on the 
top of the head. The female never has these pillared eyes. The 
three ocelli, or simple eyes, are, however, found in both sexes. 
The male is usually smaller and of darker colours than the female. 
These insects are usually described as undergoing quadruple 
metamorphosis, because when they first appear with wings, they 
are encased in a thin pellicle which covers the whole body, includ- 
