Part 6
These eggs, like those of the insects first mentioned, though connected, are expelled in succession; but other tribes, as the _Libellulidæ_, with the exception of _Agrion_, many _Ephemeræ_, _Trichopterous_ insects, &c. expel the whole at once, as it were in a mass. In those first mentioned they are gummed together in an oblong cluster[92]. In one _Ephemera_ mentioned by Reaumur[93], they formed two oblong masses, each containing from three to four hundred eggs, and three and a half or four lines long. These animals as soon as their wings are developed eject these masses by two orifices, and are aided in the process by two vesicles full of air, wherever they happen to alight or to fall; in most instances it is the water, their proper element, that receives them, but the animal does not appear to know the difference between a solid and a liquid, and seems only anxious how to free herself from a burthen that oppresses her; all has been contrived that an insect so short-lived may finish her different operations with the utmost celerity: the term of her existence would not have admitted the leisurely extrusion of such a number of eggs in succession[94]. Some _Trichoptera_, or May-flies, as _Phryganea grandis_ L., exclude their eggs in a double packet, enveloped in a mass of jelly, (a circumstance often attending the eggs that produce aquatic larvæ,) upon the leaves of willows[95]. A similar double packet in the year 1810 I observed appended to the anus of a black species with long antennæ, probably _Phryganea atrata_ F.[96] Upon taking several of the females I was surprised to find in the above situation a seemingly fleshy substance of a dirty yellow. At first, from its annular appearance, I conceived it to be some parasitic larva, but was not a little surprised upon pulling it away that it was full of globular transparent dusky eggs: it was about two lines and a quarter in length and nearly one in breadth. Being bent double it was attached to the animal by the intermediate angle, and when unfolded was constricted in the middle[97]. Each half, which was roundish, had about ten sharp transverse ridges, the interstices of which appeared as if crenated, an appearance produced by the eggs which it contained. Upon more than gentle pressure it burst and let out the eggs. Though resembling the packet of _P. grandis_ in shape and other circumstances, it was nothing like jelly, but had rather a waxy appearance, and seems to have been covered by a membrane: so that the excluded larvæ must probably have eaten their way out of it. I have still by me, in 1822, specimens of these egg-packets, which, after the lapse of so many years, retain their original form and colour. It is not improbable that other species extrude their eggs in a similar case. Scopoli says of _P. bicaudata_ L., that the female carries about under her belly her eggs united into a globe, like _Lycosa saccata_[98]. The eggs of _Geometra Potamogata_ F. are also enveloped in a gelatinous substance, and the mass is covered with leaves[99].
Insects of the _Diptera_ order also, like frogs and toads, commit their eggs to the water imbedded in masses of jelly. Dr. Derham describes two different kinds of them, in one of which the eggs were laid in parallel rows end to end, and in another in a single row, in which the sides were parallel[100]. But the most remarkable and beautiful specimen of this kind that I ever saw was one that, many years ago, I took out of a pond at Wittersham in Kent, from which I requested a young lady to make the drawing I send you[101]. The mass of jelly, about an inch and a quarter long, and rather widest in the middle, was attached by one end to some aquatic grass, and from one end to the other ran a spiral thread of very minute eggs, the turns of the screw being alternately on each side.
The mode of exclusion of the eggs of the _Blattæ_, which are engaged for a whole week in the business of oviposition, is very singular: the female deposits one or two large suboviform capsules, as large as half their abdomen, rounded on one side, and on the other straight and serrated, which at first is white and soft, but soon becomes brown and hard. This egg-case, as it may be called, contains sixteen or eighteen eggs arranged in a double series, and the cock-roaches when hatched make their escape through a cleft in its straight side, which shuts so accurately when they have quitted it, that at first it appears as entire as before[102]. The insects of the genus _Mantis_ also, or what are called the _praying insects_, when they deposit their eggs, eject with them a soft substance, which hardens in the air and forms a long kind of envelope resembling parchment, in which the eggs are arranged also in a double series. And the Locusts (_Gryllus Locusta_ L.) are said by Morier[103] to deposit in the ground an oblong substance, of the shape of their abdomen, which contains a considerable number of eggs arranged neatly in rows. The peristaltic motion observed in the females of some insects during oviposition has been before described[104].
ii. _Situation._ Under this head I include the situation in which the female insect places her eggs when extruded, whether she continues her care of them and carries them about till they hatch, or whether she entirely deserts them, placing them either without a covering within reach of their food, or enveloping them in hair or otherwise protecting them from accident or the attack of enemies. I shall consider them under two views: _first_, as depositing their eggs in _groups_, whether covered or naked; and _secondly_, as depositing them _singly_.
