78 M. A. Vayssiére on the 
The following are the modifications observed in the nymph 
when it is on the point of being transformed. During the 
whole of the aquatic period known to us the integuments of 
Prosopistoma present a very light chitine-yellow tint, nearly 
white if the insect has just moulted, and more or less marked 
at other times; but at the commencement of the transformation 
the colour becomes darker from day to day, and soon becomes 
very brown. It is especially in the posterior part of the cara- 
pace above the respiratory chamber that this coloration 
becomes very strongly marked, which is easily explained, as 
it is at this point that the wings are formed, and these, as we 
shall see hereafter, are iron-grey in the subimago. 
At the end of about a fortnight, when the general colour 
becomes blackish, it is possible, under a low power, to observe 
by transmitted light the outlines of the perfect sect through 
the nymphal envelope; and it is then necessary to watch the 
animal, as the metamorphosis will soon take place. In two 
or three days one may see the animal cling to a stone partly 
out of the water, and divest itself of its nymphal integuments ; 
and it is to be remarked that the metamorphosis takes place 
very early in the morning. 
The two parts of the carapace begin to separate in the 
median line of the body, under the pressure from below 
upwards exerted by the animal; then the anterior margins of 
the same region and the posterior part of the cephalic integu- 
ments (the epicranium) also separate. ‘The insect can then 
free its head and the whole of its thoracic part; the buccal 
organs and the legs escape easily from the nymphal envelope, 
owing to the state of atrophy in which they always are in the 
perfect animal. The Prosopistoma afterwards frees its abdo- 
men ; and at the same moment we see issue from it the wings, 
which, originally folded longitudinally in three parts, imme- 
diately acquire their definitive form. The insect can then 
take to flight, to go and rest on some point at a distance from 
the water ; and there it divests itself of its subimago envelope. - 
I have been unable to observe this last metamorphosis 
of the adult Prosopistoma, the two female subimagos that I 
had having died soon after stripping off their nymphal enve- 
lope; it is quite possible that the females of this genus 
oviposit in the state of subimago, and do not attain that of 
imago. According to Mr, Eaton the females of various types 
of Hphemerina only present a single stage in the pertect 
state *. 
_* At the end of p. 42 of the Monograph of Ephemerina of the Eng- 
lish naturalist (doc. cit.) we find the following phrase :—‘In certain 
genera the subimago is the permanent aerial stage of the females.” 
