69 
Description of the Skin cast by an EphemeroN;, in its 
Pseud-imago Condition. By Tuffen West, F.L.S.^ 
&c. 
(Read March 14th, 1866.) 
The following observations appearing to have some bearing 
on the disputed question as to the exact nature of the pen- 
ultimate change in the Ephemeridse, are brought before the 
notice of the Microscopical Society with the hope that they 
will prove to be a small contribution towards our knowledge 
of a subject confessedly requiring further elucidation. 
In wandering near water in the country on a summer^s 
evening, some of the members of our Society may have found 
themselves speedily covered by small whitish looking flies, 
with two or three long tails a piece, which after alighting on 
some portion of the dress, remain quiet for a brief period, 
and then fly off", leaving behind them an alter ego, in shape 
of a perfect cast of their integuments. These casts may some- 
times be found on railings, branches of trees, &c., in the 
vicinity of water. 
On Frensham Common are two considerable pieces of 
water, known in these parts by the names of the Great 
and the " Little Poiid,^^ which are favoured resorts of nu- 
merous aquatic insects, and from the latter of which on a fine 
summer's evening, clouds of a small species of May-fly arise, 
and settling on the hat and upper portions of the dress, soon 
cover him with their exuviae."^ So rapidly does the operation 
take place, that it was not till the sight had become educated 
by attempts on several occasions to observe the whole, that I 
became able to witness the entire process, whilst the (to the 
naked eye) complete disappearance of the pellicle covering 
the wings left a mystery on the mode of their reaching me 
which I was long unable to solve. The distance from the 
water at which the clouds that so thickly covered one occur- 
red was considerable ; I counted 230 paces from the water's 
edge to the boughs of a Scotch fir, which was fairly whitened 
with them, the tree being the nearest of a clump growing on 
a neighbouring hill. 
From their extreme delicacy the bringing home uninjured 
of these cast- skins for microscopic examination is a very difii- 
cult matter; but an individual of a larger species having 
* In Westwood's 'Introduction to the Modern Classification of lusects,' 
vol. ii, at p. 27, is a graphic description of the process, and in a foot-note 
on p. 28 a discussion of the nature of the raetamor])hosis- 
