CHLOEON (EPHEMERA) DIMIDIATUM. 75 
Eleventh State (fig. 11). 
This specinien moulted again, and so entered the eleventh stage, on the 30th of Sep- 
tember, when it measured f ths in length; sometimes, however, the insect attains a 
length of +%%ths before it moults again. 
The posterior angles of the mesothorax cover rather more of the metathorax, the 
angles of which also are a little more prominent than before. 
In a east skin, which was ;555ths in length, the central tail was ths, and consisted 
of thirty-seven segments, without, however, being quite perfect. The circles of spines 
were, as before, most conspicuous round the third, fifth, seventh, eleventh, and fifteenth, 
to which was now added the nineteenth segment; here again, therefore, the sixth and 
seventh had divided into two, while the other two new segments at the base had been 
produced by the first and third. 
Compared with the description given of a specimen in the preceding state, this still 
leaves three segments unaccounted for; the above specimen was, however, imperfect. 
The sixth and seventh segments already show indications of a division—so much so, 
indeed, that I have some doubt whether they should not be counted as four. 
The darkened part extends from the nineteenth to the twenty-fourth segment. The 
fringe extends to the thirty-second. 
In the lateral tails the fringe extends to the thirty-sixth segment, beyond which 
are nine more, though the organ is evidently imperfect. "The principal circles are now 
the third, fifth, seventh, eleventh, fifteenth, nineteenth, and twenty-third. "The twenty- 
fourth and three following segments are darkened. 
Here again, therefore, the plan of development is evidently the same as before. 
This stage lasts for about a week, and the insect increases to about sogths in length. 
Some specimens, however, like the above, moult again, and so enter the twelfth stage, 
when they are no more than goths. 
Twelfth State (fig. 12). 
At the next change of skin there is even less alteration in form than has been, up to 
this time, usual. One of my specimens, which moulted on the 3rd of November, seemed 
to have the wing-rudiments almost exactly of the same length as before. | 
The fringe on the lateral tails extends to the thirty-eighth, and that on the central 
one to the thirty-fourth segment ; but they are otherwise without alteration. 
In this stage the insect attains a length of about ths to $55ths. 
Thirteenth State (fig. 13). 
In this state the posterior angles of the mesothorax, or, as they may now be fairly 
called, the rudimentary wings, cover three-quarters of the metathorax, while the pos- 
terior angles of the latter segment have undergone little alteration. 
The number of new segments added to the antennz at each moult is larger than at 
first ; and as the growth of the third segment has not increased proportionately in rapidity, 
both itself and the new segments which are produced from it are much shorter than they 
were at an earlier period. 
