Ocelli

“ Head, body, and abdomen dark brown. Anterior wings varied with dark brown and orange; in some the dark brown occupying the greater part of the wings, in others the orange colour is predominant; but the tips and external edges in all are dark brown; in some two ocelli are seen close to the anterior edge, and near the lower corner; but in others these ocelli are not to be discovered. ”

“ The ocelli are seen in their simplest form as a pigmented patch of ectoderm, which consists of two kinds of cells— (1) pigment-cells, which are ordinary indifferent cells of the epithelium containing pigment-granules, and (2) visual cells, slender sensory epithelial cells of the usual type, which may develop visual cones or rods at their free extremity. The ocelli occur usually either on the inner or outer sides of the tentacles; if on the inner side, the tentacle is turned upwards and carried over the ex-umbrella, so as to expose the ocellus to the light ”

“ But if the ocelli are intended for near objects, it is difficult to understand why they are surrounded by a growth of hair so dense as to permit unobstructed vision only in a very narrow field, and why they are so placed on the top of the head as to be debarred from seeing any objects in the neighborhood of the mandibles and the proboscis, the ability to see which objects would appear to be very necessary in the constant and delicate labors of the worker honey-bee among the flowers. ”

“ Genal angles produced into short bluntly acute spines. Between the prominent reniform compound eyes there is a pair of elongate elevations which border a median depression in which lies a circular ocellar mound 1.3 mm. in diameter. This mound is situated directly in front of a line tangential to the posterior borders of the eyes and was apparently the seat of the ocelli. A second pair of elevations run obliquely from the middle of the posterior border towards the lateral margins, so that the eyes are situated in triangular, depressed areas of the test. ”

“ Carapace olive, having large ocelli in center but smaller ocelli or spots at sides; ocelli often interrupted; pale rim of carapace not four or five times wider posteriorly than laterally, separated from darker ground color of carapace by one dark marginal line; large females often having remnants of ocelli at sides of carapace on mottled and blotched background; pattern on snout of pale, dark-bordered stripes that unite forming acute angle in front of eyes; well-defined dark markings in subocular and postlabial region ”

“ Its two huge eyes extend in two vast oval patches from the center of the top of the cranium down either side of the head to below the roots of the horns, so that these weapons really grow out from the lower part of the eyes, which are composed of several thousand ocelli each. Each ocellus is furnished with its own lid, and the apt can, at will, close as many of the facets of his huge eyes as he chooses. ”

“ The ocelli are absent in the females of certain insects, as in the Mutillidae; and here the females are likewise wingless. But we are chiefly concerned with structures by which one male is enabled to conquer another, either in battle or courtship, through his strength, pugnacity, ornaments, or music. ”

“ The colour beneath is dark brown, the upper wings with three large ocelli having a white pupil surrounded with ferruginous and violet, the iris yellow, 175 enclosed in a green circle which has a crescent of the same colour on the inner side; under wings with four similar ocelli, three of them contiguous, the interior one insulated. Towards the base of both wings are several transverse flexuose green stripes, and along the outer edge three greyish lines, more or less interrupted with red, especially in the hinder wings. The body is black above and brown beneath. ”

“ At Coalburgh, W. Va., Nephele is never seen, but Alope is the only form; and so on southward.
2.—Alope was described by Fabricius, Ent. Syst., 1793, as fuscous (fusca) with a yellow (flava) band; with two ocelli on fore wings; on hind wing one ocellus above, six below. The band is very broad in the female, usually narrower in the male, pale yellow in both sexes. The ocelli resemble those of Nephele and vary in same manner. Usually they are round, but sometimes oval; are either small or large, often equal, but sometimes the upper is larger, at others the lower. ”

“ Young.—“An immature male resembles the female, but has tail and greater coverts like those of adult male, though the ocelli are much smaller and absent on inner webs of all the tail-feathers except three middle pairs; one or two feathers of mantle have a metallic bluish green patch in the middle and traces are apparent of black plumage on mantle, wing-coverts, throat, and under parts.” ”

