May 5, 1892] 



NATURE 



side is greatly reduced, while the lower side is weighted, 

 as before, by the long, water-logged roots. Once restored 

 to the surface, the bubble bursts, and the little drops into 

 which it is instantly resolved roll off like drops of quick- 

 silver. Azolla, which is found in most hot countries, 

 and is often grown in hothouses, behaves in a very similar 

 way. Here the leaves are far smaller, and crowded 

 together upon a branching stem of minute size. There 

 are a few hairs upon the upper surface, and between the 

 leaves are narrow clefts, connected with globular cavities, 

 which occupy the centre of every leaf. These cavities, 

 which are often closed, and never possess more than an 

 outlet of extreme minuteness, are always filled with air ; 

 so are the clefts between the leaves. No water can lodge 

 on the upper surface, apparently because the surface-film 

 is stretched from the raised edge of one leaf to that of the 

 next; and thus buoyancy, self-righting, and repulsion of 

 water are efficiently secured. 



Many plants which ordinarily float on the surface of the 

 water (Salvinia, Azolla, Duckweed, Potaviogelon nutans, 

 &c.) sink on the approach of winter. At this lime it is 

 very curious to see how completely they lose both their 

 buoyancy and their power of repelling water. I do not 

 know how this change is brought about, but the result is 

 one of obvious advantage. The leaves, or in some cases 

 the entire plants, sink to the bottom, and hibernate there, 

 out of the reach of frost. Many perish ; some are broken 

 up by decay into isolated buds. When spring returns, 

 the few survivors float up, and soon cover the surface with 

 leaves. It would be interesting to know something of 

 the mechanism by which these seasonal changes are 

 effected. 



One of the commonest objects in Nature, which is apt 

 to escape our notice on account of its minute size, for it 

 is less than one-quarter of an inch in length, is the egg- 

 raft of the gnat. This was beautifully described 150 

 years ago by Rdaumur. The eggs of the gnat are cigar- 

 shaped, and 250 or 300 of them are glued together, so as 

 to make a little concave float, shaped like a shallow boat. 

 The upper end of each egg is pointed; the lower end 

 is provided with a lid, through which the larva will 

 ultimately issue into the water. The gnat in all stages, 

 even while still in the egg, requires an ample supply of 

 air. It is therefore necessary that the egg-raft should 

 float at the surface ; it is also necessary that it should 

 always float in the same position, so as to facilitate the 

 escape of the larva. This is effectually secured by a 

 provision of almost amusing simplicity. Let us first 

 notice how efficient it is. If we take two or three of 

 these tiny ^gg rafts, and place them in a jug of water, we 

 may pour the water into a basin again and again ; every 

 time the egg-rafts float instantly to the surface ; and the 

 moment they come to the top, they are seen to be as dry 

 as at first. The fact is that the surface-film cannot pene- 

 trate the fine spaces between the pointed ends of the eggs. 

 The cavity of the egg-raft is thus overspread by an air- 

 bubble, which breaks the instant it comes to the top. 

 The larva of the gnat, when it escapes from the egg, 

 floats at the surface, and it is enabled to do so in conse- 

 quence of the properties of the surface-film. When the 

 larva changes to a pupa it becomes buoyant, and floats 

 at the surface, except when alarmed. To enable it to 

 free itself without unnecessary effort from the surface of 

 the water, the respiratory tubes of the pupa are furnished 

 with a valvular apparatus, which can cut the connection 

 with the air in a moment, and restore it at pleasure, when 

 the pupa again floats to the surface.^ 



Another Dipterous insect, whose larva inhabits rapid 

 streams, makes an ingenious use of the properties of the 

 surface-film. This is the larva of Simulium, of which I 

 have given some account in the lecture just quoted. At 



' The larva and pupa of the gnat are more fully described in mv Dritish 

 Association lecture on '• Some Difficulties in the Life of Aquatic Insects," 

 reported in Nature, vol. xliv. p. 457. 



NO. I 175, VOL. 46] 



the time of the delivery of that lecture, I was wholly unable 

 to explain how one difficulty in the life of the insect is 

 surmounted. The larva clings to the water-weeds found 

 in brisk and lively streams. The pupal stage is passed in 

 the same situation. But a time comes when the fly has 

 to emerge. Now the fly is a delicate and minute insect, 

 with gauzy wings. How does it escape from the rushing 

 water into the air above, where the remainder of its life 

 has to be passed ? This was a question upon which I 

 had spent much thought, but in vain. It appeared to 

 me for many months completely insoluble. However, I 

 was informed last year by Baron Osten Sacken of a paper 

 written by Verdat, seventy years ago, in which the 

 emergence of the fly of Simulium is described. Guided 

 by Verdat's description, I had little difficulty in seeing 

 for myself how the difficulty is actually overcome. During 

 the latter part of the pupal stage, the pupa-case becomes 

 inflated with air, which is extracted 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 con- 

 struct for itself a home beneath the surface, is the water- 

 spider {Argyroneta aquaticd). 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 prin- 

 ciples concerned in the formation of its crystalline home.^ 

 Plateau remarks that the water-spider, like most other 

 spiders, is an air-breathing animal. It dives below the sur- 

 face, 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 open- 

 ings, which are placed on that surface which floats upper- 

 most. 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 predatory 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. 

 Relying 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 water-weeds with a few threads, 

 in such a way that they meet at one or more points. It 

 then fetches from the surface a fresh supply of air, and 



' "Observations sur I'Argyronete aquatique," />«//. Acad. Roy. tie 

 Belgique, 2me. s6r., torn, xxiii., 1867. 