1. Those that deposit their eggs in _groups_ are first to be considered. I shall begin with those that protect them with some kind of _covering_.
I have already mentioned in a former letter[105] the silken bag with which _Lycosa saccata_ Latr., a kind of spider, surrounds her eggs, and in which she constantly carries them about with her, defending them to the last extremity. Many other spiders, indeed nearly the whole tribe, fabricate similar pouches, but of various sizes, forms, texture, and colours. Some are scarcely so big as a pea, others of the size of a large gooseberry; some globular, some bell-shaped; others, the genus _Thomisus_ Walck. in particular, depressed like a lupine; some of a close texture like silk; others of a looser fabric resembling wool: some consisting of a single pellicle, but most of a double, of which the interior is finer and softer[106]; some white; others inclining to blue; others again yellow or reddish; most of them are of a whole colour, but that of _Epeira fasciata_ is gray varied with black[107]. And while the parent spider of some kinds (the _Lupi_) always carries her egg-bag attached to her anus, others hold them by their palpi and maxillæ; and others suspend them by a long thread, or simply fasten them in different situations, either constantly remaining near them (the _Telariæ_), or wholly deserting them (the _Retiariæ_). The eggs of one of these last Lister describes as often fixed in a very singular situation--the cavity at the end of a ripe cherry; and thus, as he expresses it--"_Stomachi maxime delicatuli quoties hanc innocuam buccam non minus ignoranter quam avide devorarunt_[108]."
Herman informs us, that the species of the genus _Chelifer_ carry their eggs in a mass under their belly[109].
Madam Merian gives an account of two species of _Blatta_, which she affirms carry an egg-pouch about with them--one species (_B. gigantea?_) she describes as carrying its eggs in a globular pouch of web like certain spiders, and the other in a brown bag, which, when alarmed, it drops and makes off[110]. But this admirable paintress of natural objects was not always correct in her statements[111]: it seems very improbable, from the habits of those species of which we know the history, that any of them should spin a pouch of web for their eggs.
The only insects certainly known to spin an egg-pouch like the spiders, are the _Hydrophili_, a kind of water-beetles. Some of these, as _H. lividus_, carry them about with them, like _Lycosa saccata_, attached to the under side of their body, as M. Miger observed[112]; and others when they are finished desert them. That of the great water-beetle (_Hydrophilus piceus_) was long ago described and figured by Lyonnet[113]; and a more detailed account of it has since been given by M. Miger[114]. In form it somewhat resembles a turnip when reversed, since it consists of a pouch of the shape of an oblate spheroid, the great diameter of which is three quarters of an inch; and the small, half an inch, from which rises a curved horn, about an inch long and terminating in a point[115]. The animal is furnished with a pair of anal spinners, which move from right to left, and up and down, with much quickness and agility: from these spinners a white and glutinous fluid appears to issue, that forms the pouch, which it takes the animal about three hours to construct. The exterior tissue is produced by a kind of liquid and glutinous paste, which by desiccation becomes a flexible covering impermeable to water; the second, which envelops the eggs, is a kind of light down of great whiteness, that keeps them from injuring each other. The tissue of the horn is of a silky nature, porous and shining, and greatly resembling the cocoons of _Lepidoptera_. This part, contrary to what Lyonnet supposes, appears calculated to admit the air, the water soon penetrating it when submerged. At its base is the opening prepared for the egress of the larvæ, when hatched, which is closed by some threads, that, by means of the air confined in the cocoon or pouch, hinder the water from getting in[116]. This nidus does not float at liberty in the water till after the eggs are hatched, the parent animal always attaching it to some plant. By means of this anomalous process for a beetle, which this insect is instructed by Providence thus to perfect, the precious contents of its little ark are secured from the action of the element which is to be the theatre of their first state of existence, from the voracity of fishes, or the more rapacious larvæ of its own tribe, until the included eggs are hatched, and emerge from their curious cradle.
I shall next amuse you with a few instances, in which the Allwise Creator instructs the parent insect, instead of defending her eggs with a covering furnished by her internal organs, to provide it from without, either from her own body or from some other substance. Most commonly, indeed, the female leaves her cluster of eggs without any other covering than the varnish with which in this case they are usually besmeared. Either they are deposited in summer and will soon be hatched, or they are of a substance calculated to encounter and resist the severities of the season. But many species, whose eggs are more tender or have to resist the cold and wet of winter, defend them in the most ingenious manner with a clothing of different kinds of substance.
