

THE EPHEMERIDA, OR MAY-FLIES. 3 

segments, viz.: fifteen. The tails are also longer, and consist of 
four more segments, viz.: twenty-four. The eyes have become 
differentiated, the posterior pair being larger than the rest, so that 
the distinction between the oculi and ocelli is now observed. The 
oculi, however, have not yet any facets. The posterior angles of 
the second to the sixth abdominal segments are less produced than 
before, but each has a small oval appendage at its extremity, the first 
appearance of branchiz. ‘The middle ones are the most developed. 
The other gill-bearing segments now have their posterior angles 
elongated, in preparation for the remaining gills, which make their 
appearance after the next moult. No trachez have yet made their 
appearance. . : 
In two or three days the larva casts its third skin, when tke tails 
and antennz are found to be longer, and to consist of more 
segments than before. In this state, the five intermediate pairs of 
gills begin to vibrate. ‘Trachez are now seen in these gills, and 
communicate with two longitudinal trunks in the body. After the 
next moult the first pair of gills begin to vibrate. The seventh 
pair never vibrate in this genus. After the fifth moult, the antennze 
were found to contain two segments less in number than in the 
preceding state, two fresh ones having been produced, and four 
others having coalesced. The posterior eyes now consist of dark 
spots on a pale ground. 
After the sixth moult, the larger gills have small lobes attached 
to them, which eventually form double gills. The seventh pair 
remain single'in this genus.- The two lateral tails now consist of 
thirty-two segments, and the central knob has expanded into a tail 
of six segments. ‘The antennz consist of two more segments, the 
third segment having divided into three. After the next moult, 
the middle tail consists of twelve segments, and after another 
moult of eighteen segments. The third segment of the antennz 
has again divided into four parts, and the posterior dorsal angles 
of the meso and meta-thorax are slightly produced in preparation 
for the wing-cases of the nymph. 
Some of the larvee arrived at the tenth state in September, about 
twenty-five days after leaving the egg ; they then took about a week 
before re-moulting. This period, however, much depends upon 
the temperature, as the larva moults much more seldom in winter, 
and there is then less difference in the.growth between the moults. 
Sir John Lubbock found that in January, February, and March, 
Cloéon took from fourteen to thirty days before re-moulting. 
After the thirteenth moult, a secondary lobe appears on the first 
pair of gills. In the fifteenth state, the rudimentary wing-cases 
cover the metathorax. In the seventeenth state, the secondary 
lobes of the anterior gills are almost as large as the primary ; the 
tails consist of more than sixty segments, and are covered with a 

