STATES OF INSECTS. (Pupa.) 267 
degree of viscidity in the fluid that forms them *, have 
thicker and more impervious skins than those disclosed 
at an earlier period? Or are we to refer the difference 
to some unknown peculiarity of organization? On any 
supposition, the fact remains equally wonderful; and I 
know of none the illustration of which is more worthy of 
the-patient investigation of the physiologist. 
As the period of maturity of the perfect insect is thus 
in some cases not fixed even to years, and as in many 
it seems dependent upon such variable causes ; nothing 
appears more improbable than that it should ever be 
so strictly determined, that even the week in which the 
fly will leave its pupa-case can be pretty accurately pre- 
dicted. Such, however, is the fact with regard to the 
Ephemera so interestingly described by Reaumur, the 
myriads of which that issue from the banks of the Seine 
all appear in two or three days, somewhere between the 
10th and 18th of the month of August® in every year; 
at which time the fishermen regularly expect them. <A 
like regularity attends the appearance of those described 
by Swammerdam, which every year, for three days about 
the feast of St. John, issue in clouds from the Rhine <— 
not only is the week fixed, but in several instances even 
the hour. The Ephemerz observed by Reaumur ap- 
pear at no other time than between ezght and fen o’clock 
in the evening ; and so unalterably is their exclusion 
fixed, that neither cold nor rain can retard it. Between 
these hours, in the evenings on which they appear, you 
* See above, p. 244. 
» The appearance of them sometimes continues to near the end 
of the month ; it began on the 19th, when Reaumur observed them. 
vi. 480. 488. 
© Bibl. Nat. E. Transl. i. 103—. 
