191;^] on The Life-History of a Water-Beetle 761 



In closing up the cocoon u peculiar plate-like structure is formed 

 of very closely woven silk, and this ends in an upward projection 

 known as the " mast." The purpose of this " mast " is not known. 

 It is not a tubular structure, but merely a band of silk. It has been 

 stated that if it is cut off the eggs die, but in the case of another 

 species I have hatched eggs removed from the cocoon and submerged, 

 so that the suggestion that the mast is necessary for keeping up the 

 air-supply is without foundation. 



I shall not weary you with details of the development of tiie 

 embryo beyond mentioning that, unlike Dytiscus, the embryo from 

 the first occupies the whole length of the egg, and that the nerve 

 chord, again unlike Dyti^cus, grows witli the embryo as it develops. 

 The only other point I need mention is that in the cocoon all the 

 embryos develop head downwards. 



Tlie egg-laying period of Hydrocharis extends from about the 

 middle of May until about the middle of July in my tubs, but it 

 may perhaps be rather longer in the south-eastern parts of England. 

 The incubation of the egg occupies nine or ten days, and, as in the 

 case of Dyfiscus, towards the end the embryo is very tightly packed 

 within the shell. There is, however, no special hatching apparatus 

 that I have been able to find. The pulsating organ or sucking-pump 

 in the head is visible, and there are also movements of the embryo, 

 but at the end the skin splits along the back and the larva treads it 

 off, giving a peculiar backward wriggle. 



Now, under normal conditions the newly-hatched larva does not 

 at once leave the cocoon, in fact it does not appear for one or even 

 two days after hatching. As soon as it bursts the egg-shell it 

 wriggles backwards out of the egg into the space above all the eggs, 

 and it is interesting to note that the hairs on the l)ody of the newly- 

 hatched larva all point forwards. As the larvae hatch, the empty 

 shell and the silk bindings become broken down — I think they are 

 chewed by the larvge— and the whole cocoon ultimately becomes filled 

 with the larvae. 



In those cases where I dissected the eggs out of the cocoon and 

 allowed them to develop on the wet cotton wool, the newly-hatched 

 larvjB congregated into a mass and remained so for a day or two, 

 after which they became active in search of food. 



You will notice that the larva possesses on each body segment a 

 pair of lateral processes, and on the last segment a pair of ventrally 

 placed processes of a different kind. These latter, which are possessed 

 by all water-beetle larvse which come to the surface for their air, 

 have probably some connexion with raising the tail to the surface 

 for breathing, but the hairy lateral processes have been called gills. 

 Many larvae of the Palpicornia have lateral processes, usually smaller 

 than those of Hydrocharis, but in no case are they really gills, and 

 the larva; quickly drown if prevented from bringing their tails to the 

 surface to renew their air-supply. 



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