CHLOEON (EPHEMERA) DIMIDIATUM. 73 
The lateral tails are about ths in length, and consist of about thirty-six segments. 
The circles of spines are most strongly marked on the second, third, fifth, seventh, ninth, 
eleventh, and thirteenth segments. The fourteenth and five following segments are 
slightly darker than the rest. The fringe begins on the ninth segment, and extends to 
the twenty-sixth, inclusive. None of the'segments have as yet more than four sete. 
The middle tail has increased to 25;ths, and possesses twelve segments. At its base 
it is almost as broad as the lateral ones. As in them, the apex of each segment is sur- 
rounded by a circle of small spines, and has two or three small hairs, in addition to 
which the tenth and eleventh have on each side, near the apex, a longer hair. These 
are the commencement of the swiniming-fringe. The skin of the central tail has the 
usual shagreened appearance. 
Ninth State (fig. 9). 
While large specimens are often as much as $% ths when they enter the eighth stage, 
small ones, even in the present condition, are only ths in length. This, together with 
the fact that out of the specimens which I examined not one in twenty had escaped some 
injury to the tails or antenne, rendered a precise determination of the exact length and 
number of segments in each appendage very difficult and very slow *. 
I do not indeed suppose that mutilation is so frequent while the insects are in their 
native ponds, but the.process of capture must necessarily be very injurious to such 
delicate little creatures. 
A pee sooths in length had antenns of almost exactly the same length, and 
lateral tails x{{5ths in length. 
The iori consist of about twenty-six segments, the third having again divided into 
four. I say, however, about twenty-six, because the upper part of the third segment 
shows indications of another joint, which will become distinct at the next moult, and 
which some naturalists might even consider so now. 
Up to this time no traces of wings have been present. Now, however, the posterior 
dorsal angles of the meso- and metathorax are slightly produced, so slightly, however, 
that it is indeed scarcely perceptible. The mesothorax is a little larger than the meta- 
thorax, but in form they are almost exactly similar. 
The apical circles of spines on the lateral tails are most strongly marked on the 
second, third, fifth, seventh, and eleventh segments. On some of the segments the 
fringe is represented by five setze. 
The central tail is about #3,ths in length, and consists of eighteen segments. The 
fringe is appearing on the twelfth and five following segments. The circles of spines are 
most strongly marked on the second, fourth, sixth, and tenth segments. 
* As the insects grow older, these mutilations become more deceptive. It is always easy to distinguish them at 
first; but when after several moults the injury is almost repaired, individuals thus altered may easily be taken for 
normal specimens. 
