ADEPHAGA — WATER-BEETLES 



21 I 



degrees, the legs in the genera Bytiscus and Hyclro'porus being 

 but slender, while those of CyMster are so broad and powerful, that 

 a single stroke propels the Insect for a considerable distance. 



The wing-cases fit perfectly to the body, except at the tip, so 

 as to form an air-tight space between themselves and the back of 

 the Insect ; this space is utilised as a reservoir for air. When 

 the Bytiscus feels the necessity for air it rises to the surface and 

 exposes the tip of the body exactly at the level of the water, 

 separating at the same time the abdomen from the wing-cases 

 so as to open a broad chink at the spot where the parts were, 

 during the Insect's submersion, so well held together as to be air- 

 and water - tight. The ter- 

 minal two pairs of spiracles 

 are much enlarged, and by 

 curving the abdomen the 

 beetle brings them into con- 

 tact with the atmosphere ; 

 respiration is effected by this 

 means as well as by the store 

 of air carried about under the 

 wing - cases. The air that 

 enters the space between the 

 elytra and body is shut in 

 there when the Insect closes 

 the chink and again dives beneath the water. The enlargement 

 of the terminal stigmata in Bytiscus is exceptional, and in forms 

 more highly organised in other respects, such as Cijbister, these 

 spiracles remain minute ; the »presumption being that in this 

 case respiration is carried on almost entirely by means of the 

 supply the Insect carries in the space between the elytra and the 

 base of the abdomen.^ The structure of the front foot of the male 

 Bytiscus, and of many other water-beetles, is highly remarkable; 

 the foot being dilated to form a palette or saucer, covered beneath 

 by sucker-like structures of great delicacy and beauty ; by the aid 

 of these the male is enabled to retain a position on the female for 

 many hours, or even days, together. Lowne has shown that the 



1 For many particulars as to respiration of Dytiscus, and peculiarities of the 

 larva see Miall, Aquatic Insects, 1895, pp. 39, etc. (In the figure given on p. 60 

 the large stigma on the terminal segment of the abdomen is omitted, though it is 

 referred to in the text.) 



Fig. 97. — Hind- or swimming-leg of CyMster 

 tripunctatus. A, The whole leg detached ; 

 B, the movable parts in the striking posi- 

 tion, a, Coxa ; 6, trochanter ; c, lemur ; 

 d, tibia ; e, last joint of tarsus. 



