iv. Number. The fertility of insects far exceeds that of birds, and is surpassed only by that of fishes[147]. But the number of eggs laid by different species, sometimes even of the same natural family, is extremely various. Thus the pupiparous insects may be regarded as producing only a single egg; Musca Meridiana L., a common fly, lays two[148], other flies six or eight; the flea twelve; the burying beetle (Necrophorus Vespillo[149]) thirty; May-flies (Trichoptera K.) under a hundred; the silk-worm moth about 500; the great goat-moth(Cossus ligniperda) 1,000; Acarus americanus more than 1,000[150]; the tiger-moth (Callimorpha Caja) 1,600; some Cocci 2,000, others 4,000; the female wasp at least 30,000[151]; the queen bee varies considerably in the number of eggs that she produces in one season, in some cases it may amount to 40,000 or 50,000 or more[152]; a small hemipterous insect, resembling a little moth (Aleyrodes proletella Latr.) 200,000. But all these are left far behind by one of the white ants (Termes fatale F. bellicosus Smeath.)—the female of this insect, as was before observed[153], extruding from her enormous matrix not less than 60 eggs in a minute, which gives 3,600 in an hour, 86,400 in a day, 2,419,200 in a lunar month, and the enormous number of 211,449,600 in a year: probably she does not always continue laying at this rate; but if the sum be set as low as possible, it will exceed that produced by any other known animal in the creation.
v. Size. The size of the eggs is in proportion to that of the insect producing them, though in some instances small ones produce larger eggs than those laid by bigger species. Thus the eggs of many Aptera, as those of that singular mite Uropoda vegetans, and of the bird-louse found in the golden pheasant, are nearly as large, it is probable, as the parent insect; while those of the ghost-moth (Hepialus Humuli) and many other Lepidoptera, &c. are vastly smaller. This circumstance perhaps depends principally on the number they produce: the majority of them, however, are small. The largest egg known, if it be not rather an egg-case, is that of a spectre insect (Phasma dilatatum), figured in the Linnean Transactions[154], being five lines in length and three in width, which probably approaches near the size of that of some humming-birds. The largest egg of any British insect I ever saw was that of the common black rove-beetle (Staphylinus olens) sent me by Mr. Sheppard—this is a line and half long by a line in width. But we do not often meet with insect-eggs exceeding a line in length. A vast number are much smaller: those of Ephemeræ are more minute than the smallest grains of sand[155], and some almost imperceptible, as those of the subcutaneous moths, to the naked eye. Commonly the eggs laid by one female are all of the same size; but in several tribes, those containing the germe of the female are larger than those that are to give birth to a male. This appears to be the case with those of the Rhinoceros beetle (Oryctes nasicornis[156]), and according to Gould with those of ants[157]. As the female in a vast number of instances is much bigger than the male, it is not improbable that this law may hold very extensively. It is stated, however, by Reaumur[158], that the reverse of this takes place in the eggs of the hive-bee, those that are to produce males being larger than the rest.
Another peculiarity connected with the present head is the augmentation in bulk which takes place, after exclusion, in the eggs of the great tribe of saw-flies (Tenthredo L.), the gall-flies (Cynips L.), the ants (Formica L.) and the water-mites (Hydrachna Maïll. Atax F.). Those of the two former, which are usually deposited in the parenchymous substance of the leaves, or of the young twigs, of various plants, imbibe nutriment in some unknown manner, through their membranous skins, from the vegetable juices which surround them[159], and when they have attained their full size are nearly twice as large as when first laid. Except in the eggs of fishes, whose volume in like manner is said to augment previously to the extrusion of the young, there is nothing analogous to this singular fact in any other of the oviparous tribes of animals, the eggs of which have always attained their full size when they are laid.
It is to M. P. Huber that we are indebted for the knowledge of the fact that the eggs of ants grow after being laid, a circumstance favoured probably by the moist situation in which the workers are always careful to keep them. By an accurate admeasurement he found that those nearly ready to be hatched were almost twice as big as those just laid[160]. A similar observation was made on the red eggs of a water-mite (Hydrachna abstergens) by Rösel, who conjectured that they draw their means of increase from the body of the water-scorpions (Nepæ), of which they form so singular an appendage[161], which opinion is confirmed by De Geer, who observes that when the water-scorpions are covered by an unusual number of the eggs of the water-mites, they grow weak and languid, and endeavour to rid themselves of their parasitic appendages[162]. It is most probable that the mite lately named (Uropoda vegetans), which is often found planted as it were upon the bodies of various beetles, by means of a long pedicle, through which, as the fœtus by an umbilical chord and placenta, it derives its nutriment from the above animals, is at first so fixed in the egg state, though before it is disengaged from the pedicle it is hatched, since it is often found with its legs displayed and quite active—this is the more probable, as the eggs of the water-mite are fixed by a pedicle to the animals to which they are attached[163]. I have met with a remarkable instance, in which pedunculated eggs seem to draw nutriment from the mother, which brings the pedicle still near to the nature of the umbilical chord. Those of the small hemipterous insect which infests the larch before alluded to, are attached to the anal end of the mother by a short foot-stalk not longer than the egg.
