1913] on The Life-History of a Water-Beetle 757 



at infrequent intervals the whole body moves slightly in the shell, 

 the tendency being to push the head into the end. One other move- 

 mend is to be noted, and that is an up-and-down motion of the head, 

 at first very slight, but later becoming very marked. 



On either side of the head is a small papilla, at the apex of which 

 is a minute, slightly curved spine. When the embryo is at rest, this 

 papilla lies in a slight depression, but when the sucking-pump is at 

 work the papilla bulges outward, so that the spine touches the shell. 

 Thus when the head moves up and down and the sucking-pump 

 works at the same time, the two spines scrape along the inside of the 

 shell and ultimately burst it open. They are, therefore, " hatching 

 spines," and similar instruments differently situated have been 

 observed in a few insect embryos of other orders. 



Yon see, therefore, that the shell bursts open at the head end ; 

 immediately it bursts the compressed larva bulges out, and by slight 

 writhing movements works its way clear of the shell, the whole 

 operation taking less than two minutes. As soon as the larva is 

 clear of the shell the tail straightens out, and the legs and mouth 

 parts assume their natural position. In the embryo there is a 

 peculiar fold in the upper part of each jaw, but within two or three 

 minutes of the larva's escape this fold has completely disappeared. 



From the moment the larva escapes it begins to grow in length 

 and breadth. The long air-tubes in the body are flat, but have a 

 bright silvery appearance, suggesting that some gas has been secreted 

 in them ; but the larva is heavier than the water, and therefore sinks 

 to the bottom. For a time, half an hour or more, it rests quietly 

 and shows no desire to get to the surface, but sooner or later it gets 

 restless and swims to the surface, using its feathered legs as oars, 

 and raises its tail to the surface film and remains suspended for a 

 few minutes. After this the newly -hatclied larva is buoyant, and 

 cannot remain away from the surface without holding on to the 

 submerged vegetation. The buoyancy is, however, only temporary, 

 as older larvae frequently require to swim to the surface to renew 

 their air-supply. 



In the insect, breathing and blood-circulation are normally not 

 intimately associated as in other animals. In a human being or a 

 fish, or even in a snail, air is taken into special organs — lungs or 

 gills — where the blood takes up the oxygen and carries it through 

 the whole body. In the insect the blood has usually nothing to do 

 with the aeration of the different organs, the whole body being 

 permeated by innumerable air-tubes. 



In all the water-beetle larvge which come to the surface to 

 obtain their air, these innumerable air-tubes communicate with two 

 large air-tubes which run the length of the body, one on each side, 

 and these open on the last segment. Hence, when a larva requires 

 to renew its air-supply it comes up tail first, bringing the openings 

 of the two lateral tracheae into communication with the air, and by 