_Cassida viridis_, a tortoise beetle, Rösel tells us, covers her group of eggs with a partially transparent membrane. _Arctia Salicis_ F., a moth, common on willows, wholly conceals hers with a white frothy substance, which when dry is partly friable and partly cottony, and being insoluble in water effectually protects them from the weather[117]. The female of _Lophyrus Pini_ (a saw-fly), having by means of her double saw made a suitable longitudinal incision in the leaf of a fir, and placed in it her eggs in a single row end to end, stops it up with a green frothy fluid mixed with the small pieces of leaf detached by her saws, which when dry becomes friable: a necessary precaution, since these eggs are extremely brittle[118]. _Arctia chrysorhœa_, _Hypogymna dispar_, and several other moths, surround theirs with an equally impervious and more singular clothing--_hair_ stripped from their own bodies. With this material, which they pluck by means of their pincer-like ovipositor, they first form a soft couch on the surface of some leaf: they then place upon it successively layers of eggs, and surround them with a similar downy coating, and when the whole number is deposited cover the surface with a roof of hairs, which cannot be too much admired; for those used for the interior of the nest are placed without order, but those employed externally are arranged with as much art and skill as the tiles of a roof, and as effectually keep out the water, one layer resting partly on the other, and all having the same direction, so that the whole resembles a well-brushed piece of shaggy cloth or fur. When the mother has finished this labour, which often occupies her for twenty-four hours, and sometimes even twice that period, her body, which before was extremely hairy, is almost wholly naked--she has stripped herself to supply clothing to her offspring, and having performed this last duty she expires. The female moths which thus protect their eggs are often furnished with an extraordinary quantity of hair about the anus for this express purpose; and Reaumur conjectures, that the singular anal patch of scales resembling those of the wings, but considerably larger, which is found in the female of _Lasiocampa Pityocampa_, is destined for the same purpose[119].
Reaumur had once brought to him a nidus of eggs clothed still more curiously: they surrounded a twig in a spiral direction, like those of _Lasiocampa Neustria_, but were much more numerous, and were thickly covered with fine down, not pressed close, but standing off horizontally, which assumed much the same appearance as a fox's tail would if twisted spirally round a branch[120].
A procedure nearly similar was observed by De Geer in some species of Aphides (_A. Alni_ and _A. Pruni_), which covered their eggs with a white cottony down detached from their belly by means of their hind legs[121]. In this case, however, the eggs were separately coated with the down, but there was no general covering to the group.
Several insects make the leaves and other parts of plants serve as coverings for their eggs. _Tenthredo Rosæ_ L., a saw-fly, and other species of the same genus, with their saws make an incision in the green twigs of shrubs and trees, and fill it with a line of eggs placed end to end, taking care that, as the eggs grow after they are laid, they are placed at such distances as to leave room for their expansion[122]. _Rhynchites Bacchus_, a brilliant weevil, well known to the vine-dressers for the injury it does[123], rolls with much art the leaves of the vine, so as to form a cavity, in which it places its eggs; other species practise similar manœuvres; and some probably place their young progeny in the interior of twigs, making an opening for that purpose with their rostrum--at least, I once saw _Rhynchites Alliariæ_ L. with its rostrum plunged up to the antennæ in the twig of a crab-tree. Others of this tribe, as we know, place their eggs in the interior of fruits and grain, as the nut, acorn, and common weevils.
It is probable that most of the above coverings serve another purpose besides the protection of the eggs from wet and cold--that of sheltering them from the action of too great light, which, as Dr. Michellotti by numerous experiments has ascertained, is fatal to the included germe[124]. On this account it is perhaps that so many insects fasten their eggs to the under side of leaves. Those exposed in full day have usually an opaque and horny texture.
