THE LIFE-HISTORY OF HYDROBIUS FUSC1PES, L. 331 



one and a half times the length of the egg from which it comes. But such larvae are 

 never seen under normal circumstances. As it comes from the cocoon it is distinctly 

 larger and darker and the head, with its extended jaws, seems also larger in proportion 

 than in the newly hatched individual. Lyonnet states that the larvae of Hydrophilus 

 remain a day in the cocoon before breaking out. Those of Hydrobius also remain in the 

 cocoon for some time, but I do not know how long. They ultimately make their way 

 out by breaking through the cocoon, usually along the edge of the flat, lid-like surface 

 which is at the base of the ribbon and just about on the water-level. 



I hatched a number of larvae from eggs which had been taken from the cocoon and 

 allowed to sink in a tumbler of water. The larvae crawled about aimlessly for two or 

 three days, but a number of them died. I took some out of the tumbler and placed 

 them in a drop of water upon a slide to examine them under the microscope, and they 

 immediately began to crawl about and at last succeeded in getting their heads out of 

 the water. Once this was accomplished the head and anterior end of the body was 

 raised up, and the larvae remained in this attitude for thirty seconds or more. During this 

 short time they visibly expanded, and I found, when I replaced them in the water, that 

 they now floated at the surface instead of sinking. Examination under the microscope 

 revealed that the gut contained through most of its length a series of air-bubbles. 

 These larvae had, in fact, taken a "drink" of air at the first opportunity and had 

 expanded in consequence, and I found by an examination of larvae of all stages that 

 the gut almost always contained these air-bubbles. If a larva lying quietly out of the 

 water or at the surface is suddenly submerged, it quite probably sinks, but if it is first 

 gently disturbed it at once raises its head and takes an " air-drink," after which it is 

 secure against drowning if it is placed in the water. 



From the fact that larvae can be taken unawares and sunk, it is obvious that the 

 air in the gut disappears and has to be renewed from time to time, but whether such air 

 is of any use in respiration I do not know. 



It is possible, then, to account for several things by this peculiarity of the larva. 

 First, the air-space within the cocoon is probably for the purpose of enabling the larvae 

 to get an " air-drink." A similar space occurs in the cocoon of Hydrophilus, and it has 

 been suggested that it is to enable the cocoon to float. It no doubt makes the cocoon 

 buoyant, but neither in the case of Hydrophilus nor in that of Hydrobius is the cocoon 

 ever built free in the water ; it is always attached to some floating object. There 

 seems, therefore, no good reason for regarding the air as specially for the purpose of 

 buoying up the cocoon. It has also been suggested that the air is for the eggs, but the 

 eggs of Hydrobius— and probably also those of Hydrophilus — hatch perfectly well if 

 taken from the cocoon and submerged. 



Lyonnet observed that the larvae of Hydrophilus swell up three or four times 

 the size of the egg from which they emerge before they have taken any food. This 

 also is probably to be accounted for by assuming that they, like the larvae of Hydrobius, 

 take an " air-drink " in the cocoon. 



