Ocellus

“ It is not very unusual to find examples in which a black point, or what may be considered as a rudimentary ocellus, presents itself. On the upper side of hind wing is often a small but complete ocellus near inner angle, but in many cases it is partly or wholly wanting ”

“ The upper ocellus is generally bipupiled, that is to say, the black spot is twinned, and there are two small light spots in it. On the under side the fore wings are as on the upper side. The hind wings are broadly sown with gray scales, giving them a hoary appearance. The base is more or less gray, and there is a broad, regularly curved mesial band of dark gray, which in some specimens is very distinct, in others more or less obsolete. The female does not differ from the male, except that the ocelli on the fore wings are larger and more conspicuous. ”

“ 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) ”

“ Development of the eyes.—Compound eyes do not appear until the beginning of pupal life, the single eye (ocellus) being the primitive organ of vision. The ocellus of Acilius, according to Patten, arises as a pit or depression of the ectoderm (Fig. 542) . The long hypodermal cells which form the walls of this pit or hollow are arranged in a single layer, and bear at their free ends a striated cuticular edge (c) , while from their inner or basal end arise the fibres destined to form the common optic nerve. ”

“ An ocellus in an intermediate condition between the elliptic ornament and the perfect ball-and-socket ocellus. (d fig. 59) , manifestly forms, by its contraction and equalisation, the thickened portion of the ring above the white shade on a perfect ball-and-socket ocellus. The lower part of the ring is invariably a little thicker than the other parts (see fig. 57) , and this follows from the lower black mark of the elliptic ornament (b fig. 59) having originally been thicker than the upper mark (c) . Every step can be followed in the process of confluence and modification ”

“ The caudal horn is shed at the second moult, after which the colour becomes darker, the adult larva (figured by Madame Mérian, in her work on Surinam, pl. 34 and Sepp., pl. 32) being mottled brown. In addition to the ocellus on the fourth segment, there is another slightly larger on the eleventh segment, so that this species may perhaps be another transition to the third group; but our knowledge is still too imperfect to attempt to generalize with safety. ”

“ It is difficult to demonstrate [460] ontogenetic variation because specimens of corresponding size from the same general area may have ocelli of different sizes. The gradient in size of ocelli is also indicated by specimens from other states. I have the subjective impression that there is least variation in specimens from Michigan (Great Lakes-St. Lawrence River drainage) , but this is not clearly shown by Figure 11.
Fig. 11. Ratio of diameter of ocellus to length of plastron (OD/PL) in T. spinifer from some states in the upper Mississippi River and Great Lakes drainages. ”

“ The feathers of the common guinea-fowl offer a good instance of white spots surrounded by darker zones; and wherever the white spots are large and stand near each other, the surrounding dark zones become confluent. In the same wing-feather of the Argus pheasant dark spots may be seen surrounded by a pale zone, and white spots by a dark zone. Thus the formation of an ocellus in its most elementary state appears to be a simple affair. By what further steps the more complex ocelli, which are surrounded by many successive zones of colour, have been generated, I will not pretend to say. ”

“ 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 ”

“ Day-flying Lepidoptera have smaller facets than moths (Lubbock) .
Fig. 260.—Section through the ocellus of a young Dyticus larva: ct, cuticula; l, corneal lens; gh, cells of the vitreous body, being modified hypodermal cells (hy) ; st, rods; re, retinal cells; no, optic nerve.—After Grenacher, from Lang.
The simple, or single-lensed eye (ocellus) .—Morphologically the simple eye is a modified portion of the ectoderm, the pigment enclosing the retinal cells arising from specialized hypodermal cells, and covered by a specialized transparent portion of the cuticula, forming the corneal lens. ”

“ The male has short wings, and the compound eye is replaced by an ocellus on each side of the head, the female having fully developed wings and eyes. A variation may occur in the metamorphosis of this Insect, inasmuch as when the growth is completed during the month of August, the Insect changes to a pupa, the imago appears ten or twelve days thereafter, and the perfect Insect then hibernates for seven or eight months ”

“ Each ocellus consists of a capsule placed at right angles to the surface of the body in the parenchyma, below the dorsal muscles, and with its convex face outwards. It is a single cell in which pigment granules have accumulated. The light, however, can only reach the refractive rods, which lie within it, obliquely at their outer ends. These rods are in connexion with the retinal cells, and thus communicate by the optic nerve with the brain. The cerebral eyes are really paired, and are directed some upwards, some sideways, some downwards. ”

