Dr. F. Plateau on Argyroncta aquatica. 285 



air brought in, and great numbers of new threads added to the 

 preceding ones by the animal, gradually give the cell its defini- 

 tive form and solidity. I have not seen the construction of 

 the superior dwelling, or the nest properly so called ; but I 

 think we may assume that the Argyroncta proceeds in it as in 

 the first case ; only it would fix the threads but little below the 

 surface of the water, and give the walls a much greater thick- 

 ness. The air accumulated in the nest by the spider would 

 soon raise the top of the habitation a icw millimetres above 

 the surface, the aquatic plants, which serve as points of attach- 

 ment, yielding more or less to the pulling of the threads. 



The Argyroneia, having no tracheal branchise like Hydrachna^ 

 must respire the free air; for this purpose it surrounds itself, 

 as every one knows, with a stratum of air enveloping the abdo- 

 men and clothing the lower surface of the thorax ; this stratum 

 of air adheres firmly, notwithstanding all the movements of the 

 animal. 



De Lignac and, after him, Latreille were puzzled as to the 

 cause of this adherence, and assumed that a grease or varnish 

 secreted by the spider covers the portions destined to receive 

 the gaseous envelope. Some experiments having enabled me 

 to demonstrate that neither grease nor varnish exists at the 

 surface of the body of the Argyroncta, I sought the cause of 

 the phenomenon in the fine short hairs with which the animal 

 is clothed. Professor Duprez, of Ghent, has shown, in a most 

 interesting memoir "On a peculiar case of Equilibrium of 

 Liquids^^ (Mem. de PAcad. Roy. de Belg. tomes xxvi. & xxviii.), 

 that the surface of contact between the air and a liquid presents 

 very great stability when the extent of this surface is suffi- 

 ciently small ; on the other hand, I have ascertained, by experi- 

 ments detailed in my memoir, that the hairs of the Argyroncta 

 are easily wetted. My observations have also shown me that 

 these hau-s become entangled in groups, forming little closely 

 placed bundles, which project, in the living animal, beyond 

 the general stratum of air. These bundles themselves, con- 

 taining air which is in continuity with this stratum, constitute 

 so many points of adherence for the surrounding water; the 

 points in a manner subdivide the surface of the gaseous enve- 

 lope into portions of small extent, and thus give it stability. 



A great number of experiments have shown me that any 

 hairy surfaces, fragments of skins of mammalia, wadding, vel- 

 vet, &c. are covered, when immersed in water, with a stratum 

 of air of greater or less thickness, which continues permanent 

 whenever it is divided into sufficiently small portions by suit- 

 able points of adherence, and which, on the contrary, soon 

 detaches in the form of bubbles when this condition does not 