Some insects are spared all trouble in providing a covering for their eggs, their own bodies furnishing one in every respect adapted to this purpose. Not to mention the _Onisci_, or wood-lice, since they rather belong to the _Crustacea_, which have a four-valved cell under the breast, in which they carry their eggs, as the kangaroo does its young in its abdominal pouch, the whole body of the female of those strange animals the _Cocci_ becomes a covering for her eggs, which it incloses on every side. To make this intelligible to you, further explanation is necessary. You must have noticed those singular immovable tortoise-shaped insects, which are such pests to myrtles and other greenhouse plants. These are the young of a species of _Coccus_ (_C. Hesperidum_ L.), and _their_ history is that of the whole race. Part of them never become much bigger than the size of which you ordinarily see them, and when full-grown disclose minute two-winged flies, which are the males. The size of the females, which glue themselves to a twig or leaf as if lifeless, now augments prodigiously, and the whole body, distended with the thousands of eggs which it includes to the bigness of a large pea, without any vestige of head or limb, resembles a vegetable excrescence or gall-apple rather than an insect. If you remove one of them, you will perceive that the under part of its abdomen is flat and closely applied to the surface of the branch on which it rests, only a thin layer of a sort of cotton being interposed between them. In laying her eggs the female _Coccus_ does not, like most insects, protrude them beyond her body into day-light; but as soon as the first egg has passed the orifice of her oviduct, she pushes it between her belly and the cottony stratum just mentioned, and the succeeding eggs are deposited in the same manner until the whole are excluded. You will ask how there can be found space between the insect's belly and the cotton, to which at first it was closely applied, for so large a mass of eggs? To comprehend this, you must consider that nearly the whole contents of its abdomen were eggs; that in proportion as these are extruded a void space is left, which allows the skin of the under side of the body to be pushed upwards, or towards that of the back, affording room between it and the cottony web for their convenient stowage. If you examine the insect after its eggs are all laid, you will find that they have merely changed their situation; instead of being on the upper side of the skin forming the belly, and within the body, they now are placed between it (now become concave and nearly touching the back) and the layer of cotton. As soon as the female _Coccus_ has finished her singular operation she dies; but her body, retaining its shape, remains glued upon the eggs, to which it forms an arched covering, effectually protecting them, until they are hatched, from every external injury. Some species lay so many eggs, that the abdomen is not sufficiently large to cover the whole mass, but merely one side of it, the remainder being enveloped in cottony web[125].
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I am next to consider the situation of those eggs that are excluded by the mother in groups _without any_ other _covering_ than the varnish with which they are usually besmeared in their passage from the oviduct. The females only place them upon or near the food appropriated to the young larvæ, to which they adhere by means of the varnish just mentioned. These groups consist of a greater or less number; and when the eggs are hatched by the heat of the sun, the larvæ begin to disperse and attack with voracity the food that surrounds them. It is thus that most butterflies and moths attach their eggs to the stems, twigs, and leaves of plants; that the lady birds (_Coccinellæ_), the aphidivorous flies (_Syrphi_ &c.), and the lace-winged flies (_Hemerobii_), deposit them in the midst of plant-lice (_Aphides_); that the eggs of some flesh-flies are gummed upon flesh; those of crickets and grasshoppers buried in the earth; those of gnats and other Tipulidans set afloat upon, or submerged in, the water.
Frequently the whole number of eggs laid by one female is placed in one large group, more commonly, however, in several smaller ones, either at a distance from each other on the same plant, or on distinct plants. The object in the latter case seems to be, in some instances, to avoid crowding too many guests at one table, in others to protect the unhatched eggs from the voracity of the larvæ first excluded, which would often devour them if in their immediate neighbourhood.
In the disposition of the eggs which compose these groups much diversity prevails. Sometimes they are placed without order in a confused mass: more frequently, however, they are arranged in different, and often in very beautiful modes. The common cabbage-butterfly (_Pieris Brassicæ_) and many other insects place theirs upon one end, side by side, so as, comparing small things with great, to resemble a close column of soldiers, in consequence of which those larvæ which, on hatching, proceed from the upper end, cannot disturb the adjoining eggs. Many indeed have a conformation purposely adapted to this position, as the hemisphærical eggs of the puss-moth (_Cerura Vinula_), which have the base by which they are gummed membranous and transparent, while the rest is corneous and opaque. The same ready exit to the larva is provided for in the oblong eggs of the emperor moth (_Saturnia Pavonia_), which are piled on their sides in two or more lines like bottles of wine in a bin[126].
Where the larva does not emerge exactly from the end of the egg other arrangements take place. The whirlwig-beetle (_Gyrinus natator_) and the saw-fly of the gooseberry &c. (_Tenthredo flava_ L.) dispose theirs end to end in several rows; the former upon the leaf of some aquatic grass, the rows being parallel[127], the latter gummed to the main nerves of gooseberry or currant leaves, the direction of which they follow[128].