perfect State of Prosopistoma punctifrons. 83 
sequence finds itself, of flying throughout the whole of its 
perfect state. 
It has already been stated that the wings are formed above 
the respiratory organs, in sheaths which only become deve- 
loped a little while before the metamorphosis of the ani- 
mal. The sheaths destined to produce the upper wings 
originate in the interior of the integuments which cover the 
tracheo-branchie—that is to say, under the posterior part 
of the chitinous envelope of the carapace; the hinder wings 
are formed in the interior of those two plates which, in the 
aquatic state of Prosopistoma, appear to be formed to protect 
the points of insertion of the first two pairs of tracheo- 
branchie (fig. 3,/). These plates must be formed very early 
in our animal; for I have always observed them in the same 
state of development in more than fifty individuals of all sizes. 
The upper wings, or wings of the anterior pair, affect the form 
of right-angled triangles, of which the hypotenuses are repre- 
sented by the anterior margin ; their inner margin, as in all 
Ephemerina, forms the shortest side (see fig. 7). 
The longitudinal nervures of these superior wings are not 
numerous; and transverse nervures are entirely wanting. The 
base of the two strongest nervures (anterior nervures) consti- 
tutes, with a thickening of the margin of the wing, the prin- 
cipal part of the articulation with the two strongest projections 
observed on the sides of the thorax. Below the two anterior 
nervures we find three others which do not reach the articula- 
tion of the wing; the five following nervures, which succeed 
one another at nearly equal distances apart, as they approach 
the inner margin of the wing, terminate at the base of the 
wing, with the exception of the last, which is shorter and 
more delicate than the others. The anal nervure starts from ~ 
a chitinous thickening of the basal region of the organ of 
flight, and terminates on the inner nargin, nearly at the point 
where this becomes rounded to form the posterior margin. 
The wings of the second pair, of an oval form, are much 
smaller and more delicate (fig. 11) ; their principal point of 
insertion 1s upon the nodosity which may be noticed on each 
side of the metathorax at the level of the points of attachment 
of the last pair of legs. When the fore wings are extended, 
the apical extremities of the others slightly exceed the abdo- 
men on each side; when the animal is in repose the ordinary 
position of the wings is the same that is observed in all the 
Ephemerina: they are placed vertically above the body, and a 
little inclined towards each other. The inner and posterior 
margins of these different wings presented long delicate hairs 
(fig. 10) ; but the surface itself of the wings was covered with 
