272 HOUSE, GARDEN, AND FIELD 



the minute algae, growing all the more vigorously because 

 of the air brought to them, altogether hide the habitation. 

 The mouth of the dome, which is of course beneath, 

 is narrowed to a small circle, and then the spider constructs 

 a cylindrical horizontal tube, seven to eight millimetres 

 in diameter, by which she is able to enter or leave her 

 home without being observed. The air within is renewed 

 as required by the regular visits of the spider to the surface. 



Besides this home, which is the ordinary lurking-place 

 of the spider, another is required at the time when the 

 young are to be hatched. The new-born spiders are 

 devoid of the velvety covering of hairs, and would drown 

 if placed in a nursery with a watery floor. The female 

 spider therefore makes a special nest for this occasion, 

 a strong, bell-shaped nest, which floats on the surface 

 of the water and rises well out of it. The upper part is 

 partitioned off, and contains the eggs. Beneath the floor 

 of the nursery the mother takes her station, ready to 

 defend her brood against predatory insects. 



Where animals of a terrestrial, air-breathing stock 

 become adapted to a submerged life, forms less perfectly 

 equipped for aquatic conditions will usually be found 

 among allied species. We know of insects so entirely 

 aquatic in their early stages that they quickly perish 

 when removed from the water, and many gradations 

 can be found to lead from these to purely terrestrial forms. 

 In the same way there are several spiders which connect 

 the water-spider with ordinary hunting spiders. One 

 of these is Dolomedes, which used to be found in our 

 fen-country. The female is large, being 20 mm. (f in.) long, 

 and therefore much bigger than a house spider ; the male 

 is much smaller. They run about on the surface of stand- 

 ing water, and dive when pursued. But Dolomedes has 

 not learned how to make herself a crystalline home beneath 

 the water, a home whose walls consist of nothing more 

 substantial than the surface-film which forms wherever 

 air and water meet. 



