on the Bphemeride. 43 
It is well known that, in some genera, differences be- 
tween the sexes are apparent in the oculi, setz, and 
tarsi. The oculi are always smaliest in the female, and 
in the male are sometimes divided into two parts. The 
female usually has shorter sete than the male. The 
middle seta is often shorter than the two outer setz, or 
altogether absent. Sometimes the male wants the middle 
seta, whilst the female has it equal to, or only shorter 
than, the others; frequently neither sex possesses the 
central seta. The proportions of the tarsal joints of the 
male are not the same as those of the female; and his 
anterior legs exceed hers in length. 
The composition of the abdomen of Hphemeride has 
been the subject of much dispute. Some consider it to 
be ten-jointed, others reckon nine joints. The ‘tenth 
joint” throughout this paper means the dorsal arcus 
immediately above the sete, the part from which the 
setze proceed, and the ventral arc often formed of two 
valve-like pieces under the anus—the intermediate appen- 
dices of M. Pictet. The ninth joint is that which bears 
the forceps in the male, and which, in the female, is 
sometimes prolonged behind into a broad lobe beneath, 
concealing the tenth joint. This lamina has been mis- 
taken for the egg-valve in some species. The eighth 
joint is the first posterior to the opening of the oviducts. 
From the apical edge of the seventhjoint, beneath, the egg- 
valve grows out. The next five segments are not pecu- 
har in point of structure. The first jot is immoveably 
united with the metathorax, and often resembles it in 
colour. Hence certain Entomologists have reckoned 
, this joint a part of the thoracic region. That this joint 
belongs to the abdomen may, however, be demonstrated 
in the nymphs of some genera, in which it is furnished 
with a moveable pair of branchial plates. Now the 
branchial out-growths of the thorax are always fixtures 
in the nymph. In certain four-winged genera, those 
branchial plates of the first segment are present, and may 
be contrasted with the rudimentary hind-wings. 
In drying, the colour and form of Hphemeride soon 
change. Colour is of little importance, even in fresh 
examples; but form is necessary to the distinguishing 
of the species. They are, therefore, best preserved in a 
hquid. It is sufficient for ordinary purposes, to dip the 
freshly-killed specimen into dilute spirits, and then 
