ARACHNIDA. 451 



indicate a square or a trapezium, and may be compared 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 has usually a downward aspect, whilst the pos- 

 terior looks backwards ; the variety in the arrangement of the ocelli 

 of spiders always bears a constant relation to the general conformation 

 and habits of the species. Dujes has observed that those spiders 

 which hide in tubes, or lurk in obscure retreats, either under-ground 

 or in the holes and fissures of walls and 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 {fig. 165), 

 the clothos, &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 cephalothorax. Tliose spiders 

 which rest in the centre of a free web, and along which they fre- 

 quently 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 am- 

 buscade 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 cephalothorax. The structure of these simple 

 eyes resembles that which has been so well described by Miiller in 

 the scorpion ; Lyonnet had recognised the crystalline lens. The iris, 

 or process of pigment which advances in front of the lens, is green, 

 red, or brown in the diurnal spiders, and black at the back part of 

 the eye. The nocturnal species, as Mygale and Tarantula^ have a 

 brilliant tapetum, but no dark pigment. 



In the scorpion the transparent prominence which indicates each 

 ocellus is a thick dermal cornea, not divided into facets ; it is deeply 

 excavated at the middle of its inner surface for the lodgment of the 

 spherical lens : the back part of this body rests upon, without sinking 

 into, the anterior surface of a hemispherical vitreous body. The 

 interspace between this body and the lens forms a circular channel 

 tilled with aqueous humour and receiving a circular process of the 

 thick pigmental chorion which defines the pupil and confines the lens 

 to the anterior chamber. The pigment coats the retina and covers 

 part of the optic nerve. 



Spiders have the sense of hearing, but neither the organ nor its 

 situation are known. The same may be said of the sense of smell. 

 The membrane lining the mouth and pharynx may have the faculty 

 of taste, and influence the Arachnids in their choice of food. The 

 soft and often hairy integument must be to a certain degree sen- 

 sitive, but touch would appear to be exercised principally by the 



GG 2 



