THE CHLOEON DIMIDIATUM. 369 
The genus Rhyacophilus has a species, the larvee of which spin 
a cocoon in the interior of the shelter before undergoing meta- 
morphosis. 
The importance of the moulting or skin shedding in relation 
to metamorphosis has been treated of by Sir John Lubbock with 
his usual care and intelligence. He has investigated the larve of 
an Lphemera (Chloéon dimidiatum), which is common in the ponds 
of the county of Kent, and has shown that their growth takes 
place step by step during a series of moults. There are no less 
than seventeen tolerably well pronounced grades of development 
before the larva attain maturity, and the wingless creatures obtain 
the simplest rudiments of the organs of flight during the eighth 
stage of their growth. During five other stages the rudimentary 
wings increase so as to cover three-quarters of the metathorax ; 
and by the time the insects have passed through the whole of the 
stages, the wing-caSes cover more than half of the first segment 
of the abdomen. 
At first the larve had legs adapted for walking, but not for 
swimming, 
abdomen consisted of ten segments, and no trachee were visible, 
nor could they support the body out of the water. The 
so that respiration took place by the outer surface of the body. 
In moulting, the skin on the upper part of the head splits, and 
the larva comes through the minute crack. After a third moult, 
branchie or gills appeared, but still no tracheze were visible within 
them; but during the next stage the respiratory organs began 
to vibrate and to contain tracheze; moreover, the circulation of 
the blood and the beating of the heart were visible. Ihe insect 
thus undergoes a progressive development in the larva state until 
the time comes for a more decided change into the state of 
the nymph, and in fact a metamorphosis has gone on from birth 
until the maturity of the larva. When the nature of the growth 
within the egg is ascertained in this Ephemera the immature 
condition of the young larva will be more evident than ever. 
