Part 7

But the lackey-moths (_Lasiocampa Neustria_, _castrensis_, &c.) adopt a different procedure. As their eggs, which are laid in the autumn, are not to be hatched until the spring, the female does not, like most other moths, place them upon a leaf, with which they might be blown by the winter's storms far from their destined food, but upon the twig of some tree, round which she ranges them in numerous circles. If you examine your fruit-trees, you can scarcely fail to find upon the young twigs collections of these eggs, which are disposed with such admirable art, that you would take them rather for pearls, set by the skilful hand of a jeweller, than for the eggs of an insect. Each of these bracelets, as the French gardeners aptly call them, is composed of from 200 to 300 pyramidal eggs with flattened tops[129], having their axes perpendicular to the circumference of the twig to which they are fastened, surrounding it in a series of from fifteen to seventeen close spiral circles, and having their interstices filled up with a tenacious brown gum, which, while it secures them alike from the wintry blast and the attack of voracious insects, serves as a foil to the white enamel of the eggs that it encompasses. It is not easy to conceive how these moths contrive to accomplish so accurately with their tail and hind feet an arrangement which would require nicety from the hands of an artist; nor could Reaumur, with all his efforts and by any contrivance, satisfy himself upon this head. He bred numbers of the fly from the egg, and supplied the females after impregnation with appropriate twigs; but these, as though resolved that imprisonment should not force from them the secret of their art, laid their eggs at random, and made no attempt to place them symmetrically[130].

This illustrious Entomologist was more successful in discovering the mode in which another insect, the common _gnat_, whose group of eggs is, in some respects, as extraordinary as that last described, performs its operations. The eggs of this insect, of a long phial-like form, are glued together, side by side, to the number of from 250 to 300, into an oblong mass, pointed and more elevated at each end, so as considerably to resemble a little boat in shape. You must not here suppose that I use the term _boat_ by way of illustration merely; for it has all the essential properties of a boat. In shape it pretty accurately resembles a London wherry, being sharp and higher, to use a nautical phrase, fore and aft; convex below and concave above; floating, moreover, constantly on the keel or convex part. But this is not all. It is besides a _life-boat_, more buoyant than even Mr. Greathead's: the most violent agitation of the water cannot sink it; and what is more extraordinary, and a property still a desideratum in our life-boats, though hollow it never becomes filled with water, even though exposed to the torrents that often accompany a thunder-storm. To put this to the test, I yesterday (July 25, 1811) placed half a dozen of these boats upon the surface of a tumbler half full of water; I then poured upon them a stream of that element from the mouth of a quart bottle held a foot above them. Yet after this treatment, which was so rough as actually to project one out of the glass, I found them floating as before upon their bottoms, and not a drop of water within their cavity.

This boat, which floats upon the surface of the water until the larvæ are disclosed, is placed there by the female gnat. But how? Her eggs, as in other insects, are extruded one by one. They are so small at the base in proportion to their length that it would be difficult to make them stand singly upright on a solid surface, much more on the water. How then does the gnat contrive to support the first egg perpendicularly until she has glued another to it--these two until she has fixed a third, and so on until a sufficient number is fastened together to form a base capable of sustaining them in their perpendicular position? This is her process. She fixes her four anterior legs upon a piece of leaf, or a blade of grass, and projects her tail over the water. She then crosses her two hind legs, and in the inner angle which they form, retains and supports the first laid egg, as it proceeds from the anus. In like manner she also supports the second, third, &c., all of which adhere to each other by means of their glutinous coating, until she feels that a sufficient number are united to give a stable base to her little bark; she then uncrosses her legs, and merely employs them to retain the mass until it is of the required size and shape, when she flies away, and leaves it to its fate floating upon the water[131].

It may not be out of place to mention here a remarkable circumstance which not seldom attends a kind of water-scorpion (_Naucoris_ F.) occasionally to be met with in collections of Chinese insects. Its back is often covered with a group of rather large eggs, closely arranged; but whether these are its own eggs or those of some large species of water-mite (_Hydrachna_ Maïll.) has not been clearly ascertained. On the former supposition, the ovipositor must be remarkably long and flexile to enable the animal to place the eggs on its back. In confirmation of the latter it may be observed, that the species of the genus _Hydrachna_ usually attach their eggs to the body and legs of aquatic insects, as for instance _H. abstergens_ to the water-scorpion (_Nepa cinerea_), &c.[132]

2. After having thus laid before you some of the procedures of those insects that usually deposit their eggs in groups, either naked or defended by coverings of various kinds, I next proceed to a rapid survey of those of the species that commonly deposit them _singly_. Some of these, as for instance the Admiral Butterfly (_Vanessa Atalanta_), glue each egg carefully to its destined leaf by alighting on it for a moment. Another butterfly (_Hipparchia Hyperanthus_) whose caterpillar is polyphagous, drops hers at random on different plants. In general it may be observed, that all those larvæ which live in solitude, as in the interior of wood, leaves, fruits, grain, animals, &c., proceed from eggs laid singly by the female, which is usually provided with an appropriate instrument for depositing them in their proper situation. Thus the nut-weevil (_Balaninus Nucum_ Germ.) and also that of the acorn (_B. Glandium_) pierce a nut or an acorn with their long beak, and then deposit in the hole an egg, from which proceeds the maggot that destroys those fruits. Leeuwenhoek asserts that the common weevil (_Calandra granaria_) adopts the same process, boring a hole in every single grain of corn before it commits an egg to it, and at the same time, by this manœuvre, prepares a small quantity of flour to serve for the food of the tender grub when it is first hatched[133]. It is probable that the Rhyncophorous or weevil tribe in general chiefly use their beaks for the purpose of depositing their eggs in different vegetable substances, and perhaps principally in fruit or grain. The tribe of gall-flies (_Cynips_) on the contrary, whose economy, detailed in a former letter[134], interested you so much, bore an opening for the egg with their spiral oviduct, which also conveys it.

