Chapter XIII: Letter XXXII: States of Insects (1)

IMAGO STATE.

When the insect has quitted the exuviæ of the pupa, it has attained the last stage of its existence. It is now termed an _Imago_, or perfect insect; and is capable of propagation.

Just after its exclusion, it is weak, soft, and languid: all its parts are covered with moisture; and, if a winged insect, its wings have so little the appearance, either in shape, size, or colour, which they are about to assume, that it might be taken for a mutilated abortion, rather than an animal in the most vigorous stage of life. If it be a beetle, its elytra, instead of covering the back of the abdomen, are folded over the breast: their substance is soft and leathery, and their white colour exhibits no traces of the several tints which are to adorn them. If the insect be a butterfly or a moth, the wings, instead of being of their subsequent amplitude, and variegated and painted with a variety of hues and markings, are in large species scarcely bigger than the little finger nail, falling over the sides of the trunk, and of a dull muddy colour, in which no distinct characters can be traced. If the excluded insect be a bee or a fly, its whole skin is white and looks fleshy, and quite unlike the coloured hairy crust which it will turn to in an hour or two; and the wings, instead of being a thin, transparent, expanded film, are contracted into a thick, opaque, wrinkled mass.

These symptoms of debility and imperfection, however, in most cases speedily vanish. The insect, fixing itself on the spoils of the pupa, or some other convenient neighbouring support, first stretches out one organ, and then another: the moisture of its skin evaporates, the texture becomes firm, the colours come forth in all their beauty; the hairs and scales assume their natural position; and the wings expanding, extend often to five or six times their former size--exhibiting, as if by magic, either the thin transparent membranes of the bee or fly, or the painted and scaly films of the butterfly or moth, or the coloured shells of the beetle. The proceedings here described I witnessed very recently with regard to a very interesting and beautiful butterfly, the only one of its description that Britain has yet been ascertained to produce--I mean _Papilio Machaon_. The pupa of this being brought to me by a friend early in May this year (1822), on the sixteenth of that month I had the pleasure to see it leave its puparium. With great care I placed it upon my arm, where it kept pacing about for the space of more than an hour; when all its parts appearing consolidated and developed, and the animal perfect in beauty, I secured it, though not without great reluctance, for my cabinet--it being the only living specimen of this fine fly I had ever seen. To observe how gradual, and yet how rapid, was the development of the parts and organs, and particularly of the wings, and the perfect coming forth of the colours and spots, as the sun gave vigour to it, was a most interesting spectacle. At first it was unable to elevate or even move its wings; but in proportion as the aërial or other fluid was forced by the motions of its trunk into their nervures, their numerous corrugations and folds gradually yielded to the action, till they had gained their greatest extent, and the film between all the nervures became tense. The ocelli, and spots and bars, which appeared at first as but germes or rudiments of what they were to be, grew with the growing wing, and shone forth upon its complete expansion in full magnitude and beauty.

To understand more clearly the cause of this rapid expansion and development of the wings, I have before explained to you that these organs, though often exceedingly thin, are always composed of two membranes, having most commonly a number of hollow vessels, miscalled nerves, running between them[683]. These tubes, which, after the French Entomologists, I would name _nervures_, contribute as well to the development of the wings, as to their subsequent tension. In the pupa, and commonly afterwards, the two membranes composing the organs in question do not touch each other's inner surface, as they afterwards do: there is consequently a space between them; and being moist, and corrugated into a vast number of folds like those of a fan, but transverse as well as longitudinal, and so minute as to be imperceptible to the naked eye, the wings appear much thicker than in the end. Now as soon as the insect is disclosed, a fluid enters the tubes, and being impelled into their minutest ramifications, necessarily expands their folds; for the nervures themselves are folded, and as they gradually extend in length with them, the moist membranes attached to them are also unfolded and extended. In proportion as this takes place, the expanding membranes approach each other, and at last, being dried by the action of the atmosphere, become one. To promote this motion of the fluid, seems the object of the agitations which the animal from time to time gives to its unexpanded wings. That a kind of circulation, or rather an injection of an aqueous fluid into these organs, actually takes place, may be ascertained by a very simple experiment. If you clip the wings of a butterfly during the process of expansion, you will see that the nervures are not only hollow, but that, however dry and empty they may subsequently be found, they at that time actually contain such a fluid[684]. Swammerdam, who appears to have been the first physiologist that paid attention to this subject, was of opinion that an aëriform as well as an aquiform fluid contributes to produce the effect we are considering. He had observed that, if a small portion be cut off from the wing of a bee, a fluid of the latter kind exuded from its vessels in the form of pellucid globules, becoming insensibly drops--which he concluded proved the action of the latter; and he noticed, also, that the wings were furnished with tracheæ, which were at that time distended by the injected air; whence he justly surmised, that the action of the _air_ was also of great importance to produce the expansion of the wing[685]. And Jurine found that every nervure contains a trachea, which, proceeding from the interior of the trunk in a serpentine direction, follows all the ramification of the nervure, though it does not fill it[686]. Though Reaumur attributes the expansion of the wings chiefly to an _aqueous_ fluid, yet he suspects that the _air_ on some occasions contributed to it[687].

