Part 8

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].

Many eggs assume a very different colour after being laid a few days. In general upon their first exclusion they are white. Those of the chameleon-fly (_Stratyomis Chamæleon_) which I once found in great numbers, arranged like tiles on a roof one laid partly over another, on the under side of the leaves of the water-plantain, from white become green, and then change to olive green. Those of the hemipterous enemy of the larch, more than once mentioned in this letter, are first mouse-coloured, then they assume a reddish hue, and lastly a blackish one. Those of the gnat from white in a short time assume a shade of green, in a few hours they are entirely green, and at length become gray[191]. Those of the silk-worm, which at first are of a yellow or sulphur colour, acquire a violet shade. The eggs of that rare moth _Endromis versicolor_, are at first sulphur-coloured, then green, next rose-coloured, and lastly blackish. The colour of almost all eggs changes when they are near hatching; but this change depends more frequently upon the colour of the included larva, which appears through the transparent shell of the egg, than upon any actual alteration in the egg itself.

viii. _Period of hatching._ The general rule for the hatching of the eggs of insects is the absorption by the embryo of all the superabundant moisture included in them; but the time varies according to the state of the atmosphere, to the action of which they are subjected. Like those of other animals, they require a certain degree of heat for the due evolution of the included larva. This heat in much the greater number of instances is derived from the temperature of the air, but often also from other sources. The eggs of the gad-fly tribe are hatched principally by the heat of the body of the animal to which they are committed; and doubtless the vital heat of various larvæ, small as it may be, must contribute something to the hatching of the eggs deposited in them by various Ichneumons. In the fermenting bark in which the instinct of the rhinoceros beetles (_Oryctes nasicornis_ &c.) impels them to place theirs, the dung which the _Scarabæidæ_ select for that purpose, and the decaying vegetables chosen by many other insects, a degree of artificial heat must exist: and the eggs, or rather egg-like pupæ, of the spider-fly of the swallow (_Ornithomyia Hirundinis_) are hatched by the heat of those birds which sit upon them along with their own eggs.

Fabricius says, "Insects never sit upon their eggs[192];" but certainly, as I formerly related to you[193], the female earwig does this, and one would be induced to suppose, from the circumstance of the young ones following their mother, as chickens do the hen, that _Pentatoma grisea_ (_Cimex_ Linn.), formerly mentioned, may do the same[194].

With these exceptions, the eggs of all insects are hatched by atmospheric heat alone, the variations in which determine the more speedy or more tardy disclosure of the included insect. The eggs of such species as have several broods in the year, as the nettle butterfly (_Vanessa Urticæ_) when laid in summer are hatched in a few days; but if not laid till the close of autumn, they remain dormant through the winter, and are only hatched at the return of spring. That this difference is to be attributed to the influence of heat has been often proved by experiment: the autumnal eggs if brought into a warm room may be hatched as soon as those laid in the height of summer. Silk-worms' eggs naturally are not hatched till they have been laid six weeks, but in countries where they are reared, the women effect their exclusion in a much shorter period by carrying them in their bosoms: yet to retard their hatching with particular views is in many circumstances impossible. When the heat of the atmosphere has reached a certain point, the hatching cannot be retarded by cellars; and M. Faujas has remarked, that in June the silk-worm's eggs would hatch in an ice-house[195].

The period of exclusion does not, however, depend solely upon temperature: the hardness or softness of the shell, and possibly differences in the consistence of the included fluid, intended to serve this very purpose, cause some eggs to be hatched much sooner than others exposed to the same degree of heat. Thus the eggs of many flesh-flies are hatched in twenty-four hours[196]; those of bees and some other insects in three days; those of a common lady-bird (_Coccinella bipunctata_) in five or six days; those of spiders in about three weeks; those of the mole-cricket in a month; while those of many _Lepidoptera_ and _Coleoptera_ require a longer period for exclusion. The hard eggs of _Lasiocampa Neustria_ and _castrensis_, noticed above, remain full nine months before being hatched[197], as do those of another moth (_Hypogymna dispar_), which, though laid in the beginning of the warm month of August, do not send forth the included caterpillar till the April following[198]. We know no more of the cause of this difference than of that which takes place in the period of exclusion of the eggs of the different species of birds.

Some eggs change considerably both their form and consistence previously to being hatched. M. P. Huber found that those of different species of ants when newly laid are cylindrical, opaque, and of a milky white; but just before hatching their extremities are arched, and they become transparent with only a single opaque whitish point, cloud, or zone, in their interior[199]. An analogous change takes place in the eggs of many spiders, which just before hatching exhibit a change of form corresponding with that which the included spider receives when its parts begin to be developed, the thin and flexible skin of the egg moulding itself to the body it incloses[200].

