THE SENSES AND THEIR ORGANS. 



291 



FIG. 285. 



On the theory that the dark colored eyes are of the greatest advan- 

 tage in the light, and the light colored eyes most valuable in the dark, 

 one would expect that in the case of cave species the eyes first to disap- 

 pear would be the middle ones, and those longest to persist the side ones; 

 the rear eyes longest of all. I was anxious to test this theory, but unfor- 

 , tunately had but a scant amount of material to do it. However, the few 

 facts at hand are valuable for comparison, and are quite in harmony with 

 the above inference. 



Pavesi has observed 1 that while the species of Nesticus possess nor- 

 mally eight eyes, in a cave dwelling species, Nesticus speluncarum, there 

 are only four, the four middle eyes being atrophied. This suggests that 

 the four central eyes serve especially in daylight. 



The above observation of Pavesi corresponds substantially w r ith Emer- 

 ton's studies of the spider fauna of some of the large caverns of America. 2 

 Out of six species of Lineweavers described, five 

 show some unusual condition of the eyes. Three 

 species have the front middle pair very small ; 

 one has all the eyes small and colorless, with the 

 front middle pair wanting in the males and some 

 females; and one species is entirely without eyes. 

 The complete obliteration of all the front middle 

 pair in some specimens, and the partial atrophy 

 of the same eyes in others, would seem to indi- 

 cate that the organs so situated are of most ben- 

 efit in full sunlight, or, at all events, that sun- 

 light is more necessary to their preservation than 

 the others. 



Several figures are here presented, made from 

 Emerton's drawings, 3 which will illustrate the progressive atrophy of the 

 eyes in the case of some of the spider fauna of the caverns of Kentucky 

 and Virginia. Fig. 286 shows the face of a female Anthrobia mam- 

 mouthia, from which the eyes have been entirely obliterated. Fig. 285 is 

 a drawing of the eyes of a female Linyphia inserta from Fountain Cave. 

 Here the middle eyes of the front row are extremely small, but, neverthe- 

 less, are quite manifest. In contrast with this is Fig. 284, w r hich repre- 

 sents the head and mandibles of a male of the same species (Linyphia 

 inserta), from which the middle front eyes have entirely disappeared. The 

 absence of this or any pair of eyes, so far as my knowledge extends, is in 

 no case a sexual characteristic, so that the disappearance of these eyes, if 

 we suppose the figures to have been drawn correctly, can only be attributed 



1 Sopra una nuova specie de Ragni appartenente alle collezioni dei Museo Civico di 

 Geneva, Ann. Mus. Civ., 1873, page 344. 



2 Notes on Spiders from Caves, Am. Naturalist, Vol. IX., page 278. 



3 Op. cit, plate i., Figs. 5, 18, 21. 



FIG. 284. FIG. 286. 



FIG. 284. Face of Linyphia inserta, 

 with two eyes wanting. FIG. 285. 

 Eyes of another individual, same 

 species, all present. FIG. 286. 

 Face of Anthrobia mammouthi, 

 with eyes atrophied. 



