EXTERNAL ANATOMY OF INSECTS. 49] 
peiana, a spider, the two smallest are round and the rest 
ovals, In the trapdoor or mason spider (Cteniza ce- 
mentaria), the four small internal ones are round, and 
the large external ones oval>; and those that are cir- 
cumscribed posteriorly with an impressed semicircle, 
are shaped like the moon when gibbous °¢. 
The sztuation and arrangement of simple eyes are also 
various. In many they are imbedded, as usual, in the 
head; but in the little scarlet mite, formerly noticed 4 
(Trombidium holosericeum), they stand upon a small foot- 
stalk ©: 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 Phalangide 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’. If their eyes 
were not ina vertical and elevated position, the sight of 
* Walck. Aran. t. i. f. 2. b bid. tia fod « 
© Tbid. t. ii. f. 18, 20. 4 Vou. I. p. 326. 
© De Geer vii. 138. ¢. viii. f. 15. yy. £ Ibid. 4. xl. fi 3. 00, yy. 
& Prate XXVI. Fic. 43. h. 
