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THE INTELLECTUAL OBSERVER.

DECEMBER,

1867.

INSECTS' EGGS: THEIR STRUCTURE, VARIETY, AND BEAUTY.

BY JABEZ HOGG, F.L.S., HON. SEC. R.M.S., ETC.

(With a Coloured Plate.)

IN our country rambles, some among us may have been tempted to examine what appeared to be curled-up leaves, or patches of whitish dots adhering to branch or stem, many so like the bark of the tree itself, that at a first glimpse, even the practised eye might have failed to detect that they were organised bodies. Upon more closely and attentively scrutinizing these little adhesions, they are seen to be collections of insects' eggs, thus skilfully deposited and concealed with maternal care. To such apparently unimportant objects I would direct the reader's attention, and more particularly to their structure, variety, and beauty, as disclosed by the aid of the microscope. My observations will be mainly confined to the eggs of the Lepidopterous insects, which at this period of the year may be secured in large numbers, for the purpose of making a careful comparison of their formation, watching the development of the germinal vesicle into that of the fully-formed embryo, and noting the transformation of the creeping caterpillar to a thing of wondrous life and beauty, taking its flight among the loveliest of nature's handiworks, and sipping honey from every flower "from morn to dewy eve.'

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Nearly all insects are oviparous; the few instances in which this is believed not to be the case, are not positive deviations from the general law. The eggs of insects, however, do not often fall under notice, for in consequence of their smallness, they escape observation; and from the scrupulous care taken by the parent to conceal them from the depredations of their numerous enemies, they are not easily discoverable. The situation mostly selected by the female moth, is the leaf or bark of such trees and plants as will serve their young for food. Sometimes, with an instrument provided by Nature for the

VOL. XII.-NO. V.

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purpose, she bores a hole in the bark in which to deposit them. Not infrequently the interior of fruit or grain, or even a dung-heap is selected, and some few commit their store to water, there to await the heat of the summer sun to hatch the brood.

Their defence against cold, and atmospheric changes consist in coating the inside with a varnish-like substance, while the outside is often covered over with a denser material, as portions of vegetable fibre and the hairs or feathers from the body of the insect, which, together with the leaf on which the eggs are laid, form groups of tiny nests. Others form stronger and more durable receptacles. The female cock-roach constructs a strong horny bag, or purse, in which having deposited her eggs, she carefully carries it about with her. The coccus converts her whole body into a shield or covering for her eggs, so thoughtful does she appear for the future safety of her brood. As a general rule, the female insect having deposited her full number of eggs, leaves them to be hatched by the heat of the sun, which may be accomplished in a few days, or not until the following spring, as is the case with the eggs of the silkworm, and all those laid late in the summer or autumn. The number of eggs produced by different species is very varied, some depositing only two, while others, and by far the largest number, lay them by hundreds. The ant is said to lay from thirty to forty thousand in a year. The queen-bee, fifty thousand; but I believe the ordinary number is much below this. The silkworm moth mostly lays about five hundred eggs. The gcat-moth, about a thousand, and the tigermoth some fifteen hundred, arranged with the most uniform and symmetrical order. Insects' eggs bear great extremes of temperature without losing their vitality; want of air and light appear to be far more speedily detrimental in this respect than the extremes of heat or cold. I have exposed silkworm's eggs to severe frosts, and also plunged them into scaldingwater, without in the least affecting them, either for good or evil. Several genera of moths with wonderful instinct, cover their eggs with soft vegetable materials; and some, it is asserted, pluck the hair and feathers from their bodies for the same purpose. A writer states, I think, without sustaining the statement by sufficient evidence, that moths use a pair of pincers, placed at one extremity of the body, for plucking out their hairs to cover their eggs; but this would scarcely seem to be needful, since the eggs are nearly always deposited in groups, and securely fastened up in the leaf on which they have been deposited, so that frequently it requires some force to separate the bundle, and expose them to view.

