254 INTRODUCTION TO ZOOLOGY chap. 



abdomen, and so into the thoracic and large first abdominal 

 spiracles j at the same time the large reservoir of air below 

 the elytra is renewed for use later on, when the beetle is 

 submerged again. There is a row of spiracles on either 

 side of the abdomen opening into the dorsal air reservoir. 

 When the beetle is at the surface, the interchange of 

 air is assisted by the rhythmic movements of the elytra, 

 which alternately lessen and increase the capacity of the air 

 reservoir. 



These beetles feed chiefly on various water- 

 weeds, but apparently they are also carnivorous to 

 a slight extent, though only attacking quite small creatures. 

 In April, the female Hydrophilus may be seen 

 Tion"° at the surface of the water, where the vegetation 

 is fairly thick, preparing the cocoon in which she 

 lays her eggs.^ She has at the end of her body two tubular 



spinnerets from each of which 

 a silk thread issues, and, clinging 

 to the weeds upside down, she 

 weaves a little concave sheet of 

 silk which just covers the up- 

 turned lower surface of her 

 abdomen, attaching the silk to 

 a floating leaf ; then, turning 

 over, she weaves a similar piece, 

 and joins the two together into 

 a cocoon closed at one end (Fig. 

 _-, ™ „ ,.,, 185). She keeps her abdomen 



Fig. 185.— TlieEgg-eocoon of the ,■■,!, . ., K. , 



Silver Water Beetle. hidden inside this cocoon, and 



s, Hollow spike. for the next two hours she lays 



neat rows of eggs in it, until about 

 fifty or sixty eggs hang vertically from its roof with their 

 pointed ends uppermost and with an air space below them. 

 When this process is completed, she begins to close the open 

 end by spinning fresh threads round it; finally a little 

 upright hollow spike is woven which may project nearly an 

 inch above the water (Fig. 185, s). The function of the 



1 As I have not yet had the good luck to watch this whole process, the 

 following description is taken from Lyonnet, M^novres du Mushim, vols, 

 xviii.-xx., and from Miger's account in the Ann. du Muslum d'Hist. Nat., 

 tom. xiv., 1809. 



