2<x> Falcofwr : A bnormality in Spiders. 



eye eminence irregularly shaped and angularly produced in 

 striking contrast to the rounded and symmetrical contour which 

 is characteristic of a properly equipped specimen. 



I have unfortunately not made notes of all the cases of 

 partial blindness I have seen, but have noticed that the male 

 of Tiso vagans Bl., in which the eyes are naturally minute, 

 seems to be more liable to this kind of deformity than any other 

 species, of which I have had experience, as I have at various 

 times taken several examples of it thus afflicted. 



In a short paper, ' East Coast Spiders,' ' Naturalist,' January 

 1906, p. 29, I recorded a very curious specimen of a female, 

 Hilaira excisa Camb., from Hayburn Wyke, North Riding, 

 which had only half its eyes in a serviceable condition, two 

 laterals on the same side being altogether obsolete, the two 

 centrals next to them greatly reduced in size and imperfectly 

 formed, and the caput in some particulars abnormal also. 



The effects of this form of blindness are, of course, restricted 

 to the individual, and do not in the least affect the race. It 

 is, therefore, quite different in character from that which 

 prevails amongst certain exotic cave-dwelling spiders, in which 

 it is commonly supposed to be the cumulative effect, through 

 countless generations, of darkness so intense as to render the 

 faculty of sight futile and the possession of eyes useless, with 

 the result that these organs have gradually degenerated to 

 mere pigment cells, and, in course of time, all traces of them 

 have entirely disappeared. The blindness is here a character- 

 istic of the species and not merely of the individual. This is, 

 however, an extreme case. All cave-dwellers are not blind, 

 though some have their eyes reduced in number or diminished 

 in size. Neither are all those species with eyes lessened in 

 number or in size troglodytes. Several British spiders, for 

 example, Tiso vagans BL, Thyreosthenitis hiovatus Camb., and 

 the members of the genus Porrhomma Sim., are particularly 

 noticeable for the small size and often imperfect structure of 

 their eyes. Some of the Porrhommata were originally discovered 

 in caves and coalmines, having been conveyed to the latter in 

 fodder, and the condition of darkness under which they there 

 existed, was held to be sufficient to account for the peculiar 

 state of their eyes. We now know that they are not confined 

 to such places, but may be found in the open at the roots of 

 grass, amongst fallen leaves and beneath stones, and that 

 these characteristics are not altered thereby. We may con- 



Naturalist^ 



