1892.] on the Surface-Film of Water, dc. 545 



the difficulty is actually overcome. During the latter part of the 

 pupal stage, the pupa-case becomes inflated with air, which is ex- 

 tracted from the water, and passed through the spiracles of the fly into 

 the space immediately within the pupal skin. The pupal skin thus 

 becomes distended with air, and assumes a more rounded shape in 

 consequence. At length it splits along the back, in the way usual 

 among insects, and there emerges a small bubble of air, which rises 

 quickly to the surface of the water and there bursts. When the 

 bubble bursts, out comes the fly. It spreads its hairy legs, and runs 

 upon the surface of the water to find some solid support up which it 

 can climb. As soon as its wings are dry, it flies to the trees or 

 bushes overhanging the stream. 



A very interesting inhabitant of the waters, which makes use of 

 the properties of the surface-film to construct for itself a home 

 beneath the surface, is the water-spider (Argyroneta aquatica). This 

 interesting little animal has been described by many naturalists, some 

 of whom, judging from their accounts, had no personal acquaintance 

 with its habits. But among the number is the eminent naturalist 

 Felix Plateau, son of the physicist to whom we are so much indebted 

 for our knowledge of the phenomena of surface-tension. I need 

 hardly say that in his account of the water-spider. Prof. Plateau gives 

 a full and adequate account of the scientific principles concerned in 

 the formation of its crystalline home.* Plateau remarks that the 

 water-spider, like all other spiders, is an air-breathing animal. It 

 dives below the surface, and spends nearly its whole life submerged. 

 In order to do this without interruption to its breathing, the spider 

 carries down a bubble of air, which overspreads the whole abdomen 

 as well as the under side of the thorax. These parts of the body are 

 covered with branched hairs, so fine and close that the surface-film of 

 water cannot pass between them. The spider swims on its back, and 

 the air lodges in the neighbourhood of the respiratory openings, 

 which are placed on that surface which floats uppermost. When the 

 spider comes to the top, as it does from time to time to renew its 

 supply of air, it pushes the abdomen out of the water, and we can 

 then see that this part of the body is completely dry. When it sinks, 

 the water closes in again at a little distance from the body, and the 

 bubble forms once more. 



It would be inconvenient to the water-spider to be obliged to 

 come frequently to the surface for the purpose of breathing. A pre- 

 datory animal on the watch for its victims must lie in ambush close 

 to the spot where they are expected to appear, and the water-spider 

 accordingly requires a lurking-place filled with air, beneath the 

 surface of the water. It has its own way of supplying this want. 

 Eelying on the fact, already illustrated by our muslin bag, that the 

 surface-film of water will not readily pass through small openings, 

 the spider proceeds as follows. It begins by drawing together some 



* " Observations sur I'ArgyroDcte aquatique," Bull. Acad. Roy. de Belgique, 

 2me ser. torn, xxiii. 1867. 



