£68 STATES OF INSECTS. 



the fly happened nearly two months later than in ordi- 

 nary circumstances. Are we to conjecture that those ol 

 the moth just mentioned, or of E. lanestris, that are lat- 

 est matured, from a greater degree of viscidity in the 

 fluid that forms them % have thicker and more imper- 

 vious skins than those disclosed at an earlier period? 

 Or are we to refer the difference to some unknown pe- 

 culiarity of organization ? On any supposition, the fact 

 remains equally wonderful ; and I know of none the illus- 

 tration of which is more worthy of the patient investiga- 

 tion 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 5 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 c — 

 Not only is the week fixed, but in several instances even 



a See above, p. 245. 



b 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. 



c Bill. Nat. E. Transl. i. 103-—, 



