CHLOEON (EPHEMERA) DIMIDIATUM. 69 
number, but in the later stages they become very numerous. A specimen which I found 
on the 21st of September, and which then resembled fig. 1, changed its skin on the fol- 
lowing morning, and then remained without much alteration until the 25th, by which 
time it had grown to ;5ths, when it moulted once more, and so entered the 
Third State (fig. 8), 
which lasts for two or three days, during which the insect grows from #2,ths to ~$;ths 
or gg goths in length. 
The posterior eyes are now about twice as large as the three front ones; so that the 
distinction between eyes and ocelli begins at this early stage, though we have seen that 
at first they are very similar. I could not distinguish any facets. 
The antenn:e (fig. 20) are 3%oths in length, and now consist of fifteen segments, the 
two new ones being formed at the expense of the third, in which also almost the whole 
increase of length has taken place. The apices also of these three segments differ from 
those of all the others in being surrounded by a circle of teeth, as was, indeed, the case 
with the third segment in the preceding state, when, however, the teeth were smaller 
and less conspicuous. Of the two new segments, the basal one is rather the shorter. 
The terminal segments have increased slightly in size, but not in number; indeed, 
several of them are even less strongly marked than before; this is particularly the case 
with the two which are now the tenth and eleventh, and between which scarcely any 
division is visible. 
The tarsi have three supporting hairs. 
The posterior angles of the second and four following abdominal segments are much 
less produced than before; but then, on the other hand, each of them supports a small, 
oval, leaf-like appendage, the first appearance of the branchie. Those attached to the 
third and fourth segments are the largest, and are almost as long as the segments them- 
selves; the second and fourth are somewhat smaller; the fifth still less; while the first 
is almost rudimentary. The seventh segment has the posterior angles somewhat pro- 
duced, in preparation for the posterior branchiz, which will make their appearance after 
the next moult. I could see no trachez either in the body or in the gills, even in this, 
the third stage in the animal’s free existence. 
The two tails are now @4;ths in length, and consist of twenty-four segments,—the 
whole increase in the number of joints, and almost all that in the length, being due to 
the basal portion, which is almost double as long as it was before, and constitutes now 
just half of the totallength. I believe that what was in the last stage the second segment 
now forms the fifth and sixth, the four basal being due to the subdivision of the first. 
Each of these segments has a ring of teeth round the apex; but the rings are not all 
equally well marked, those round the second, fourth, and sixth being larger than the 
other three. Each of the six basal segments has also one or two hairs near the apex. 
The eighteen terminal segments are almost unaltered. 
The little knob between the two tails is larger than before, and pyriform, 
VOL. XXIV. " 