Dr. Derham seems to have observed, that the eggs of some Diptera, of the tribe of Tipulidæ, also increase in size before the larva is excluded[164]. It seems to me likely enough, that in this and many of the above cases in which the egg is supposed to grow, it is rather an extension of the flexile membrane that forms their exterior proportioned to the growth of the included embryo from food it finds within the egg, than from any absorption from without.
vi. Shape. We are accustomed to see the eggs of different species of oviparous animals so nearly resembling each other in form, that the very term egg-shaped has been appropriated to a particular figure. Amongst those of birds, with which we are most familiar, the sole variations are shades of difference between a globular and oval or ovate figure. The eggs of insects, however, are confined by no such limited model. They differ often as much, both as to their shape, sculpture, and appendages, as one seed does from another; and it is not improbable that, if duly studied, they would furnish as good indications of generic distinctions as Gærtner has discovered in those of plants. Their most usual form indeed is globular, oval, or oblong, with various intermediate modifications. We meet with them ovate, or of the shape of the common hen's egg, flat and orbicular, elliptical, conical, cylindrical, hemispherical, lenticular, pyramidal, square, turban-shaped, pear-shaped, melon-shaped, boat-shaped, of the shape of an ale-stand, of a drum, &c.[165], and sometimes of shapes so strange and peculiar, that we can scarcely credit their claim to the name of eggs. Thus the eggs of the gnat are oblong and narrow, or nearly cylindrical, having at the top a cylindrical knob[166], so as to give them the precise form of the round-bottomed phial sometimes used by chemists: those of the common water-scorpion (Nepa cinerea) are oblong, and at the upper end are surrounded by a sort of coronet, consisting of seven slender rays or bristles of the length of the egg[167], so as to resemble somewhat the seeds of Carduus benedictus (Cnicus acarna[168]) of the old botanists. One would think this spinous circlet a very awkward appendage to bodies which are to be gradually extruded through the fine membranous ovaries and oviduct which inclose them: but they are so admirably packed, the unarmed end of each egg fitting closely into the space inclosed by the spines of the one next below it, or, rather, the spines which are moveable, embracing it closely, that not only is no room lost, but the ovaries are perfectly secure from injury. The eggs of another species of this tribe (Ranatra linearis) have only two of these spines or bristles—they are inserted in the stem of a water-rush (Scirpus) or other aquatic plant, so as to be quite concealed, and are only to be detected by the two bristles which stand out from it[169]. The eggs of the beautiful lace-winged flies (Hemerobius), those golden-eyed insects so serviceable in destroying the plant-lice (Aphides[170]), are still more singular. Those of H. Perla are oval, and each of them attached to a filiform pedicle not thicker than a hair, and seven or eight times as long as the egg. By this pedicle (which is supposed to be formed by a glutinous matter attached to one end, which the female draws out by abstracting her ovipositor with the egg partly in it from the leaf, to which she has previously applied it, to a proper length, when the gluten becoming sufficiently solid she wholly quits the egg,) the eggs are planted in groups of ten or twelve on the surface of leaves and twigs, from which they project like so many small fungi, to some of which they have a remarkable resemblance. When the included larva has made its way out of them by forcing open the top, they look like little vases, and were actually once figured by a Naturalist, as we learn from Reaumur, as singular parasitic flowers growing upon the leaves of the elder, for the origin of which he was extremely puzzled to account[171]. Eggs similarly furnished with a pedicle are also laid by other insects; but as most of these have been before alluded to, it is not necessary to describe them here[172]. The cause of these differences of form is for the most part concealed from us: in many instances it may perhaps be referred to that will to vary forms, and so to glorify his wisdom[173] and power, independently of other considerations, which, as Dr. Paley has well remarked[174], seems often to have guided the Great Author of Nature. But in some cases the end to be answered is sufficiently evident. The long footstalks of the eggs of the Hemerobius just mentioned, there can be little doubt, are meant to place them out of the reach of the hosts of predaceous insects which roam around them, from whose jaws, thus elevated on their slender shaft, they are as safe as the eggs of the tailor bird in its twig-suspended nest from the attack of snakes. Reaumur has described the eggs of a kind of fly, common upon the excrements of the horse and other animals (Scatophaga vulgaris Latr.), or one related to it, that requires to be immersed in the dung to which it is committed, on which the future grubs are to feed. He found that if not thus surrounded with moisture, they infallibly shrivelled up and came to nothing; but it is equally necessary that they should not be wholly covered: if they were, the young larva would be suffocated at its first exit from the egg. In what way is this nice point secured? In this manner. Each egg is provided at its upper end, at which the animal when hatched comes out, with two diverging horns[175]; these prevent it from being stuck into the excrement, in which the female deposits the eggs one by one, more than three-fourths of its length: and when examined they resemble not badly, as Reaumur remarks (except that their colour is white), a parcel of cloves stuck into a pudding, as they are neatly inserted at due distances in the disgusting mass[176]. The French Naturalists found these eggs in swine's dung; I have observed them in cow-dung. Latreille thinks that the bristles above described attached to the eggs of Nepa and Ranatra have a similar use, as the female plunges them all but these bristles into the stems of aquatic plants[177]: but may not this have something to do with their oxygenation? Reaumur has figured another egg of a dipterous insect which has a longitudinal wing or lateral margin attached to it, giving it the form of an oblong square, the object of which, he conceives, is to give a greater surface by which it may be more firmly fixed to the substance against which the fly attaches it[178].
