202 THE UNIVERSE. 



serves as a lair, from which the bloodthirsty little creature 

 watches for its prey, in order to throw itself upon it as it 

 passes. This miniature bell adheres to the adjoining grass 

 by a considerable number of threads ; and 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 entirely 

 aquatic life that they owe their name of naiads (Naiadece), 

 given them by Walckenaer, their ingenious historian. A 

 layer of air, fixed to the hair of their bodies, and which 

 gives them under water the lustre of a living pearl, materi- 

 ally assists their power of swimming by lightening them. 

 It is by means of this that they succeed in filling their little 

 bell with respirable gas so soon as it is built. For this pur- 

 pose the spider comes to the surface of the stream, takes a 

 bubble of air under its abdomen, and carries it to its sub- 

 merged refuge ; and it repeats these voyages till the bell is 

 completely filled with air. 



Entomologists are acquainted with other hydraulic en- 

 gineers also, but none of them equal in intelligence the 

 naiads, of which we have just been speaking. 



One of the great French Coleoptera, the water-beetle {Hy- 

 droplulus piceus), whose name is suggestive of its aquatic 

 habits, also builds an impermeable silken retreat under the 

 water, 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 with more solid materials. 

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

 sense of the term ; but, instead of working in the fens, set 



