330 MR FRANK BALFOUR-BROWNE ON 



The position of this organ in the head (immediately below the aorta and just at 

 the point where the aorta divides into two vessels passing down the frontal region), and the 

 apparent relationship between its pulsations and the peristaltic contractions of the gut, 

 lead me to think that there is no special embryonic organ, but that the pulsations are 

 due to the working of a pharyngeal pump. The larva, as we shall see, feeds by suction, 

 and there is no reason why the pharyngeal pump should not be of service before the 

 embryo escapes from the egg. I imagine that the fluid which surrounds the embryo 

 within the shell is taken in by the mouth and pumped by the pharynx into the gut. 

 Thus the embryo swells up and space is afforded it within the shell by the removal of 

 the surrounding fluid.* 



For a short time before the shell bursts the embryo is so tightly packed that it is 

 difficult to make out the limits of head and tail (vide fig. 31). The head lies well into 

 the anterior end of the egg, while the first, second, and third abdominal segments occupy 

 the posterior end. The shape of the egg is often somewhat altered by the packing of the 

 embryo which, although usually as described, is sometimes turned in other positions. 



The shell ultimately bursts somewhere in the region of the maxillary palpi and the 

 larva immediately straightens out, the shell falling off its back as it does so. All the 

 trophi and legs at once assume their normal positions, the labium folding forwards and 

 coming to lie immediately behind the mouth, the maxillae likewise folding forward and 

 lvino- on either side of the labium. 



I have described the maturation of the egg as taking fourteen days from the time 

 it is laid until the escape of the larva. This seems to be the usual time in April or 

 May, but the rate of development varies with the temperature. Thus eggs laid in 

 February — which is exceptionally early — took twenty days to mature, while in one or 

 two cases during the summer thirteen days were sufficient. 



Kowalevsky (187l) and Heider (1889) found that the eggs of Hydrophilus 

 developed in twelve days, while Miger (1809) gives twelve to fourteen days as the usual 

 period. In my experience with this species twenty days and more was the usual time 

 from the building of the egg-cocoon to the appearance of the larva, but as my specimens 

 were kept in the north of Ireland — far beyond the normal range of the species — this 

 may account for the slower rate of development. 



I have not followed the development of the embryo of Hydrophilus, but the 

 statements of Kowalevsky, etc., indicate that the stages are practically identical with 

 those of Hydrobius. None of these authors, however, describe the swelling of the 

 embryo prior to the bursting of the shell nor do they refer to the cephalic pulsating 

 organ. 



The Larva. (Figs. 5 to 9.) 



The larva which emerges from the egg is a small white worm-like creature with a 

 pale testaceous head and darker eyes, the eyes being the most visible part, and is about 



* On ihis point see " The Lii'e-history of the Agrionid Dragonfly," Proc. Zool. Soc. London, Aug. 1909. 