“ On either side, just above the mandibles, are located the eyes, or ocelli, which in the caterpillar are simple, round, shining prominences, generally only to be clearly distinguished by the aid of a magnifying-glass. These ocelli are frequently arranged in series on each side. The palpi are organs of touch connected with the maxillæ and the labium, or under lip, and are used in the process of feeding, and also when the [ 7] caterpillar is crawling about from place to place. The larva appears to guide itself in great part by means of the palpi. ”

“ The Eyes consist of a very large number of simple lenses arranged in the form of two hemispheres, one on each side of the head. The ocelli, or simple eyes, are situated on the crown, and are usually almost entirely covered by scales. ”

“ These are the eyes. A section cut through [Pg 150] one of these eyes and placed under the microscope would show that the surface of the eye was formed by a transparent body like a lens, and that behind this lay a complicated arrangement of rods passing gradually into the nerves of sight. Only ocelli, as these eyes are called, are found in the spider and his kind. But in true insects, like the dragon-fly, or the butterfly, we meet with eyes of another kind, in addition to ocelli. These are known as compound eyes. ”

“ The wing-feathers, bearing the ocelli, are covered with dark stripes (fig. 57) or with rows of dark spots (fig. 59) , each stripe or row of spots running obliquely down the outer side of the shaft to one of the ocelli. The spots are generally elongated in a line transverse to the row in which they stand. They often become confluent, either in the line of the row—and then they form a longitudinal stripe—or transversely, that is, with the Fig. 57. Part of secondary wing-feather of Argus pheasant, shewing two perfect ocelli, a and b. ”

“ Two large males (TU 11580, 13025) have large ocelli (approximately five mm. in diameter) that have nearly black centers. In general, there is more dark pigmentation than farther north; some specimens have extensive pigmentation on the ventral surface of the carapace and soft parts of the body (TU 156, 5648) . ”

“ They may, however, come into close contact and form one single, large, compound eye. Such ocelli, in a very large number of cases, retain each its own dioptric apparatus, and therefore the external appearance of the compound eye represents not a single lens, but a large number of facets, as is seen in the eyes of insects. ”

“ Larger ratios are usually derived from measurements of larger individuals. Seemingly, there should be a clinal tendency in ontogenetic variation paralleling the size of ocelli and dependent on it; ontogenetic variation should be least in western populations in which the size of ocelli does not change appreciably with increasing size, and should be greatest in eastern populations in which the ocelli on adult males are larger than those on the carapace of young turtles. ”

“ The two posterior ocelli are placed on a transverse ridge, and the central lobe of the labium is rather large, while the two lateral ones are divergent, armed with a strong tooth, and having a spinous appendage attached to them. The larva and nymph are more elongated than in Libellula, and the structure of the mask is somewhat different. About half a dozen species occur in Britain. Æ. grandis is two inches and a half in length, of a fulvous brown colour, with two yellow stripes on each side of the thorax, the abdomen variegated with green or yellow. ”

“ Immature insects rarely possess compound eyes; they are only known to occur in the nymphs of Odonata and Ephemeridæ, and in the larvæ and pupa of Corethra.
Mode of vision by single eyes or ocelli.—In their simplest condition, the eyes of worms and other of the lower invertebrates, probably only enable those animals to distinguish light from darkness. The ocelli of spiders and of many insects, however, probably enable them, as Lubbock remarks, to see as our eyes do. The simple lens throws on the retina an image, which is perceived by the fine terminations of the optic nerve. ”

“ Most free-living forms are minute, and all have thin tests.
[111]
The eyes of copepods are of interest, in that they suggest the paired ocelli of the Harpedidæ and Trinucleidæ. In the Copepoda there are, in the simplest and typical form of these organs, three ocelli, each supplied with its own nerve from the brain. Two of these are dorsal and look upward, while the third is ventral. In some forms the dorsal ocelli are doubled, so that five in all are present (cf. some species of Harpes with three ocelli on each mound) . ”