Some of the Coleopterous insects resort to curious and inge

nious methods of concealing their eggs-for example, the burying-beetle deposits hers in the decaying carcasses of such animals as moles and mice, which they bury, if not already beneath the surface. It is stated by an observer of their operations, that in order to effect this object, several beetles unite their labours and remove the earth from beneath the dead body, which gradually sinks; they then proceed to cover it up, and frequently run backwards and forwards, apparently for the purpose of ramming down the earth. According to an eye-witness of the operations of these grave-diggers, four beetles were observed to inter in a very small space of earth, no fewer than twelve carcasses of various small animals. The object of all this care and solicitude, is not alone the security of their eggs, but to ensure an early supply of food for their young. Another example is afforded from among the Scarabaida, or larger kind of Dung-chaffers. The earth-borer (Geotrupes stercorarius), an insect whose "drowsy hum" falls so often on our ear, during a walk in the country in the stillness of an autumnal evening, digs round holes in the earth, often of considerable depth, then conveys a small quantity of dung to the bottom, in which the eggs are deposited. Each of these is placed in the centre of a small ball or pellet carefully prepared for this purpose; when dry enough, the pellet is transported, it may be to some distance from its place of preparation, to be buried in the hole dug for its reception, and what is somewhat remarkable, when it is unable to raise its load from the ground, it rolls it along, or pushes the pellet backwards with its hind legs. When the surface of the ground is irregular, the labour is proportionably increased, and not unfrequently the beetle is obliged to call in the assistance of its help-mate, before it can overcome the obstacles which impede it. According to some writers, the incessant and arduous labour which these beetles were observed to undergo, led the ancient Egyptians to regard them with a sort of sacred awe, and as symbolical of the labours of Osiris.

In form, colour, character, and beauty of design, the eggs of insects are more surprisingly varied than those of the feathered tribes; but our acquaintance with the composition of either exterior or interior is certainly not so complete as in the case of birds. The eggs of the animal series differ considerably in their external characteristics, nevertheless, all closely resemble each other, while yet a part of the ovarian ovum. At one period of their formation, all eggs consist of three nearly similar parts. First, The internal nucleated cell or germinal

The elaboration of structure and variety of forms in a large number of eggs, might be turned to a practical account, as many suggest patterns of great beauty and delicacy for art-designs.

vesicle, with its macula. Second, The vitellus or yolk-substance; and Third, The vesicular envelope or vitelline membrane. The germinal vesicle is first produced, and may be regarded as the ovigerm; the yolk-substance next gradually envelopes it, or is deposited around the germinal vesicle, and the vitelline membrane which encloses the whole, is the latest formed.

The chemical constituents of the ovum is albumen, fatty matters, and a large proportion of a substance precipitable by water. "The production of the chorion or shell-membrane does not take place until the ovum has attained to nearly its full size, and it appears to proceed, in part, from the consolidation over the whole surface of one or more layers of an albuminous fluid secreted from the wall of the oviduct. The observations of Herman Meyer have shown that a part of the outer membrane is also derived from a conversion into it, of the inner cellular or epithelial lining membrane of the oviduct, at the place where it is in closest contact with the surface of the ovum. And many of the varieties in the appearance and structure of the external covering, may probably depend on the different modes of development of these cells."*

The embryo cell appears to be so directly connected with the germinal vesicle, that at a certain period it is absorbed and entirely disappears; or rather, "the germinal yolk becomes the nucleus of the future embryo, when a greater degree of compactness is observed to take place in the yolk, and all that remains of the germinal vesicle is one or more highly refracting fat globules." In insects' eggs, as in those of the higher animals, a clear space is seen between the surface of the yolksubstance and the enclosing vitelline membrane.

The shell is furnished with a lid, to facilitate, it is said, the egress of the mature worm; but since we find that the whole integument offers little resistance to the strong and well formed mandible of the creature, an operculum, or lid, seems to be unnecessary for the purpose stated by continental writers. The chorion, in many instances, is so very thin and translucent, that even the changes taking place within can be readily seen; indeed, I have often watched the young silkworm eat its way through the chorion, or egg shell, and this its first trial of a formidable cutting instrument, well supplied with muscular apparatus, it uses with admirable dexterity. The head of the young caterpillar, according to the statement of Meissner, lies towards the dot, or central opening in the lid, and which he has termed the micropyle,† from its resemblance

Dr. Allen Thomson. "Ovum, Cyclopa lia of Anatomy and Physiology." The term micropyle (a little gate) has heretofore only been used in connection with the vegetable kingdom; it is used to denote the opening, or foramen, towards which the radicle is always pointed.

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