The wings of the other tribes of insects probably differ from the _Lepidoptera_ in the manner in which they are folded. It should seem from Reaumur's description, that those of some flies, instead of the straight transverse folds of the former, have angular or zigzag folds[688]; which equally shorten the wing. Many _Hymenoptera_ have wings without any nervures except the marginal. We may conjecture that these are more simply folded, so as to render their expansion more easy; but even in these wings there are often tracheæ, which appear as spurious nervures, and help to effect the purpose we are considering.

The operation of expanding their wings, in by far the larger number of insects, takes place gradually as described above; and, according to their size, is ended in five, ten, or fifteen minutes; in some butterflies half an hour, in some even an hour. A few species, such as _Sphinx Œnotheræ_ F., require several hours, or even a day, for this operation; and, from the distance to which they creep before it has taken place, a considerable degree of motion seems requisite for causing the necessary impulse of the expanding fluids[689]. In a few genera, however, as the gnat, the gnat-like Tipulidæ, and the Ephemeræ, this process is so rapid and instantaneous, that the wings are scarcely disengaged from the wing-cases before they are fully expanded and fit for flying. These genera quit the pupa at the surface of the water, from which, after resting upon it for a few moments, they take flight: but this would evidently be impracticable, and immersion in the fluid, and consequent death, would result, were not the general rule in their case deviated from.

Some species of the last of these genera, _Ephemera_, are distinguished by another peculiarity, unparalleled, as far as is known, in the rest of the insect world. After being released from the puparium, and making use of their expanded wings for flight, often to a considerable distance, they have yet to undergo another metamorphosis. They fix themselves by their claws in a vertical position upon some object, and withdraw every part of the body, even the legs and wings, from a thin pellicle which has inclosed them, as a glove does the fingers; and so exactly do the exuviæ, which remain attached to the spot where the Ephemera disrobed itself, retain their former figure, that I have more than once at first sight mistaken them for the perfect insect. You can conceive without difficulty how the body, and even legs, can be withdrawn from their cases; but you must be puzzled to conjecture how the wings, which seem as thin, as much expanded, and as rigid as those of a fly, can admit of having any sheath stripped from them; much less how they can be withdrawn, as they are, through a small opening at the base of the sheath. The fact seems to be, that though the outer covering is rigid, the wing inclosed in it, notwithstanding it is sometimes more than twenty-four hours before the change ensues, is kept moist and pliable. In proportion, therefore, as the insect disengages itself from the anterior part of the skin, the interior or real wings become contracted by a number of plaits into a form nearly cylindrical, which readily admits of their being pulled through the opening lately mentioned; and as soon as the insect is released from its envelope, the plaits unfold, and the wing returns to its former shape and dimensions. Thus our little animal, having bid adieu to its shirt and drawers, becomes, but in a very harmless sense, a genuine _descamisado_ and _sansculotte_. It does not seem improbable, that the pellicle we have been speaking of is analogous to that which, in addition to the outer skin, incloses the limbs of _Lepidoptera_, &c. in the pupa state, but which they cast at the same time with the puparium, and leave adhering to it[690].

The body of newly-disclosed insects commonly appears at first of its full size; but the aphidivorous flies (_Syrphus_ F. &c.), and some others, in about a quarter of an hour after leaving the pupa become at least twice as large as they were at their first appearance: this apparent sudden growth, which is also noticed by Goedart, Reaumur found to depend upon the expansion of the previously compressed segments of the animal by means of the included air[691]. Both in this instance and in that of insects whose wings only require expansion, the size of the _imago_ often so greatly exceeds that of the _pupa_, that we can scarcely believe our eyes that it should have been included in so contracted a space. The pupa of one of the beautiful lace-winged flies (_Hemerobius Perla_) is not so big as a small pea, yet the body of the fly is nearly half an inch long, and covers, when its wings and antennæ are expanded, a surface of an inch square[692].

