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XIX. On the Development of Chloéon (Ephemera) dimidiatum. 
By Ste Joux LUBBOCK, Bart., F.R.S., V. P. Linn. Koc., Pres. Ent. Soc., V. P. Eth. Soc. &c. 
(Plates LVIII. & LIX.) 
PART IL 
Read December 21st, 1865. 
THE Linnean Society have done me the honour of publishing, in the 2nd part of the 
2th volume of their Transactions, a memoir on the development of a very common 
freshwater larva—that, namely, of Chloéon dimidiatum. | 
I traced it through seventeen stages, and described the gradual alterations which it 
underwent. The object of the present communication is, in the same manner, to de- 
scribe the subsequent and very remarkable changes which take place during the develop- 
ment of the perfect insect. 
As will be seen from the Table given at the end of this memoir, the changes of skin 
during the winter months are separated by longer intervals than is the case in other 
periods of the year, and sometimes two or even three moults will take place without 
any material change of form. In December, January, and February there are generally 
at least 14 (and sometimes as many as 30) days between each moult. Thus a speci- 
men which on the 6th of November entered the 17th stage remained without any ma- 
rial change until the 18th of January, during which period it changed its skin four 
times, at intervals respectively of 19, 16, 19, and 19 days. My different specimens, 
“er, did not appear to agree exactly as to the number of moults. 
how- 
Eighteenth State. 
ri Wing-cases now cover the whole of the first ab 
Es is on an average about y of an inch. The so 
"antenne and the caudal appendages continue to increase IN a 
üficult to obtain specimens ^d shik these organs are perfect; nor when this is done 
a the number of segments be very easily determined. Plate LVIIL fig: 1 represents 
part of the basal portion of one of the caudal appendages, and shows the steps in zi 
Process by which the segments are formed. Probably no two observers would exact y 
‘Sree as to the number of segments which compose one of these organs. » : 
The anterior plate of the anterior branchia is now about as long as the other, an 
SOMewhat bro à 
Nin ader than in the preceding state. EG 
dominal segment, and the length 
gments which compose 
number; but it is very 
