492 EXTERNAL ANATOMY OF INSECTS. 



oval a . In the trapdoor or mason spider (Mygale cce- 

 mentaria), the four small internal ones are round, and 

 the large external ones oval b ; and those that are cir- 

 cumscribed posteriorly with an impressed semicircle, are 

 shaped like the moon when gibbous c . 



The situation and arrangement of simple e3'es are also 

 various. In many they are imbedded, as usual, in the 

 head ; but in the little scarlet mite, formerly noticed d , 

 ( Trombidium holosericeum), they stand upon a small foot- 

 stalk e : the hairiness of this animal might otherwise have 

 impeded its sight. In spiders they are planted on the 

 back of the part that represents the head, sometimes four 

 on a central elevation or tubercle, and the remaining 

 four below it — as in Lycosa,- sometimes the whole eight 

 are on a tubercle, as in Mygale ; and sometimes, as in 

 the common garden-spider (Epeira Diadema), upon 

 three tubercles, four on the central one and two on each 

 of the lateral ones. Other variations in this respect might 

 be named in this tribe. In the scorpions a pair are placed 

 one on each side, on a dorsal tubercle, and the other four 

 or six on two lateral ones of the anterior part of the 

 head f . In the Phalangida the frontal eyes of the scor- 

 pion cease, and only a pair of dorsal ones are inserted 

 vertically in the sides of a horn or tubercle, either bifid 

 or simple, often itself standing upon an elevation which 

 emerges from the back of the animal s. If their eyes 

 were not in a vertical and elevated position, the sight of 



* Walck. Aran. t. If. 2. b /jy. ,;_ \^ 7> 



c Ibid. t. u.f. 18, 20. d Vol. I. p. 323. 



e De Geer vii. 138. t. viii. f. 15. y y, f Ibid. t. xl./. 3. oo,yy. 



g Plate XXVI, Fig. 43. h. 