When the development of the perfect insect is complete, and all its parts and organs have attained the requisite firmness and solidity[693], it immediately begins to exercise them in their intended functions; it walks, runs, or flies in search of food; or of the other sex of its own species, if it be a male, that it may fulfill the great end of its existence in this state--the propagation of its kind. Previously to thus launching into the wide world, or at least immediately afterwards, almost all insects discharge from their intestines some drops of an excrementitious fluid, often transparent, and sometimes red. I have before related to you the alarm that this last circumstance has now and then produced on the minds of the ignorant and superstitious[694]. Whether this excrement is produced indifferently both by males and females I cannot positively assert; but a circumstance related by Jurine affords some ground for a suspicion that it is peculiar to the latter. A specimen of a female of _Lasiocampa Rubi_, when killed emitted some of this fluid, which dropped upon the floor: this appeared to attract the males to the apartment in which it happened, and to the very spot--from whence it may be conjectured, that the scent of the fluid brought them there, and that the use of it is to bring the sexes together soon after exclusion from the pupa[695].

The colour, sculpture, and other peculiarities which distinguish insects in this state I shall consider at large in another letter, when I treat of their external parts and organs. Under the present head I shall confine myself to pointing out the characters by which the _sexes_ of many species are distinguished from each other; as likewise the _duration of their life_ in their perfect state; together with the circumstances on which this duration depends.

I. _Sexual Distinctions._ The first general rule that may be laid down under this section is,--That among insects, contrary to what mostly occurs in vertebrate animals, the _size_ of the female is almost constantly larger than that of the male. Even in the larva and pupa states, a practised eye can judge, from their greater size, which individuals will become females. There are, however, some exceptions to this rule. Thus amongst the _Coleoptera_, the male _Dynastidæ_, remarkable for their horns, as you may see in _D. Alocus_, _Antæus_, _Actæon_, &c., as likewise those of _Lucanus_, are larger than the unarmed females[696]. In the _Neuroptera_ the female _Libellulidæ_ are sometimes sensibly smaller, and never larger, than their males[697]. In the _Hymenoptera_ the male of the hive-bee, but more particularly that of _Anthidium manicatum_ and other bees of that genus, is much more robust than the other sex[698]. In the _Diptera_, the same difference is observable in _Syrphus Ribesii_, and some other aphidivorous flies, and also in _Scatophaga stercoraria_[699]. And amongst the _apterous_ tribes, we are informed by De Geer that the male of _Argyroneta aquatica_, which builds an aërial palace in the bosom of the waters[700], usually exceeds the female in bulk[701]. The reason of this rule seems in some degree connected with the office of the female as a mother, that sufficient space may be allowed for the vast number of eggs she is destined to produce; and it is when impregnation has taken place, and the eggs are ready for extrusion, that the difference is most sensible. In the majority of cases this sexual disproportion is not very considerable, but in some few it is enormous. Reaumur mentions a beetle, of which he intended to give the history, the male of which is so small compared with the female, that a bull not bigger than a sheep, or even a hare, set by the side of the largest cow, would aptly contrast with them. This little beetle, he says, has wings and elytra, while the giant female has no vestige of either, having the upper surface of its body naked and membranous[702]. The species to which this illustrious Naturalist here alludes, does not appear to have been ascertained. The female of many gall-insects (_Cocci_) is so large in comparison with the male, that the latter traverses her back as an ample area for a walk[703]. But this is nothing compared with the prodigious difference between the sexes of _Termes fatale_, and other species of white ants, whose males are often many thousand times less than the females, when the latter are distended with eggs[704]. Accidental differences in the size of the sexes sometimes arise: as when the female larva has, from any cause, been deprived of its proper supply of food, it will occasionally be less than the male. De Geer has stated a circumstance with respect to the _Aphides_ that produce galls, that should be mentioned under this head--the first, or _mother_ female, is larger than any of her progeny ever become[705].

The second observation that may be generally applied to the sexes of insects is, that, size excepted, there is a close resemblance between them in other respects. But to this rule the exceptions are very numerous, and so important that it is necessary to specify examples of each under distinct heads.

i. In some species the sexes are either partly or wholly of a different _colour_. Thus, in the order _Coleoptera_, the elytra of the male of _Rhagium meridianum_ F. are testaceous, and those of the female black. _Leptura rubra_ of Linné, with red elytra, is the female of his _L. testacea_, in which they are testaceous. _Cantharis dermestoides_ of the same author is the other sex of his _Meloe Marci_; one of which is chiefly testaceous, and the other black: which seems to have so misled Linné, that he placed them in different genera. One more instance in this order, the female of _Cicindela campestris_, as was first observed to me by our friend Sheppard, has a black dot on each elytrum, not far from its base near the suture, which the male has not.