Another large tribe of insects depositing their eggs singly, are those which feed upon the bodies of other animals, into the flesh of which they are either inserted, or placed so as speedily to find their way into it. Some of these introduce them into _living_ animals, and then leave them to their fate, as the _Ichneumons_ and gad-flies: others deposit them along with the _dead_ body of an insect interred in a hole, often prepared with great labour, as the different species of sand-wasps (_Sphecidæ_), spider-wasps (_Pompilidæ_), &c.: the manners of the latter of these tribes have been already adverted to[135], and those of the _Ichneumonidæ_ will come more fully under consideration when I treat of the _diseases_ of insects.

A similar labour in providing suitable habitations for their eggs is undergone by various other insects whose larvæ live chiefly on vegetable food, some inserting their egg within the substance the larva devours, as those that prey on timber, twigs, roots, or the like, and others on its surface. One would suppose at first, that the exceedingly small egg which produces the subcutaneous larvæ would, by the parent moth, be imbedded in the substance of the leaf which is to exhibit hereafter their serpentine galleries: but this is not the case, for she merely glues it on the outside; at least such was the situation of the only egg of these very minute moths Reaumur had ever an opportunity to observe[136].

Other insects, belonging to the tribe which lay their eggs singly, bury them in the ground. Of this description are many of the lamellicorn insects, the dung-chafers (_Scarabæidæ_ MacLeay) particularly, which, inclosing their eggs in a pellet of dung, deposit them in deep cylindrical cavities. Concerning the proceedings of some of these, as well as of the whole race of bees, wasps, &c., which all lay single eggs, I have before detailed to you many interesting particulars[137]. I must not conclude this subject without observing, that the female _Pycnogonidæ_, an osculant tribe between Insects and _Crustacea_, carry their eggs upon two pair of false legs[138].

iii. _Substance._ From this long dissertation on the _situation_ of the eggs of insects and matters connected with it, I pass on to their _substance_ or their external and internal composition, giving at the same time some account of the embryo included in them. The eggs of insects, like those of birds, consist in the first place of an external coat or shell, varying greatly, as to substance, in different genera. Most commonly, particularly in those which deposit their eggs in moist situations, as in dung, earth, and the like, it is a mere membrane, often thin and transparent, and showing, as in spiders, all the changes that take place in the inclosed embryo, as the formation of the head, trunk, and limbs[139]. This membrane is sometimes so delicate as to yield to the slightest pressure, and insufficient to protect the included fluids from too rapid an evaporation, if the eggs be exposed to the full action of the atmosphere. In most _Lepidoptera_, and several other tribes, this integument is considerably stronger, in those moths whose eggs are exposed throughout the winter, as _Lasiocampa Neustria_, &c., so hard as not to yield easily to the knife. Even in these, however, its substance is more analogous to horn or a stiff membrane than to the shell of the eggs of birds. Nothing calcareous enters into its composition, and it is not perceptibly acted upon by diluted sulphuric acid. The eggs of birds are lined by a fine membrane; but I have examined several of those of insects, and have been able to discover nothing of the kind in them. I will not, however, affirm that it does not exist, though the shell of the insect egg appears more analogous to the membrane that lines that of the bird than to the outside shell itself.

Within this integument is included a fluid, on the precise nature of which, except that it is an aqueous whitish fluid, few or no observations have been made, or indeed are practicable; but it is reasonable to suppose, that like the white and yolk of the bird's egg, it serves for the development of the organs of the germe of the future insect.

But few observations are recorded that relate to the embryo included in the egg. It is stated, that it is invested with an extremely fine and delicate pellicle--supposed by some analogous to the _Chorion_ and _Amnios_ of the human fœtus, though others think the shell of the egg to correspond with the _Chorion_, and the successive integuments of the larva with the _Amnios_[140]. When the egg is first laid, nothing indeed is to be seen in it but the fluid just mentioned; the first change in this fluid is the appearance of the head of the embryo, more particularly in _Coleoptera_, of two points, the rudiments of the mandibles, and of those apertures into the tracheæ which I have called spiracles[141]; the little animal we may suppose then assumes its form and limbs. The embryo is usually so folded in the egg that the head and tail meet[142], and the head, annuli, and other parts of the larva are often visible through the shell[143]. Swammerdam even saw the pulsation of the great dorsal vessel through the shell of the egg of _Oryctes nasicornis_.

Under this head I must notice another singular circumstance peculiar I believe to the eggs of insects, that sometimes, though rarely, they are covered with down or hair. Those of a singular little hemipterous insect, of a genus I believe at present undescribed, the ravages of which upon the larch have been before noticed[144], are covered by a downy web, as is the case with the animal itself. De Geer has described the eggs of a bug, not uncommon in this country (_Pentatoma juniperina_ Latr.), which are reticulated with black veins, in which very short bristles are planted[145]. I possess also a nest of brown eggs, probably of a species of the same genus, found upon furze, which appear to be covered with very short downy hairs. The top of these is flat, and surrounded by a coronet of short bristles, each surmounted by a small white ball, so as to wear the appearance of a beautiful little _Mucor_. But hairy eggs are not confined to the Hemiptera Order, for, according to Sepp, those of the figure-of-eight moth (_Bombyx cæruleocephala_) are of this description[146].

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.