“ Upper Side. Antennæ black. Thorax and abdomen dark brown. Anterior wings chocolate-coloured; the lower corners being tipped with orange, to the upper part of which joins a spot of the same colour, and another small one is placed nearer the tips. The basal parts of the posterior wings chocolate, the apical parts orange; they are doubly angulated, and at the abdominal corners have a black ocellus with two white pupils, and a small orange spot below them; three other small eyes are also placed along the external edges. ”

“ 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. ”

“ But I have elsewhere described (50. ‘Variation of Animals and Plants under Domestication,’ vol. i. p. 254.) an exactly analogous variation in the hackles of a sub-variety of the game-cock, in which the tips, having a metallic lustre, “are separated from the lower part of the feather by a symmetrically shaped transparent zone, composed of the naked portions of the barbs.” The lower margin or base of the dark-blue centre of the ocellus is deeply indented on the line of the shaft. ”

“ The eyes of the butterflies, and indeed of almost all insects, afford materials for a study which is complete in itself. When examined with a tolerably high power, instead of finding each eye with an unbroken spherical surface, it is seen that many are composed of thousands of hexagonal divisions, each being the outer surface of a separate portion termed the ocellus. ”

“ However this may be, appearances strongly favour the belief that on the one hand, a dark spot is often formed by the colouring matter being drawn towards a central point from a surrounding zone, which latter is thus rendered lighter; and, on the other hand, that a white spot is often formed by the colour being driven away from a central point, so that it accumulates in a surrounding darker zone. In either case an ocellus is the result. The colouring matter seems to be a nearly constant quantity, but is redistributed, either centripetally or centrifugally. The feathers ”

“ The average size of the ocelli on the carapace of the subspecies spinifer decreases westward toward the Mississippi River; ocelli of different sizes occur on different individuals from the same state and presumably from the same population. For example, INHS 2281, plastron 9.9 centimeters in length, from Effingham County, Illinois, has some ocelli eight millimeters in diameter, whereas a larger male from the same locality, UI 1322, plastron 11.6 centimeters in length, has the largest ocelli only five millimeters in diameter. ”

“ 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. ”

“ These eggs soon hatch, probably during the heat of summer. The larva is very active in its habits, being provided with six legs, attached to the thorax, on the back of which are the little wing-pads, or rudimentary wings. The large head is provided with enormous eyes, while a pair of simple, minute eyelets (ocelli) are placed near the origin of the small bristle-like feelers, or antennæ. Seen from beneath, instead of the formidable array of jaws and accessory organs commonly observed in most carnivorous larvæ, we see nothing but a broad, smooth mask covering the lower part of the face ”

“ 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. ”

“ Guibé stated that a specimen in the Museum of Natural History, Paris, No. 6957, having a carapace 17 centimeters in length, conforms to the characters of ocellatus as mentioned by Lesueur, and was obtained from the Wabash River by Lesueur. Two of the specimens mentioned by Lesueur (loc. cit.) are stated to be females. No. 6957 is an adult male and clearly shows the juvenal pattern; it is regarded as the lectotype of T. ocellatus Lesueur, a name-combination, which is a synonym, based on a secondary sexual difference in pattern. ”

“ 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. ”

“ The reddish head and neck are nearly bare, shewing wrinkled warty skin and a pendent erectile process on the forehead; a bunch of long black bristles decorates the chest of the male, which has a stout spur on each metatarsus. The bill and feet are red. M. ocellata of Yucatan, British Honduras, and Guatemala, has black plumage, tipped with brassy-green, and fringed with greenish-copper, that becomes redder below; the rump region is steel-blue, and brilliant ocelli of green-blue margined with copper mark the ends of the greyish rectrices and their coverts. ”

“ The muscles and the nerves are resorbed by histolysis, then the external part of the imaginal envelopes and the old parietal hypodermis, reduced very thin and degenerated, is detached in shreds. The antenna becomes external throughout its whole extent. The transformation is in this case, then, almost as complete as in the thorax of Diptera or Hymenoptera. It is necessitated by the change of form and of volume of the head. The region of the ocelli persists unchanged almost alone from the larva to the imago also. ”

“ 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. ”