“ There are two eye-spots on the fore wings in the usual location, indistinct on the upper, distinct on the lower side of the wings. The under sides of the wings are variable. In the type they are dark; in other specimens they are paler. They may or may not have ocelli on the secondaries. The form with obsolescent ocelli has been named silvestris by Edwards. Both the fore and hind wings are abundantly and evenly marked by little striæ, and crossed on either side of the median area by obscure, irregular, transverse dark lines, either one or both of which may be wanting in some specimens. ”

“ Diagnosis.—Juvenal pattern uniform tan or brownish lacking markings, having whitish dots or spots, or having well-defined, blackish ocelli or spots; surface of carapace "sandpapery" in adult males; conical projections (in some subspecies) along anterior edge of carapace in large females; contrasting pattern of blackish marks on pale background (in some subspecies) on dorsal surface of limbs of adult males. ”

“ The node is so slightly prominent that the eyes hardly project beyond the outline of the carapace. The visual surface is crescentlike with somewhat swollen extremities [pl. 65, fig. 10] and situated along the outer edge of the ocular node. The ocelli are distinct, separated by the length of their own diameter, and situated on a flat circular tumescence that lies on a line connecting the inner angles of the compound eyes.
Abdomen. The abdomen is slender, increases slightly in width to the fourth dorsal segment and then tapers very gradually to the telson. ”

“ I may digress by adding, that if the smaller workers had been the most useful to the community, and those males and females had been continually selected, which produced more and more of the smaller workers, until all the workers were in this condition; we should then have had a species of ant with neuters in nearly the same condition as those of Myrmica. For the workers of Myrmica have not even rudiments of ocelli, though the male and female ants of this genus have well-developed ocelli. ”

“ It will be seen that the third and the fourth longitudinal veins are fused from the base to and beyond the {54}point at which in normal flies the anterior cross-vein lies. The cross-vein and the cell normally cut off by it are absent. There are a number of other features (see fig. D c) characteristic of this mutation: the wings are held out at a wide angle from the body, the ocelli are very much reduced in size or entirely absent, the bristles around the ocelli are usually small. The females are absolutely sterile, not only with their own, but with any males. ”

“ Scoliidae.
The members of this family, so far as is known, display less perfect instincts than the Sphegidae and Pompilidae, and do not construct cells or form burrows. Information as to the habits is almost confined to European forms. We adopt five sub-families.
Sub-Fam. 1. Mutillides.—The sides of the pronotum reach the tegulae: the female is destitute of wings and ocelli, frequently having the parts of the thorax so closely soldered that the divisions between them are obliterated: the males are winged, furnished with ocelli, and having the thoracic divisions distinct ”

“ F. Smith has observed, the larger workers have simple eyes (ocelli) , which, though small, can be plainly distinguished, whereas the smaller workers have their ocelli rudimentary. Having carefully dissected several specimens of these workers, I can affirm that the eyes are far more rudimentary in the smaller workers than can be accounted for merely by their proportionately lesser size ”

“ In some genera they are absent, as, for instance, in Pelagia, Cyanea and Rhizostoma. In Aurelia there are two on each rhopalium, a simple ocellus on the exumbral side, and a cupped ocellus on the subumbral side (not present in young individuals) . In Charybdaea there are no less than six ocelli on each of the four rhopalia (fig. 7) ; on the exumbral aspect there are two median ocelli (oc1, oc2) , a distal and a proximal, each of them a vesiculate ocellus with a lens, and on the sides of the rhopalium are two pairs of ocelli without lenses (oc.l) ”

“ As with many other flies and other insects, there are on the top of the head very small simple and rather inconspicuous eyes called ocelli, three in number, between the large and prominent compound eyes, which latter are said to possess each four thousand facets. ”