§ S05. THE ARACHNOIDAE. 375 



is very variable, and, in front, it terminates between the lens and the vitre- 

 ous body by a ring which resembles an Iris. When two of these stemmata 

 are contiguous, the pigment tunic is common between them.*"' 



The number, the situation, the disposition, and the direction of the eyes, 

 present so many variations, that they have been used by zoologists to char- 

 acterize the genera. Chelifer^ Erythraeus, Smaridia, Tetranychus, Arre- 

 nurus, and the Tardigrada, have two of these organs on the anterior por- 

 tion of the back, while with many Oribatea, they are lateral and anterior. 

 With Trombidivm, there are two eyes also, but they are situated directly 

 'above the first pair of legs, on clavate peduncles.'"'' With the Pycnogo- 

 nidae, and with Obisium, there are four eyes situated on the first segment of 

 the body ; there are the same number, also, with Bddla, Rkijncholophus, 

 Eijlais, Atax, Diplodontus, Hydrachna, and Limnochares, situated on the 

 anterior part of the back.*^' 



With the Opilionina, there are two median, larger, and two lateral, 

 smaller eyes. The first of these are situated on a tubercle, and their cor- 

 neae face right and left.*^' With Galeodes, there are six eyes on the 

 anterior border of the first segment of the body ; of these, the middle or 

 largest pair is directed upwards ; another, situated in front of these last, 

 forwards ; and the remaining pair, inserted above the anteriorlegs, later- 

 ally.*"' The Ai'aneae have, usually, eight eyes; only a few have but six.*''' 

 These eyes, always situated on the cephalothorax, are generally of differ- 

 ent sizes with the same individuu.1, and are either grouped symmetrically 

 upon the anterior median line of the cephalothorax, or scattered on its 

 lateral border.'*' The dorsal eyes are directed upwards, and the marginal 

 ones, forwards or laterally. 



The disposition and direction of these organs are conformable with the 

 animal's mode of life ; some species watch their prey in crevices, fissures, 

 or tubes ; while others remain motionless in the centre of their webs, or 

 lurk from side to side, — a kind of life requiring them to look in all direc- 

 tions. The color of the pigment of the eyes is based also upon the same 

 relations ; for, with the diurnal species, it is green, reddish, or of a 



2 For the structure of the eyes of Arachnoidae, in pairs, so that each pair would easily be taken 

 see Soemmerirag-, De ocul. horn, animal, sect, hor- for a single eye. With Atax, Diplodontus, and 

 izont. p. 74, Tab. III.; and Uaede, Nov. Act. Nat. Hydrachna, the two pairs of eyes are widely sep- 

 Cur. XI. p. 338 {My^ale) ; but especially iV/u/- arated. But with Eylais, and Limnochares, 

 ler, Zur. vergleich. Physiol, d. Gesicht-sinn. p. 316, they are closely appro.ximated. With the young 

 Taf. VII. tig. 8-11, or Ann. d. Sc. Nat. XVII. of these aquatic mites, their position is often dilfer- 

 1829, p. 234, PI. XII. fig. 1-4 (.Androctonufi and ent {Dugi.f, Ann. d. Sc. Nat. I. p. 144, PI. IX. 

 Galeodes). Brants (Tijds. &c. V., or Ann. d. Sc. X.). 



Nat. IX. 1838, p. 308) has confirmed Wfi/Zer's o'o- fVagner''s attributing (Lehrb. d. vergleich. 



servations for the eyes of Buthus and Mygale ; Anat. p. 431) compound eyes to certain Hydrach- 



but he observed, also, tul)es situated behind the nea, is due, without doubt, to his regarding as 



vitreous body, and analogous to those of the eyes of such the approximated simple eyes. "Dujardhi. 



Crustacea and Insecta. Midler, however (Arch. (Ann. d. Sc. Nat. III. p. 19), however, affirms 



1838, p. 139), has been unable to find them, but he that Penthaleus has a single eye, composed of 



observed that the fibres of the optic nerve, after eight to ten facets, while some of the species of 



having entered the eye, are separated by the long Oribates and Mnlgus have only a single stemma 



filamentoid pigment bodies ; and he adds, that situated on the back. 



these fibres should not be confounded with the •■> See Treviranus, Verm. Schrift. I. p. 24, Taf. 



vitreous cones of the faceted eyes, the first becoming II. fig. 10. The two lateral eyes are wanting with 



opaque in alcohol, while the second preserve their many Opilionina. According to Tulk (loc. cit. p. 



transparency. 326, PI. V. fig. 32), there is a pair of muscles iu- 



3 These pedunculated eyes, already figured by serted on the two middle eyes, by which their con- 

 Degeer (loc. cit. p. 57, Taf. VIII. fig. 15, y. y.), tents can be displaced. 



have been described by Hermann (loc. cit. p. 19, C See Mailer, Zur vergleich. Physiol. &c. p. 



PI. III. fig. E. G.), as Oculi inferi ; see, also, 332, Taf. VII. fig. 11. 



Treviranus, Verm. Schrift. I. p. 49, fig. 31, 33, 7 There are six eyes with Scytodes, Se.gestria, 



34, 0. o. Dysdera, and Uptiotes. 



4 With Bde//a, the eyes are wholly lateral. With >i See Savig-ni/, Descript. de l':^gypte, loc. cit. 

 tlie Hydrachnea, above named, they are united PI. I.-VII. and /f'a/cAeraaer, loc. cit. PI. I.-IV., &c. 