In proportion as the germe included in the egg is expanded, it becomes visible through the shell when transparent: this is particularly the case with spiders, in which, as was before observed, every part is very distinctly seen. At length, when all the parts are consolidated so as to be capable of motion, which in spiders takes place in four or five days after they begin to be visible in the egg, the animal breaks the pellicle by the swelling of its body and the movement of its legs, and then quits it, and disengages all its parts one after the other[201]. In general, at least where the shell is harder than that of spiders, insects make their way out by gnawing an opening with their mandibles in the part nearest their head, which, when the shell is very strong (as in _Lasiocampa Neustria_, &c.), it is often several hours in accomplishing[202]. In many instances, however, the larva is spared this trouble, one end of the egg being furnished with a little lid or trap-door, which it has but to force up, and it can then emerge at pleasure: such lids are to be found in the eggs of several butterflies and moths, as _Satyrus Mæra_, _Saturnia pavonia major_, &c. and the common louse[203]. In those exquisitely elegant eggs, before described, of some kind of bird-louse (_Nirmus_) found adhering to the base of the neck feathers of the golden pheasant[204], there is a lid or cap of this kind of a hemispherical form terminating in a tortuous style. Those of a species of bug (_Pentatoma_ Latr.), found by our friend the Rev. R. Sheppard, besides a convex lid are furnished with a very curious machine, as it should seem, for throwing it off. This machine is dark-brown, of a corneous substance, and of the shape of a cross-bow[205], the bow part being attached to the lid or pushing against it, and the handle, by means of a membrane, to the upper end of the side of the egg.

When the included animal has made its way out of the egg, it enters upon a new state of existence, that of _Larva_, to which I shall direct your attention in the following letter.

FOOTNOTES:

[61] The word μεταμορφοω, and its derivative μεταμορφωσις, are not extant in any Greek writer before the date of the New Testament. They are used to express any _external_ change of form or colour, and metaphorically an _inward_ change and progressive improvement of the mind. Comp. Matth. xvii. 2. Ælian. _Var. Hist._ l. i. c. 1. Rom. xiii. 2. 2 Cor. iii. 18. They are, therefore, not improperly applied, as some have supposed, to the changes of insects.

[62] _Entwickelungsgeschichte der Schmetterlinge_ 12-27. 105--.

[63] Dr. Virey's observations under the article _Embryo_ (_N. Dict. d'Hist. Nat._ x. 195.) deserve here to be considered. "Il y a donc quelque chose au dessus de l'intelligence humaine dans cette formation des êtres; en vain on veut l'approfondir, c'est un abime dans lequel on ne voit que la main de Dieu. A quoi bon s'appesantir sur le mystère de la formation des êtres, sans esperance de l'expliquer? Ne vaut-il pas mieux observer les opérations de la nature autant qu'il est permis à l'œil humain de les appercevoir?"

[64] § xiv.

[65] _N. Dict. d'Hist. Nat._ x. 193.

[66] _Œuv._ v. 279. "Il n'est pas exact de dire que le cœur, la tête, et la moelle épinière, sont formés les premiers dans les fœtus des animaux à sang rouge et vertébrés," says Dr. Virey; "mais il faut dire seulement que tel est l'ordre dans lequel ces organes commencent à devenir visibles." _N. Dict. d'Hist. Nat._ x. 196.

[67] _Ibid._ 193.

[68] _Œuvr._ viii. 315.

[69] _Hor. Entomolog._ 446.

[70] See on this subject _N. Dict. d'Hist. Nat._ xx. article _Metamorphosis_.

[71] _N. Dict. d'Hist. Nat._ xx. 349--.

[72] _N. Dict. d'Hist. Nat._ xx. 348.

[73] _Bibl. Nat._ Ed. Hill. ii. 138.

[74] _Œuvr._ v. 283--.

[75] _N. Dict. d'Hist. Nat._ xx. 355.

[76] Leeuwenhoek discovered in the incipient fœtus of a sheep, not larger than the eighth part of a pea, all the principal parts of the future animal. _Arc. Nat._ I. ii. 165, 173.

[77] Bonnet, _Œuvr._ v. 284.

[78] _N. Dict. d'Hist. Nat._ xx. 352.

