208 EVENINGS AT THE MICROSCOPE. 



seen in the centre of its perpendicular web, on shrubs 

 and in corners of our gardens, the four middle eyes 

 form a square, and the two lateral ones on each side are 

 placed in contact with each other. 



It is interesting to remark that their arrangement is not 

 arbitrary, but is ancillary to the varying instincts and 

 wants of the different kinds. On this subject I will quote 

 to you what Professor Owen says : — "The position of 

 the four median ones is the most constant ; they gene- 

 rally indicate a square or trapezium, and may be com- 

 pared with the median ocelli in hexapod insects. The 

 two, or the two pairs of lateral ocelli may be compared 

 with the compound eyes of insects ; the anterior of these 

 lias usually a downward aspect, whilst the posterior looks 

 backwards; the variety in the arrangement of the ocelli of 

 Spiders always bears a constant relation to the general 

 ronformation and habits of the species. Dujrs has 

 observed that those Spiders which hide in tubes, or lurk 

 in obscure retreats, either underground or in the holes or 

 fissures of walls or rocks, from which they only emerge to 

 seize a passing prey, have their eyes aggregated in a close 

 group in the middle of the forehead, as in the Bird-spider, 

 the Clotho, &c. The Spiders which inhabit short tubes, 

 terminated by a large web exposed to the open air, have 

 the eyes separated, and more spread upon the front of the 

 cephalo-thorax. Those Spiders which rest in the centre 

 of a free web, and along which they frequently traverse, 

 have the eyes supported on slight prominences which 

 permit a greater divergence of their axes : this structure 

 is well marked in the genus Thomisa, the species of 

 which lie in ambuscade in flowers. Lastly, the spiders 

 called Errantes, or wanderers, have their eyes still more 

 scattered, the lateral ones being placed at the margins 

 of the cephalo-thorax." * 



The shining hemisphere (or nearly a sphere) is in each 

 * " Comp. Anat." (Ed. 2), 451. 



