THE CHLOEON DIMIDIATUM. 369 



The genus RhyacopJiilus has a species, the larvre 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 larvae of 

 an Ephemera {CJiloeon dimidiatimi), 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 larvae attain maturity, and the wingless creatures obtain 

 the simplest rudiments of the jorgans 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 larvae had legs adapted for walking, but not for 

 swimming, nor could they support the body out of the water. The 

 abdomen consisted of ten segments, and no trachea were visible, 

 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, 

 branchiae or gills appeared, but still no tracheae were visible within 

 them ; but during the next stage the respiratory organs began 

 to vibrate and to contain tracheae ; moreover, the circulation of 

 the blood and the beating of the heart were visible. The 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 ^g^ is ascertained in this Ephemera the immature 

 condition of the young larva will be more evident than ever. 



