EXTERNAL ANATOMY OF INSECTS. 491 



sitans, Scorpio maurus, &c. there are eight ; and in Po- 

 dura and. Sminthyrus Latr. there are sixteen a . 



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 mor sitans 

 and many spiders, scorpions, and phalangia b , 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 c ; 

 but in the Tarantula and Sphasus, the two ox four inter- 

 nal are the largest. In Clubiona and Drassus they are all 

 nearly of the same size d ; and in the Micrommata family 

 they are very small e . 



They vary also in shape. In Scolopendra mor sitans the 

 three anterior ones are round, and the posterior one 

 transverse, and somewhat triangular. In Mygale cal- 

 peiana, a spider, the two smallest are round and the rest 



11 DeGeervii. t.'m. f. 8,9, 12. 



» Plate XXVI. Fig. 43. h. c Walck. Aran. t. If. 3. 



d Ibid. t. v.f. 42—48. e Ibid. t. iv.f. 41. 



