BIOLOGY AND STRUCTURE OF HYDROPHILUS CARABOIDES G47 



pillars, that is to say they have a structure similar in its idea 

 to that of the spiracles and the beginning of the tracheae in 

 other insects, as, for instance, in the caterpillars of Lepidoptera • 

 (Bombyx mori) (Verson and Guayat). 



In the latter instance the inner brush-like structure of the 

 vestibulum plays the part of a filter, serving to retain the 

 dust from the air inhaled. 



In the larva of H. caraboides the pillar-hke cuticula 

 of the initial parts of the tracheae evidently has no special 

 application, since the spiracles themselves are closed by 

 a chitinous membrane. That they are really closed, and there- 

 fore do not act as breathing apertures, can be well observed 

 on a living larva immersed in a fixing fluid, as for instance 

 Duboscq's mixture. The body is compressed by the action 

 of the fixative, and the air comes forth from out of the spiracles 

 in the form of silvery bubbles. Such bubbles appear in the 

 larva of H. caraboides only on the hind end of the body, 

 i. e. from the mouth of the tracheal chamber (atrium stigma- 

 tique). But the lateral spiracles join, as has been observed 

 above, the main longitudinal tracheal trunks by means of 

 narrow branches. 



The tracheal trunks play also an accessory part — they serve 

 as natatory air-bladders which facilitate the ascent of the larva 

 and passive flotation near the surface of the water. 



The larvae of H. caraboides are very sensible to lack of 

 air. During the feeding process they must breathe ; on account 

 of this necessity the larva is obliged to assume a very peculiar 

 and uncomfortable attitude when feeding, as is illustrated in 

 Text-fig. 2. 



If the larva is put into a small vial with water corked in such 

 a manner that there is no bubble of air left inside the vial, 

 and it cannot therefore find the surface of the water, it begins 

 to make restless movements. 



The larva starts up and meets with the glass wall of the vial. 

 It struggles with the end of the abdomen, swims actively 

 about, changes place in search of the surface of the water. 

 Its restless movements become swifter, it turns along the axis 



