18 



INTEODUCTION. 



surface of the water in order to obtain the requisite supply of air, 

 which they, in most cases, draw more especially through the 

 spiracles situated at the posterior end of the body. It is astonishing, 

 however, how very little air suffices for some insects. I have kept 

 Eubrychius velatus (a well-known small British water-frequenting 

 weevil, which swims like a Dy tiscid) in a very small tightly corked 

 tube of water for some days, and it was none the worse. I did 

 not observe any air-bubble at its posterior end, as is often seen in 

 the Dytiscidse and various aquatic insects when they come to the 

 surface. The amount of immersion that beetles will stand in a flood 

 shows how very different the function of their respiratory system 

 must be from those of the Vertebrates. Probably the tracheae and 

 sacs ramifying throughout the body contain air sufficient to support 

 life for a considerable time in cases of necessity. The fact that 

 Coleoptera can stand a long immersion has, of course, a very 

 important bearing upon the question of their distribution. 



The Organs of Reproduction. 



The external organs of reproduction consist of a male intro- 



mittent organ and a female 

 receptacular organ and ovi- 

 positor, the sexes being always 

 separate. These structures are 

 very varied in form, and should 

 not be spoken of in the terms 

 applied to vertebrate animals, 

 as is usually the case, for, 

 especially in the male, there is 

 no analogy whatever in structure 

 and very little in physiology ; 

 the best term to apply to the 

 male organ and its appurtenances 

 is the sedeagus. The chief in- 

 ternal organs of the female are 

 the ovaries or clusters of egg- 

 tubes ; these clusters are two in 

 number and are situated one on 

 each side of the body. The 

 tubes vary very much in number ; 

 they fill the space of the abdo- 

 men uot occupied by the ali- 

 mentary canal, and are sus- 

 pended to the tissues connected 

 with the *' heart " by thread-like 

 terminations. The formation 

 of these organs has been made 

 use of by several recent authors 

 as an important character in the 

 classification of the Coleoptera, in which order one or more 



— e 



Fig. 10. — Ovarian tubes. Meroistic 

 (on the left) ; holoistic (on the 

 right) ; c, egg-chamber ; n, nutri- 

 ment chamber. (After Lang.) 



