90 NATURAL HISTORY OF AQUATIC INSECTS CH. 



In April the female Hydrobius lays her eggs in 

 white cocoons, which are to be found in great numbers 

 at or close to the surface of the water. They are often 

 slightly attached to blades of grass. The cocoon 

 is about as large as the Beetle which constructs it, 

 and is formed of silken threads woven together in 

 various degrees of closeness. A female may some- 

 times be found with the unfinished cocoon attached 

 to her body. One end of the cocoon (that first formed) 

 is firm and smooth ; the other end, which the Hydro- 

 philus shapes into a mast, is left loose and irregular 

 by Hydrobius.^ From each cocoon several larvae 

 issue. They are of minute size, but active and in- 

 tensely predatory. The large head, armed with 

 formidable jaws, which are incessantly opened and 

 closed, reveals at a glance the instincts of the creature. 

 Though constantly immersed in water, the larva, like 

 the pupa and the imago, requires a constant supply 

 of gaseous air. This it draws in by the tail. On the 

 dorsal surface of the tail a number of valves and flaps 

 (see fig. 20) form a shallow cup fringed with longer and 

 shorter setae, and into this no water can enter. When, 

 as is usually the case, the larva lies in very shallow 

 water, the tail-end is quite dry, and air is freely drawn 

 into the long tracheal tubes, which open under cover 

 of the fore edge of the cup. If accidentally pushed 

 into deep water, the larva hangs from the surface by 

 its tail, breathing all the time, or creeps along the 



^ The eggs of Hydrobius hatch out readily when completely 

 submerged and wetted with water. The use of the cocoon in 

 this case appears to be rather to conceal and protect the egg 

 than to secure buoyancy. 



