CHLOEON (EPHEMERA) DIMIDIATUM. 67 
` The separations between the three thoracic segments are not very distinct, The pro- 
thorax is rather the igon of the three, and, as does also the metathorax, equals the 
head in breadth. 
< The legs are well developed, and are adapted for walking, but not for swimming; still 
ghey are not strong enough to support the body out of water. They consist of the usual 
five divisions: the two basal are short, and might almost be considered as one; the 
other three are long and cylindrical, the femur being the longest, and the tibia rather 
shorter than the other two. 
The claw is large, and resembles a clasp-knife in shape. "There are a few strong simple 
hairs scattered over the legs, two or three at the apex of each segment, in addition to 
which the tarsus has one near the middle. Those which are on the upper side of the 
leg are delicate and thin; but those on the under side are much stronger, as the insect 
rests on them when walking. The tarsus has only one of these supporting hairs. 
The abdomen consists of ten segments, which decrease gradually in breadth from the 
first to the last. The eighth, ninth, and tenth are rather longer than the rest. The 
second and four following segments increase slightly from in front to behind: this con- 
` dition is no doubt connected with the subsequent production of the so-called gills; it is 
most strongly marked in the third, fourth, and fifth segments, just as the gills attached 
to these segments will at first be the largest. 
^. No trachez are as yet visible ; respiration must therefore take place by the outer sur- 
face of the body. 
The posterior segment bears two long, many-jointed caudal filaments or tails. They 
are rather longer than the body, though very slightly so, and consist of nineteen seg- 
ments. The basal is s35thsin length, cylindrical, slightly enlarged on the inner side near 
ihe base, and has several transverse ridges, which have at first sight much the appearance 
of joints, but which are in reality rather comparable to those numerous minutely toothed 
ridges, which, when the insect is rather older, give the whole skin a shagreened 
appearance. The apex is surrounded by a circle of strong teeth, near which, or rather 
closer to the terminal ridge, are one or two minute hairs. 
The second division of the tail is gépths in length; it is divided into five segments by 
ridges which, though resembling those above described, are (and especially the apical 
one) more strongly marked and more persistent, As in the basal segment, the terminal 
ridge bears three or four hairs, and the apex of the division is surrounded by a circle of 
short spines. 
The third division of the organ is likewise divided by ridges into five segments, which, 
except that they are slightly slenderer, in all respects resemble those of the second division. 
The six following segments call for no special description, The twelfth, thirteenth, 
fifteenth, sixteenth, and seventeenth are of moderate length, the fourteenth being shorter. 
The sixteenth and seventeenth segments have each a hair near the apex. The eighteenth 
is longer and slenderer; the nineteenth, like the terminal segment of the sand V tapers 
to a very sharp point, and resembles a hair almost as much as à segment. 
These appendages do not appear to be as yet of much use in swimming, which is, I think, 
-at this age effected principally by means of the abdomen, ‘This latter is very moveable, 
