4 THE NORTHERN MICROSCOPIST. 

fringe of hair. In the eighteenth state, the wing-cases cover the 
first abdominal segment, and the body is one-third of an inch long. 
In this state the sexes can be distinguished. Between the large 
compound eyes of the male, may be seen, on the top of the head, 
a series of dots, the rudiments of the facets of the large pillared 
eyes. Rudimentary forceps may also be seen, and in the female 
small ovaries. In the twentieth and last state of the nymph, the 
wing-cases extend to the third abdominal segment. The small 
antennze of the subimago can be seen within the larger antenne of 
the nymph, lying loosely in their sheath, the soft tissue having 
become absorbed around them; whilst beyond the new antenne, 
the old ones retain their usual consistence. ‘The two lateral tails 
of the nymph now appear darker than the middle one, the 
subimago tails being visible inside them, whereas the contents of 
the middle tail have been absorbed. ‘The four-jointed tarsus of 
the subimago can also be seen within the single joint of the 
nymph. ‘The ovaries of the female now occupy nearly the whole 
length of the abdomen, and consist of two cylindrical bodies, each 
of which consists of a great number of minute egg-tubes. Each 
of these egg-tubes is divided into two chambers; the upper one 
long and narrow, contains vitellogenous cells, the lower, oval in 
form, contains oil globules. The mouth consists of a pair of 
mandibles with saw-like projections on the upper edge, pro- 
longed internally into a molary plate; a pair of maxilla, with two- 
jointed palpi; a labium, with three-jointed palpi, and a bilobed 
labrum. 
The insect now quits the water, and becomes a subimago, and 
in doing so, it sheds, for the first time, the inner membrane of the 
tracheal trunks, which is usually cast by other larve at each moult. 
It drops its abdominal gills, and breathes through spiracles. After 
a short time, it extricates itself from its additional skin, which it 
leaves very perfect behind it, the wing-cases being in a state of 
collapse at the sides of the thorax. It is now an imago, and 
appears brighter, lighter, and more adapted for a denizen of the 
air. Sir John Lubbock names his species “ Chloéon dimidiatum.” 
The male has four large compound eyes, the upper pair being 
raised on broad columns, pushing the lateral pair low down at the 
sides of the head. In front of these, the three simple eyes are 
placed triangularly on the forehead, the upper two being elevated 
on short pedicels. The mouth is without any external opening. 
Curtis dissected the mouth of an Ephemera danica, and was able 
to discover the rudiments of maxillee, labrum, and two sets of palpi. 
Nearly two hundred species of these insects, British and foreign, 
are already known. The British, however, are restricted to ten 
genera, containing thirty-seven species, according to the following 
classification of Mr. Eaton. 

