OF THE WATER-BEETLE PELOBIUS TARDUS. 89 



support and vapidly vibrated so tlmt :i cnrrent of water passes 

 under the l)ody and bathes the gills. 



The first stage of larval life occupies from six to eleven days, 

 seven or eight days being apparently tlie usnal period, and the 

 larva then moults. 



There is little difierence in general appearance between first 

 and second stage larvae excej)t that the head is more i-ounded 

 and less trinngular in shape, and the three '" tail " processes are 

 somewhat shorter in proportion to the body-length, but some 

 change has taken place in the number of gill-filaments. Those 

 of the prothoi'a-x remain as in the first stage, but there ai^e now 

 six filaments round the base of each of the middle legs and five 

 round the base of ea,ch of the hind legs. The only change in the 

 abdominal gills is the appearance of an additional pair of very 

 small ones on the first segment, one gill towards either side of 

 the sternite. 



This stage lasts from nine to thirteen days, when the larva 

 once more moults, reaching its third and final stage. 



In the third stage again the body-length increases more than 

 that of the " tail '" processes, which are now only about one-third 

 the body-length, and the gills are also moi^e complex in that each 

 filament shows several transverse constrictions, as if an attempt 

 had been made to produce segmented filaments such as are seen 

 in the larva of Sicdis. As to numbers of filaments, no change 

 takes place on the pro- or meso-thoracic segments, but an 

 additional pair appears on the metathorax, one iisuall}^ very 

 short gill appearing anterior to and outside each of the posterior 

 legs. The abdominal gills do not again increase in number, but 

 remain as in the second stage. A diagram showing the ari-ange- 

 ment of the gills will be found on PI. IIT. fig. 4. 



In all stages of the larva nine pairs of spiracles are to be 

 found, two thoracic and seven abdominal, but, as has been 

 mentioned previously, these all remain closed and functionless 

 throughout the larval period. 



As a rule it was quite easy to determine when a larva had 

 moulted, because the cast skin would be visible in the tumbler or, 

 if the moult had just taken place, the larva would be white, 

 though the pigment appears very quickly after the ecdysis ; but 

 individual larvas possessed idiosyncrasies, some always retiring 

 into the mud just before moulting and not appearing again until 

 their colours had been restored. I have records of quite a 

 number of larvee, amongst my earlier batches, which apparently 

 grew up without moulting at all, and it was not until I discovered 

 skins amongst the mud at the bottom of the tumblers that I 

 realized what had happened. 



4 (e) The Food of the Larva. 



I had considerable difliculty at first in keeping the larvpe alive. 

 I adopted the same method as that used in the case of the 



