THE LIFE-HISTORY OF HYDROBIUS FUSCIPES, L. 325 



produced by the rubbing of the tips of the elytra against the abdominal tergites, as 

 these regions are quite smooth. The other British species of the genus, Hydrobius 

 oblongus, Herbst, stridulates much more distinctly, and I believe in that case the 

 noise is made by both sexes, and it certainly is not specially confined to the periods of 

 courting. 



The Spinning of the Egg-Cocoon and Oviposition. 



The cocoon of Hydrobius differs somewhat in form from that of Hydrophilus, and 

 Lyonnet's description of the method by which the female of the latter spins her cocoon 

 would not apply to the female Hydrobius. Whereas Hydrophilus builds a more or less 

 spherical cocoon with a short "mast" projecting unsupported from the water, Hydrobius 

 builds a cocoon attached along one side to its support, and with a flat, ribbon-like process 

 extending to the water surface in place of the " mast." 



According to Lyonnet (1829-32), Hydrophilus builds her cocoon in two parts ; that 

 is, having built so much, she deliberately turns round and constructs a replica of the 

 first part and then weaves the two parts together. Under normal circumstances 

 Hydrobius builds her cocoon straight through without turning round. 



The favourite situation for the cocoon is on the under side of a growing blade of 

 grass floating upon the water. The female first takes up a large supply of air so that 

 her subelytral space is full, and she has also a large bubble attached to her under side. 

 She lies lengthwise along the under side of the blade of grass, and, projecting her 

 spinnerets fully, she brings them and the air-bubble into contact with the grass 

 and the silk begins to flow as a fine thread from the apex of each of the spinnerets. 

 The whole cocoon is formed on the film of the bubble which is on the under side of 

 the insect. The spinnerets work the whole time in the air, pressing against the film 

 and weaving the cocoon on its curved surface. 



The silk will not form threads in water, but comes away from or adheres to the 

 apices of the spinnerets in the form of whitish lumps. It seems to be unusual for 

 the beetle to build the cocoon far below the water. I tried the experiment of submerging 

 the grass blade while the cocoon was being built. After a time the beetle turned to 

 the side of the blade to reach the surface, but failing, she continued to work for a minute 

 or so. She then made more determined attempts, and as she was in danger of asphyxiat- 

 ing, she very reluctantly broke the air-bubble which united her with the cocoon and 

 reached the surface. Having renewed her supply she returned to the cocoon, and by 

 moving her abdomen over it endeavoured once more to take up her work, but after two 

 or three failures she gave it up and later started a new cocoon nearer the surface. 



The silk threads are first spun backwards and forwards across the grass blade and 

 the insect then begins to spin on the film immediately behind her and so raises a wall 

 which soon takes the shape of a blunt cone.- She gradually moves forward, spinning all 

 round the film, and thus extending the cone. She never turns over, but continues facing 

 the same direction all the time, occasionally turning to the side of the grass blade and 

 TRANS. ROY. SOC. EDIN., VOL. XLVII. PART II. (NO. 14). 50 



