§ 205. THE ARACHNOIDAE. 8Ty 
is very variable, aad, 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 Trombidium, there are two eyes also, but they are situated directly 
above the first pair of legs, on clavate peduncles.” With the Pyenogo- 
nidae, and with Odzsiwm, there are four eyes situated on the first segment of 
the body; there are the same number, also, with Bdella, Rhyncholophus, 
Eylais, 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 Araneae 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 individual, 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, 
see Soemmering, De ocul. hom. animal. sect. hor- 
izont. p. 74, Tab. III.; and Gaede, Nov. Act. Nat. 
Cur. XI. p. 338 (Mygale); but especially Mul- 
ler, Zur. vergieich. Physiol. d. Gesicht-sinn. p. 316, 
Taf. VII. fig. 8-11, or Ann. d. Sc. Nat. XVII. 
1829, p. 234, Pl. XII. fig. 1-4 (Androctonus and 
Galeodes). * Brants (Tijds. &c. V., or Ann. d. Sc. 
Nat. LX. 1838, p. 308) has confirmed Miiller’s ob- 
servations for the eyes of Buthus and Mygale ; 
but he observed, also, tubes situated behind the 
vitreous body, and analogous to those of the eyes of 
Crustacea and Insecta. Muller, however (Arch. 
1838, p. 139), has been unable to find them, but he 
observed that the fibres of the optic nerve, after 
having entered the eye, are separated by the long 
filamentoid pigment bodies; and he adds, that 
these fibres should not be confounded with the 
vitreous cones of the faceted eyes, the first becoming 
opaque in alcohol, while the second preserve their 
transparency. 
8 These pedunculated eyes, already figured by 
Degeer (loc. cit. p. 57, Taf. VIII. fig. 15, y. y.), 
have been described by Hermann (loc. cit. p. 19, 
PL III. fig. E.G.), as Oculi inferi; see, also, 
Treviranus, Verm. Schrift. I. p. 49, fig. 31, 33, 
34, 0. 0 
4 With Bdedla, the eyes are wholly lateral. With 
the Hydrachnza, above named, they are united 
in pairs, so that each pair would easily be taken 
for a single eye. With Ataxz, Diplodontus, and 
Hydrachna, the two pairs of eyes are widely sep- 
arated. But with Kylais, and Limnochares, 
they are closely approximated. With the young 
of these aquatic mites, their position is often differ- 
a. (Duges, Ann, d. Sc. Nat. I. p. 144, Pl. IX. 
Xx. 
Wagner's attributing (Lehrb. d. vergleich. 
Anat. p. 431) compound eyes to certain Hydrach- 
nea, is due, without doubt, to his regarding as 
such the approximated simple eyes. Dujardin 
(Ann. d. Sc. Nat. IIL. p. 19), however, affirms 
that Penthaleus has a single eye, composed of 
eight to ten facets, while some of the species of 
Oribates and Molgus have only a single stemma 
situated on the back. 
5 See Treviranus, Verm. Schrift. I. p. 24, Taf. 
II. fig. 10. The two lateral eyes are wanting with 
many Opilionina. According to T'ulk (loc. cit. p. 
326, Pl. V. fig. 32), there is a pair of muscles in- 
serted on the two middle eyes, by which their con- 
tents can be displaced. 
6 See Miller, Zur vergleich. Physiol. &c. p. 
332, Taf. VII. fig. 11. 
7 There are six eyes with Scytodes, Segestria, 
Dysdera, and Uptiotes. 
8 See Savigny, Descript. de Pfigypte, loo. cit. 
Pl. I-VIL. and Walckenaer, loc. cit. Pl. I.-LV., &c. 
