490. EXTERNAL ANATOMY OF INSECTS. 
siz. In the majority of spiders and Scolopendra mor- 
sitans, Scorpio maurus, &c. there are eight ; and in Po- 
dura and Sminthurus there are sixteen *. 
As to their structure, nothing seems to have been ascer- 
tained ; probably their organization does not materially 
differ from that of one of the lenses of a compound eye ; 
which I shall soon explain to you. 
Their colour in the many is black and shining, but 
in the bird-louse of the goose they are quite white and 
transparent. In spiders they are often of a sapphirine 
colour, and clear as crystal. In Scolopendra morsitans 
and many spiders, scorpions, and phalangia”, they ap- 
pear to consist of iris and pupil, which gives them a 
fierce glare, the centre of the eye being dark and the 
circumference paler. In the celebrated Tarantula (Ly- 
cosa Tarantula), the pupil is transparent, and red as a 
ruby; and the iris more opaque, paler, and nearly the 
colour of amber. 
Where there are more than two, they vary in magni- 
tude. In the enormous bird-spider (Mygale avicularia) 
the four external eyes are larger than the four internal °; 
but in the Tarantula and Sphasus, the two or four inter- 
nal are the largest. In Clubiona and Drassus they are 
all nearly of the same size*; and in the Micrommata 
family they are very small ¢. 
They vary also in shape. In Scolopendra morsitans 
the three anterior ones are round, and the posterior one 
transverse, and somewhat triangular. In Mygale cal- 
* De Geer vii. ¢. iti. f. 8, 9, 12. 
® Prate XXVI. Fic. 43. h. © Walck. Aran. t. i. f. 3. 
* Tbid, 1. v.f. 42—48. © Ibid. t. iv. f. 41. 
