326 MR FRANK BALFOUR-BROWNE ON 



elevating her head to the surface and renewing her air-supply. She also occasionally 

 slightly raises her elytra and separates them and then closes them down again, thus 

 enlarging her ventral bubble from her subelytral air-supply. 



The building of the body of the cocoon occupies an hour or more, but oviposition 

 always commences before the body is complete and often quite early in the building 

 process. I removed one female after she had been spinning for twenty minutes, by which 

 time she had only formed a small cone of silk and she had already deposited three eggs. 

 An egg passes rapidly from the oviduct, scarcely interrupting the spinning. It passes 

 out between the spinnerets and rubs against the fringing hairs of the grooved ventral 

 plate. The spinnerets deliberately place it in the cocoon, and then rapidly spin threads 

 backwards and forwards over it, fastening it down. The spinnerets are remarkably 

 sensitive. On several occasions T ripped gashes in partially formed cocoons and the 

 spinnerets quickly turned in the direction of the rent and closed it up. 



The number of eggs laid in a cocoon is from thirteen to nineteen, and possibly may 

 vary beyond these figures. Once they are laid the female spins a layer of silk over 

 the top and then continues the sides of the cocoon, leaving a certain amount of space 

 unoccupied by eggs but filled with air. From time to time she squeezes the cocoon 

 with her hind tibia?, shaping it ; and, as a rule, this squeezing is specially noticeable where 

 the cocoon ends off with a more or less flattened surface rather like a lid in appearance. 



Once this flat surface is complete the female continues to spin a narrow band on the 

 air-bubble, every now and again pressing it against the grass blade and thus attaching 

 it. She carries this ribbon to the edge of the blade and then spreads it out on the water 

 and on the upper surface of the grass. Its form and length depend upon the position of 

 the cocoon with regard to the surface of the water, and sometimes, instead of being 

 spread out on the surface, it is carried up the support into the air, but under normal 

 circumstances the ribbon always reaches the surface. 



As to the process of spinning, the silk comes off from the apex of each spinneret as 

 a fine thread, and the two spinnerets work side by side backwards and forwards, attach- 

 ing the threads at the end of each stroke. But although the silk thread comes from 

 the apex of the spinneret there is not, so far as I can discover, any canal running up 

 within that organ. The long apical hair, although not solid, has no aperture from 

 which the silk could flow, and the apical and basal segments are, I believe, filled up with 

 tissues. 



So far as I can make out, the silk flows along the outer surface of the spinnerets, 

 which appear wet all the time that spinning is going on. During spinning the 

 spinnerets are fully extended and the grooved ventral plate fits around their bases. The 

 fringe of hairs on the free edge of this plate appears, like the spinnerets, to be wet, and 

 I think the silk fluid flows off the two horn-like processes of the plate which are pressed 

 between the spinnerets and runs out along these. 



I found that once the beetle had fairly started making the cocoon it was possible to 

 lift the grass blade completely out of the water and even to place it under the micro- 