[79] Select Works by Hoole, i. 132. The fact is confirmed by M. L. Dufour, who, having opened the abdomen of a female scorpion, found in the midst of some eggs nearly mature a little scorpion a quarter of an inch long; it lay without motion, with its tail folded under the body. _N. Dict. d'Hist. Nat._ xxx. 426.

[80] Reaum. iv. 425--.

[81] _Ibid._ 428--. _t._ xxix. _f._ 10, 11.

[82] Busch, a German author, affirms that many _Cimicidæ_ are subject to this law. _Schneid._ i. 206.

[83] Quoted in Huber _Fourmis_, 208. Some reptiles also are at one time oviparous, and at another ovo-viviparous. _N. Dict. d'Hist. Nat._ xii. 568.

[84] I say _almost_ all insects, because the larvæ of _Hymenoptera_ and _Diptera_ are supposed not to undergo this change. _N. Dict. d'Hist. Nat._ xx. 365.

[85] Reaum. vi. _Mém._ xiv. De Geer, vi. 280.

[86] See VOL. I. Lett. xi.

[87] See VOL. II. p. 36.

[88] De Geer i. 494--.

[89] Called by M. l'Abbé Preaux, who observed it near Lisieux in Normandy, _Mouche Baliste._ _N. Dict. d'Hist. Nat._ xxi. 442.

[90] PLATE XX. FIG. 20.

[91] Reaum. vi. 509. _t._ XLV. _f._ 11, 12.

[92] Reaum. vi. 434.

[93] _Ibid._ vi. 494.

[94] The vesicles, which Reaumur thinks may be pulmonary vesicles, as well as assisting in the extrusion of the masses of eggs, he has figured _t._ xliv. _f._ 10. _u u._

[95] De Geer ii. 534. _t._ xiii. _f._ 13.

[96] Coquebert _Illustr. Ic. t._ i. _f._ A. B.

[97] PLATE XX. FIG. 25.

[98] _Ent. Carniol._ 269. _n._ 705.

[99] Reaum. ii. 401.

[100] In Raii _Hist. Ins._ 264.

[101] PLATE XX. FIG. 24.

[102] Goeze _Naturf._ xvii. 183--. _t._ iv. _f._ 16-19. Comp. _N. Dict. d' Hist. Nat._ iii. 475. and xix. 239. De Geer iii. 533.

[103] _Second Journey through Persia_, 100--.

[104] See VOL. II. p. 36.

[105] VOL. I. p. 359--.

[106] Latr. _Hist. Nat. des Fourmis_, 334. _N. Dict. d'Hist. Nat._ ii. 284.

[107] Lister _De Aran._ Tit. 13, 14. _N. Dict. d'Hist. Nat._ ii. 284.

[108] Lister _Ibid._ 56. Tit. 15.

[109] _N. Dict. d'Hist. Nat._ xxvi. 447.

[110] _Ins. Surinam, t._ i.

[111] A striking instance of this may be seen in her forty-ninth plate, in which she has clapped the rostrated head of _Fulgora laternaria_ upon the body of a _Cicada_ Latr., affirming it to be the former fly in its previous state! This might be a trick upon her.

[112] _N. Dict. d'Hist. Nat._ xv. 489.

[113] _Lesser_ L. i. 300.

[114] _Annales du Muséum_, xiv. 441.

[115] _Lesser_ L. i. _t._ ii. _f._ xvi.

[116] Miger _Ann. du Mus._ ubi supr. Comp. _N. Dict. d'Hist. Nat._ xv. 482--.

[117] De Geer i. 192.

[118] Ibid. ii. 982.

[119] Reaum. ii. 97. 159.

[120] Ibid. 107--. _t._ iii. _f._ 15.

[121] De Geer iii. 48. 51.

[122] Reaum. v. 122.

[123] See above, VOL. I. p. 196. 202.

[124] _Journ. de Phys. Philos. Mag._ ix. 244.

[125] Reaum. iv. _Mem._ i.

[126] Rösel, ix. 157. _t._ 265?

[127] Ibid. iii. 197.

[128] See above, VOL. I. p. 195.

[129] PLATE XX. FIG. 14.

[130] Reaum. i. 95-- _f._ 1-13.

[131] Reaum. iv. 615--. _t._ xliv. _f._ 2-7.

[132] _N. Dict. d'Hist. Nat._ xv. 445. Rös. iii. 156.

[133] _Epist._ 1687.

[134] VOL. I. p. 448--.

[135] See above, VOL. I. p. 344--.

[136] Reaum. iii. 8--.

[137] VOL. I. 349--. 371--.

[138] _N. Dict. d'Hist. Nat._ xxviii. 271.

[139] De Geer vii. 194.