Lepisma saccharina belongs to the same genus as Podura. This minute springtail derives its name from having been discovered in old sugar-casks. It has a spindle-shaped body covered with silvery scales, long used as test objects. The sides of the abdomen are furnished with a series of appendages with long bristle-like setæ, or hairs, at their extremities. The head is concealed under a prothorax, the antennæ are long, and the maxillary palpi are either five or seven-jointed, and very conspicuous, to enable them to cut the dry wood on which they principally feed. The scales must be mounted under thin cover-glasses; oblique illumination shows up some portions to advantage, while central light from an achromatic condenser and a wide-angled objective renders their markings more distinct. Portion of a scale more highly magnified is shown in [Fig. 405].
Eggs of Insects ([Plate VI]., Nos. 124-139).—In form, colour, and variety of design, the eggs of insects are more surprisingly varied than those of the feathered tribes; but as from extreme smallness they escape observation, an acquaintance with their structure is not so familiar as it might be. Although the eggs of the bird tribe differ much in their external characteristics, they closely resemble each other while yet a part of the ovarian ova, and prior to their detachment from the ovary. At one period of their formation all eggs consist of three similar parts:—1st. The internal nucleated cell, or germinal vesicle, with its macula; 2nd. The vitellus, or yolk-substance; and 3rd. The vesicular envelope, or vitelline membrane. The germinal vesicle is the first produced, then the yolk substance, which gradually envelops it, and the vitelline membrane, the latest formed, incloses the whole. The chemical constituents of the egg are the same in all cases, albumen, fatty matters, and a proportion of a substance precipitable by water. The production of the chorion, or shell membrane, does not take place till the ovum has attained nearly its full size, and it then appears to proceed, in part at least, from the consolidation over the whole surface of one or more layers of an albuminous fluid secreted from the wall of the oviduct.
The embryo cell is so directly connected with the germinal vesicle that at a certain period it disappears altogether, and is absorbed into the germinal yolk, or rather becomes the nucleus of the embryo, when a greater degree of compactness is observed in the yolk, and all that remains of the germinal vesicle is one or more highly refracting fat globules and albuminoid bodies. Towards the end of the period of incubation, the head of the young caterpillar is said to lie towards the dot or opening in the lid, termed the micropyle,[83] from its resemblance to a small gate, or opening through which the larva emerges forth as a butterfly.
The germinal vesicle is comparatively large and well-marked while the egg is yet in the ova-sac. By preparing sections after Dr. Halifax’s method,[84] we find that the germinal vesicle in the bee’s egg is not situated immediately near or even below the so-called micropyle, but rather more to the side of the egg; just in the position which the head of the embryo is subsequently found to occupy at maturity.
The egg membrane, or envelope, of all the Lepidoptera is composed of three separate and distinct layers: an external slightly raised coat, tough and hard in its character, a middle one of united cells, and a fine transparent vitelline lining membrane, perfectly smooth and homogeneous in structure, imparting solidity, and giving a fine iridescent hue to the surface. The germinal vesicle is of a proportionately large size for the egg, and its macula is at first single, then multiple. In the egg of the silkworm the outer membrane is comprised of an inner reticulated membrane of non-nucleated cells, in the outer layer the cells are arranged in an irregular circular form, also non-nucleated, with minute interstitial setæ or hairs projecting outward.
The outer surface of the egg-shell of Coccus Persicæ is covered by minute rings, of which the ends somewhat overlap. These rings are thought to be identical in their character with the whitish substance which exudes through pores on the under-side of the body; it is more than probable that a succession of layers of rings fully accounts for the beautiful prismatic hues they present viewed as opaque objects under the microscope, and illuminated by Lieberkühn or side-condenser. This white substance, it should be observed, forms a part of the intimate structure of the egg-shell, and is in nowise affected by methylated spirit or dilute acids. Sir John Lubbock[85] states that in the greenish eggs of Phryganea, “the colour is due to the yolk-globules themselves. In Coccus, however, this is not so; the yolk-globules are slightly yellow, and the green hue of the egg is owing to the green granules, which are minute oil globules. When, however, the egg arrives at maturity, and the upper chamber has been removed by absorption, these green granules will be found to be replaced by dark-green globules, regular in size, and about 1⁄8000th of an inch in diameter, and which appear to be in no way the same in the yolk of Phryganea eggs.” Another curious fact has been noticed, which partially bears on the question of colour: the production of parasite bodies within the eggs of some insects. In the Coccus, for instance, parasitic cells of a green colour occur, “shaped like a string of sausages, in length about the 1⁄2000th of an inch by about the 1⁄7000th in breadth.”
