I AQUATIC BEETLES 75 



under surface, and here is found a stout and long 

 spine projecting backwards from the last thoracic ring, 

 which has been known to wound the hand when the 

 Beetle is carelessly held. 



The mode of respiration of the adult Hydrophilus 

 is highly peculiar, and altogether different from that 

 of Dytiscus. It was first adequately described by 

 Nitzsch,^ whose account I find to agree in all essentials 

 with the facts which I have myself observed. 



In adult Beetles there are usually two pairs of 

 thoracic spiracles (mesothoracic and metathoracic), 

 and from six to eight pairs more are borne upon the 

 anterior segments of the abdomen. Hydrophilus and 

 Dytiscus have each seven pairs of abdominal spiracles. 

 In Hydrophilus the foremost of these is large, and 

 those behind small, while in Dytiscus the last of the 

 series, near the hinder end of the body, is much larger 

 than those in front. When the Insect rises to breathe, 

 the hinder end of its body is inclined upwards to- 

 wards the surface of the water. In this position the 

 hindmost pair of spiracles give the readiest access 

 to the great air-tubes which run along the body, and 

 it is these which are enlarged. The large spiracles 

 of Hydrophilus however are not found at the tail end 

 of the body, but at the fore end (thoracic and first 

 abdominal), an arrangement which is convenient for 

 the taking in of air by the fore part of the tracheal 

 system. 



Both in Dytiscus and Hydrophilus a large part of 

 the surface of the body is adapted to receive and 



^ Archiv fur die Pkysioloi^ie, von Reil und Autenrieth. Bd. 

 X. pp. 440-458, pi. IX. (181 1 ). 



