1. Simple Eyes[1434]. We will consider them as to their number, structure, shape, colour, magnitude, situation, and arrangement.
As to their number, they vary from two to sixteen. In the flea, the louse, the harvest-man (Phalangium), there are only a pair; in the bird-louse of the goose (Nirmus Anseris), and probably in others of the same genus, there are four[1435]; in some spiders (Scytodes, Dysdera, and Segestria Latr.[1436]), and some scorpions[1437], there are six. In the majority of spiders and Scolopendra morsitans, Scorpio maurus, &c. there are eight; and in Podura and Sminthurus Latr. there are sixteen[1438].
As to their structure, nothing seems to have been ascertained; 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 morsitans and many spiders, scorpions, and phalangia[1439], they appear 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 (Lycosa 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 magnitude. In the enormous bird-spider (Mygale avicularia) the four external eyes are larger than the four internal[1440]; but in the Tarantula and Sphasus, the two or four internal are the largest. In Clubiona and Drassus they are all nearly of the same size[1441]; and in the Micrommata family they are very small[1442].
They vary also in shape. In Scolopendra morsitans the three anterior ones are round, and the posterior one transverse, and somewhat triangular. In Mygale calpeiana, a spider, the two smallest are round and the rest oval[1443]. In the trapdoor or mason spider (Mygale cæmentaria), the four small internal ones are round, and the large external ones oval[1444]; and those that are circumscribed posteriorly with an impressed semicircle, are shaped like the moon when gibbous[1445].
The situation 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[1446], (Trombidium holosericeum), they stand upon a small foot-stalk[1447]: 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[1448]. In the Phalangidæ the frontal eyes of the scorpion 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[1449]. If their eyes were not in a vertical and elevated position, the sight of these insects would be very limited; but by means of the structure just stated, they get a considerable range of surrounding objects, as well as of those above them. With regard to the arrangement of the eyes we are considering, it varies much. Sometimes they are placed nearly in the segment of a circle, as in those spiders that have six eyes only, before noticed[1450]; sometimes in two straight lines[1451]; at others in two segments of a circle[1452]; at others, in three lines[1453], and at others in four[1454]. Again, in some instances they form a cross, or two triangles[1455]; in others, two squares[1456]; in others, a smaller square included in a large one[1457]; in others, a posterior square and two anterior triangles[1458]; sometimes a square and two lines. Though generally separate from each other, in several cases two of the eyes touch[1459]; and in one instance three coalesce into a triangle[1460]. But it would be endless to mention all the variations, as to arrangement, in the eyes of spiders.
2. Conglomerate Eyes[1461] differ in nothing from simple eyes, except that instead of being dispersed they are collected into a body, so as at first sight to exhibit the appearance of a compound eye:—they are, however, not hexagonal, and are generally convex. They occur in Lepisma, the Iulidæ, and several of the Scolopendridæ. In Scolopendra forficata the eye consists of about twenty contiguous, circular, pellucid lenses, arranged in five lines, with another larger behind them, as a sentinel or scout, placed at some little distance from the main body. In the common millepede (Iulus terrestris) there are twenty-eight of these eyes, placed in seven rows, and forming a triangle, thus
—the posterior row containing seven lenses, the next six, and so on, gradually losing one, till the last terminates in unity. Each of these lenses is umbilicated, or marked with a central depression. In Craspedosoma Leach, you will find a similar formation. In Glomeris zonata, a kind of wood-louse that rolls itself into a ball, the lenses are arranged in a line curved at the lower end, with a single one by itself at the posterior end on the outside; they are oblong and set transversely, and their white hue and transparency give them the appearance of so many minute gems, especially as contrasted with the black colour of the animal[1462]. Between these eyes and the antennæ is another transverse linear white body, but opaque, seemingly set in a socket, and surrounded by a white elevated line, like the bezel of a ring. Whether it is an eye, or what organ, I cannot conjecture[1463]. Its aspect is that of a spiracle.