The eggs of moths and butterflies present many varying tints of colour; in speaking of this quality I do not restrict the term solely to those prismatic changes to which allusion has been made, and which are liable to constant mutations according to the accident of the rays of light thrown upon them; but I more particularly refer to the several natural transitions of colour, the prevailing tints of which are yellow, white, grey, and a light-brown. In some eggs the yellow, white, and grey are delicately blended, and, when viewed with a magnifying power of about fifty diameters, and by the aid of the side-reflector (parabolic-reflector), exhibit many beautiful combinations. The more delicate opalescent, or rather iridescent, tints appear on the eggs of insects, while those of the feathered tribes furnish no like example. The egg of the mottled umber moth, Erannis defoliaria ([Plate VI]., No. 137), is in every way very beautiful. It is in shape ovoid, with regular hexagonal reticulations, each corner being studded with a knob or button; the space within the hexagon is finely punctated, and the play of colours is exquisitely delicate. In this egg no micropyle can be seen. The egg of the thorn moth, Ennomos erosaria ([Plate VI]., No. 138), is of an elongated brick-looking form, one end of which is slightly tapered off, while the other, in which the lid is placed, is flattened and surrounded by a beautifully white-beaded border, having for its centre a slightly raised reticulated micropyle. The empty egg-shell gives a fine opalescent play of colours, while that containing the young worm is of a brownish-yellow.
The egg of the straw-belle moth, Aspillates gilvaria ([Plate VI]., No. 139), is delicately tinted, somewhat long and narrow, with sides slightly flattened or rounded off, and irregularly serrated. The top is convex, and the base a little indented, in which are seen the lid and micropyle. The young worm, however, usually makes its way through the upper convex side: the indentation represented in the drawing shows the place of exit.
An example of those eggs possessing a good deal of natural colour is presented in that of the common puss-moth, Cerura vinula, a large spheroidal-shaped egg, having, under the microscope, the appearance of a fine ripe orange; the micropyle exactly corresponds to the depression left in this fruit on the removal of the stalk. The surface is finely reticulated, and the natural colour a deep orange.
The egg of the mottled rustic moth, Caradina morpheus (No. 124), is subconical, and equally divided throughout by a series of ribs, which terminate in a well-marked geometrically-formed lid. The egg of the tortoise-shell butterfly, Vanessa urticæ (No. 125), is ovoid and divided into segments, the ribs turning in towards the micropyle. The common footman, Lithosia campanula (No. 126), produces a perfectly globular egg covered with fine reticulations of a delicate buff colour. The egg of the shark moth, Cucullia umbratica (No. 127), is subconical in form, with ribs and cross-bars passing up from a flattened base to the summit, and turning over to form the lid. No. 136 is the egg of blue argus butterfly, Polyommatus argus. That of the small emerald moth, Jodis Vernaria (No. 134), is an egg of singular form and beauty—an oval, flattened on both sides, of silvery iridescence, and covered throughout with minute reticulations and dots. It is particularly translucent, so much so that the yellow-brown worm is readily seen curled up within. The lid or micropyle is not detected until the larva eats its way out of the shell. It should be noted that the series of eggs in [Plate VII]. are somewhat over-coloured, and consequently lose much of their natural transparency. The eggs of flies and parasites also present much variety in form, colour, and construction. Many of their eggs are provided with a veritable lid, which opens up with a hinge-like articulation. This lid is seen in the egg of bot-fly, [Plate VI]., No. 144, from which the larva is just escaping; No. 146, egg of Scatophaga; No. 147, egg of parasite of magpie.[86] Still more remarkable in the delicate and beautiful forms are some of the parasities which infest birds in particular: [Plate VI]., No. 145, the egg of parasite of pheasant; No. 147, that of the magpie, while that of the peacock is curiously interesting. In [Fig. 407] the larvæ of the horn-bill are seen just about to emerge from their eggs.