WEB-SPIDERS. 229 



although covered with a fold of the skin, are distinct from each other, and have 

 only migrated part of the distance over the lower surface of the abdomen. These 

 families, Desidce and Argyronetidce, have three-clawed feet like the Agalenidce, 

 but instead of being snare-spinners, fabricate a silken case to serve as a receptacle 

 for their eggs and as a place of refuge. Both have an aquatic mode of life. The 

 first family is represented by the genus Desis, found on the coasts and coral-reefs 

 of the South African, Indo-Malayan, and Australian seas. At low water the 

 reefs and rocks upon which they live are uncovered ; but at the rising of the tide 

 the spiders retreat into holes and crannies, where they surround themselves with 

 a layer of silk strong enough to keep out water. They are good swimmers 

 and feed upon small fish, crustaceans, etc. The Argyronetidai live in fresh water, 

 and are represented only by the water-spider (Argyroneta aquatica), frequenting 

 ponds and ditches in the British Islands and other parts of Europe. Amongst the 



;<^i 



water-spideks, with SEsi's (enlarged . 



waterweeds the water-spider spins a thimble or bell-shaped web, the aperture of 

 which opens downwards. Then, ascending to the surface and thrusting its 

 abdomen out of the water, it succeeds, by some process not clearly understood, in 

 enveloping the hinder part of its body in a film or bubble of air. Retaining this 

 bubble in position by means of its hinder pair of legs, the spider swims down to its 

 web, and inserting its abdomen into the aperture of the bell, sets free the bubble of 

 air, which rises to the upper part of the cavity of the web and replaces a certain 

 quantity of water. The spider then fetches down another bubble, and repeats the 

 process until the web is filled with air; it then has a water-tight chamber, in 

 which it can dwell till all the oxygen is consumed. Here the eggs are laid and 

 hatched. This spider lives on insects which it catches in the water. Belonging 

 to the family Agalenidce is the genus Agalena, of which a British species 

 (A. labyrinthica) is abundant in most districts, and spins a large sheet-like 

 web upon hedges and bushes. At its inner extremity the web ends in a tube 

 communicating at the back with the bush, into which the spider makes its 

 escape when pursued. This spider is exceedingly agile, running with great speed 



