283 



XLIII. — Observations on Argyroncta aquatica. 

 By Dr. Felix Plateau *. 



Among the numerous species of the group of Araneides Tuhi- 

 teles of Latreille^ the Diving Spider [Argyroneta aquatica, 

 Walck.) possesses a peculiar interest, both as regards its sin- 

 gular habits and the modifications induced by these habits in 

 the physiological functions of the animal. After being investi- 

 gated in 1 749 by the Abbe de Lignac, who, unfortunately, was 

 not a very good naturalist, and observed a little later in Sweden 

 by Clerck, this spider afterwards fell into a sort of neglect ; in 

 fact the authors who have since paid attention to the Arach- 

 nida only say a few words about this species, or if they describe 

 its habits, they all recur to the same sources, the memoirs of De 

 Lignac and Clerck. But these two observers, although very 

 skilful for their time, had missed some important facts, and 

 were far from having always given satisfactory explanations of 

 those which they had witnessed ; lastly, the embryonic develop- 

 ment had still to be worked out. These various gaps I pro- 

 pose to fill in the present memoir. 



The embryonic development, which I shall not here describe 

 in detail, presents the following peculiarities : — the germinal ve- 

 sicle instead of enclosing only a single spot contains several, 

 sometimes as many as ten, grouped in a variable manner. 

 Wagner has already indicated this exception in the genera 

 Epe'ira, Clubiona, and Salticus. The enigmatical dark body 

 observed by many authors beside the germinal vesicle in the 

 ova of Tegeiiaria, Lycosa, Salticus, and Thomisus is wanting in 

 those of Argyroneta. After the disappearance of the germinal 

 vesicle, which takes place early, the formation of the proligerous 

 disk is accompanied by oily drops like those seen by Kolliker 

 in Lycosa saccata. 



The eggs when laid are not spherical as in most of the 

 Arachnida, but slightly ovoid. The development of the em- 

 bryo follows the well-known course ; but nevertheless I must 

 dwell a little upon the development of the limbs, in conse- 

 quence of the difference of opinion existing upon this subject 

 among the authors who have studied other species. At each of the 

 lateral extremities of the five transverse projections which exist 

 on the ventral lamina of the embryo a dark point appears and in- 

 creases rapidly in size, taking first of all the form of a hemi- 

 spherical excrescence, and then that of a tube, which, gradually 

 elongating, penetrates into the zone formed by the albumen, 

 and only then recurves towards the ventral lamina ; the free 



* From the * Bulletin do TAcad^mie Royale de Belgiqne,' 2*"* s^rie, 

 tome xxiii. 1867. Abstract communicated by the Author. 