Besides these more striking variations in figure, their surface, though often smooth, is frequently curiously and most elegantly sculptured, a circumstance that distinguishes the eggs of no other oviparous animals. Some, as the margined egg just mentioned, are only sculptured on one side, the other being plain; or, as those of the Tusseh silk-worm[179] (Attacus Paphia) and other Bombyces, which have orbicular depressed eggs with a central cavity above and below, have their circumference crossed with wrinkles corresponding with the rings of the inclosed embryo[180]. Others again are sculptured all over. Of these, in some, the sculpture of the two sides is not symmetrical, as in those of a fly figured by Reaumur[181]: but in general there is a correspondence in this respect between the different parts of the egg. In those elegant ones before alluded to of some bird-louse attached to the golden pheasant, the shell resembles the purest wax, and is scored with longitudinal striæ, each distinguished by a series of impressed points, which give it a beautiful appearance of net-work. In the others, as in a common butterfly (Hipparchia Ægeria) and moth (Geometra cratægata), the whole surface is covered with hexagonal reticulations[182]. Others, as those of another butterfly (Hipparchia Hyperanthus), are beset with minute granules or tubercles[183]. Others again, like those of the cabbage and hawthorn butterflies (Pieris Brassicæ and Cratægi), are remarkable for beautiful longitudinal ribs, often connected by elevated lines crossing them at right angles[184]; and in some, as in another butterfly (Hipparchia Jurtina), crowned by imbricated scales[185]. Many other minor differences in this respect might be noticed, but these will suffice to give some idea of the infinite variety exhibited in this respect by these little atoms. If the Creator has wrought them with so much art and skill, can it be beneath his reasonable creatures to examine and admire them, that they may glorify those attributes which they serve to illustrate?
Some eggs after exclusion occasionally become slightly corrugated: Malpighi supposed that this occurs only when the eggs are barren, having observed that those of the moth of the silk-worm which preserved their plumpness always produced caterpillars, while those which lost their original rotundity and became wrinkled were constantly unprolific. Bonnet, however, found exactly the reverse take place in another moth[186], so that these appearances are scarcely to be depended upon. Kuhn asserts, that a virgin female of the puss-moth (Cerura Vinula) having begun to lay eggs, which were yellow above, green below, and depressed, he introduced to her an hour afterwards a male, and some minutes subsequently to the union, she again deposited eggs, which were wholly of a dark brown and convex[187].
vii. Colour. The colour of the eggs of insects is as various as their shape and sculpture. They are very often white, those of some spiders like minute pearls[188]; some are yellow, as those of the silk-worm; others orange, such are the eggs of the bloody-nosed beetle (Timarcha tenebricosa); others again of a golden hue; sometimes they are of a sanguine red. I remember once being much surprised at seeing the water at one end of a canal in my garden as red as blood: upon examining it further I found it discoloured by an infinite number of minute red eggs, belonging probably to some dipterous insect of the Tipulidan tribe. There are also eggs of every intermediate shade between red and black; some again are blue and others green. They are not always of whole colours, for some are speckled like those of many birds, of which I can show you specimens, that are also shaped like birds' eggs; these I think were laid by a common moth (Odenesis potatoria); others are banded with different colours—thus the blue eggs of the lappet-moth (Gastropacha quercifolia) are encircled by three brown zones[189]; others are brown with a white zone[190].