TRACHEAL GILLS OF LARVM 



473 



The larvae of the small water beetles of the family Elniidae (Elmis, 

 Potamophilus, Macronychus, and Psephenus) have similar habits. 

 That of Elmis has ten dorsally situated pairs of spiracles, and on the 

 end of the body bushy gills which are protruded at pleasure. The 

 young larva is without spiracles, its tracheal system being closed. 

 Macronychus and Potamophilus have similar habits. In the larva of 

 the latter genus, which has nine pairs of spiracles, there are at the 

 end of the body on each side three tufts of 

 thread-like gills which are connected with 

 the two main horizontal tracheae, while the 

 branches of the abdominal tracheae are 

 dilated into numerous (64) bladder-like 

 sacs. The larva usually breathes through 

 the caudal gills. When the water is low 

 or dried up, the air is inhaled directly FIG 453 



through the spiracles. (Kolbe.) Iuiar f ed Psei>henus ' 



FIG. 454.- The larva of Psephenus leconteL by its 



Larva of (iyri- . . * J 



nus. After broad hemispherical body, is adapted to adhere to the 



Westwood. 



smooth surface or rounded stones, in which situation we 

 have found it. Although it is said by Rolph to have two pairs of 

 spiracles, one pair on the mesothoracic and the other on the 1st 

 abdominal segment, it probably rarely rises to the surface to breathe 

 the air direct. 



a 



Fio. 455. End of body of a Psyehodes larva : A, end of body of a youne. freshly moulted larva, 

 side view : ., the three anal Drills ;'i, the left air-cavity. B, older larva of the same species, with 

 tin- open air-cavity seen from above. C. end of larva of another species as it goes down into the 

 water, with a bubble of air, ft, between the crown of hairs of the air-cavity or tube : a, the two pairs 

 of anal trills ; b, the two main trachea 1 . After F. Miiller. 



It possesses five pairs of gills on the under side of the 2d to the 

 6th abdominal segments. Each gill has finger-shaped processes on 

 its hinder edge, which are "from their constant motion evidently 

 connected with respiration." Tracheae may be seen, according to 

 H. J. Clark, entering the gills, and " the circulation of water among 



