212 THE UNIVERSE. 



all the wants of the Arachnis. Here it passes the winter 

 and rears its young; and Avhen it is pressed b)^ hunger, the 

 bell serves as a lair from which the bloodthirsty little 

 creature watches for its prey in order to throAv itself 

 upon it as it passes. This miniature bell adheres to the 

 adjoining grass by a considerable number of threads; just 

 as a balloon is held back by numerous cords till the 

 moment arrives which allows it to soar into the clouds, 

 so do these threads prevent the accumulated air from 

 carrying off the abode. 



These little spiders swim easily, and it is to their ab- 

 solutely aquatic life that they owe their name of naiads 

 {Naiadece), given them by Walckenaer, their clever his- 

 torian. A layer of air, fixed by the hair of their bodies, 

 and which gives them under water the lustre of a living- 

 pearl, materially assists their power of swimming by lighten- 

 ing them. It is by means of this that they succeed in fill- 

 ing their little bell Avith respirable gas so soon as it is built. 

 For this purpose the spider comes to the surface of the 

 stream, takes a bubble of air under its abdomen, and 

 Cannes it to its submerged refuge; and it repeats these 

 voyages till the bell is completely filled with air. 



Entomologists are accpiainted with other hydraulic 

 engineers also, but none of them equal in intelligence the 

 naiads, of which we have just been speaking. 



One of our great French Coleoptera, the water-beetle 

 {Hydrophilus irkeus) , whose name is suggestive of its aquatic 

 habits, also builds an imi^ermeable silken retreat under the 

 Avater, but does not inhabit it, and restricts itself to intrust- 

 ing its progeny to it. It is simply a shell for its eggs. 



In other cases insects build Avith more solid materials. 

 They employ mortar and paste, and are masons, in the 

 true sense of the term; but, instead of Avorking in our fens. 



