Diigesiella hentzi (Girard). 367 



there is a distinct membrane dividing- the retina into two almost 

 equal parts. a distal and a proximal. More material is, liowever, 

 necessary in order to obtain a thorougli understanding- of tliis 

 striicture. 



The dioptric apparatus of the side eyes diifers from that of 

 the posterior middle eyes. In the latter it is represented by a lens 

 which appears almost flat when looked at from above, but whicli is, 

 in reality, slightly convex-concave, of the same thickness as the 

 surrounding chitin and with the concave surface toward the retina. 

 It forms rather in distinct Images which are almost twice the size 

 of the imag-es in the anterior middle eyes, without aberration but 

 with a considerable loss of light, so that a black Square appears 

 throug-h them a lijght grey. It is interesting to note that the Image 

 shows no distortion when the Square is rotated round its axis. 

 although this eye has an oval lens. 



The dioptric apparatus of the anterior and posterior side eyes 

 consists of ellipsoidal lenses and in consequence the Images formed 

 by them show remarkable distortion. A Square appears as a 

 rectangle, the ratio of its sides being in direct proportion to the 

 i-atio of the axes of the lens. The distortion increases when the 

 Square is rotated round the eye axis and reaches its maximum 

 when the sides of the square form an angle of 45" with the long 

 and Short axes of the lens. The shape of the Image is that of a 

 long rhomb. It is easy to see that the long diagonal of this rhomb 

 occupies more rods on the retina than the long side of the rectangle. 

 With other words the Image ofan object Stretches rapidly 

 at every turn either of the tarantula or of the object 

 and in doing this, stimulates a greater part of the 

 retina. It is doubtful whether such distorted Images can convey 

 a correct conception of the object under Observation. It seems to 

 me to be more probable that these eyes serve merely to perceive 

 light and shade, for which they are certainly better adapted than 

 eyes forming correct Images under all angles. 



The follovving obversation shows that Diigesiella perceives direct 

 sunlight. As soon as a ray of sunlight strikes its eyes it Covers 

 them up by drawing all its legs together so that their patellas 

 touch. At other times there is not this reflex action but Dugesiella 

 invariably walks away and tries to get into the shade. Although I 

 have shown that Images of dark objects are formed in all eyes, yet 

 Diigesiella certainly does not perceive such objects. Neither male 