Amongst the _Orthoptera_, the male _Locustæ_ F., as Professor Lichtenstein has informed us[706], have a fenestrated ocellus, which is not to be found in the other sex. I was once attending to the proceedings of a Hemipterous species, _Pentatoma oleracea_ Latr., which I found in union: the paired insects had white spots, but another individual was standing by them, in which the spots were of a sanguine hue. I mention this by the way only--the spots in the prolific sexes being of the same colour: but might not the red spotted one be a neuter?

The sexes of many _Lepidoptera_ likewise differ in their colour. I must single out a few from a great number of instances. The males of _Lycæna Argus_ F. have the upper surface of their anterior wings of a dark blue, while in the female it is wholly brown. The wings of the former sex of _Hypogymna dispar_ are gray, clouded with brown; but those of the latter are white, with black spots. In the brimstone butterfly (_Colias Rhamni_), which is one of the first that appear in the spring, the wings of the male are yellow--of the female whitish. In the common orange-tip (_Pieris Cardamines_ F.), one sex has not the orange tip to the upper wings: and, to name no more, the male of _Lycæna dispar_, one of our rarest and most beautiful butterflies, has only a single black spot in the disk of its fulgid wings; while in the other sex, the primary pair have nine, and the secondary are black, with a transverse orange fascia near the posterior margin. But the most remarkable difference in this respect observable in the insects of the order in question, takes place in a tribe, of which only one species is certainly known to inhabit Britain--I mean the _Papiliones Equites_ of Linné: what he has called his _Trojani_ and _Achivi_ in some instances have proved only different sexes of the same species. Mr. MacLeay's rich cabinet affords a singular instance confirming this assertion;--a specimen of a Papilio is divided longitudinally, the right hand side being male, and the left hand female. The former belongs to _P. Polycaon_, a Grecian, the latter to _P. Laodocus_, a Trojan. An instance of two _Grecians_ thus united is recorded in the _Encyclopédie Méthodique_, as exhibited in a specimen preserved in the Museum of Natural History at Paris; which on the right hand side is _P. Ulysses_, on the left _P. Diomedes_[707].

In the _Neuroptera_, the _Libellulidæ_ are remarkable for the differences of colour in the sexes. In the common _Libellula depressa_, which you may see hawking over every pool, the abdomen of the male is usually slate-colour, while that of his partner is yellow, but with darker side-spots. Reaumur, however, noticed some males that were of the same colour with the females[708]. Schelver observed, when he put the skins of _Libellula depressa_ into water, that the colours common to both sexes were in the substance of the skin, and remained fixed; while those that were peculiar to one could be taken off with a hair-pencil, and coloured the water: which therefore were superficial, and, as it were, laid on[709]. The yellow males, therefore, that Reaumur observed, were probably such as had the superficial blue colour which distinguishes them washed off. In _Calepteryx Virgo_ Leach, the former are of a lovely silky blue, and the latter green. In _Agrions_ F. nature sports infinitely in the colours of the sexes.

In the order _Hymenoptera_ there are often differences equally great; the sexes of many of the Ichneumons and Saw-flies are of quite different colours. The former tribe Linné has divided into sections, from the white annulus observable in the antennæ of some, and from the colour of their scutellum: but these are often merely sexual characters[710]. The male of _Anthophora retusa_ Latr., a kind of wild bee, is wholly black, the female wholly gray, and of so very different an aspect that they were long regarded as distinct species; a mistake which has likewise occurred with regard to the sexes of _Osmia cærulescens_, another bee, of which the male has a bronzed and the female a violet abdomen[711]. The nose of male _Andrenæ_ Latr. is often yellow, or white, as in _A. hæmorrhoidalis_--when that of the female is black[712]. The _labrum_ also is often of a different colour in the sexes, as in _Ceratina_ Latr.

In the _Diptera_, _Aptera_, _Arachnida_, &c., I am not aware of any striking difference in the colours of the sexes.

ii. The sexes of insects vary (but more rarely than in colour) in their _sculpture_ also, and _pubescence_. Thus the elytra of the females of many of the larger water-beetles (_Dytiscus_) are deeply furrowed, while those of the males are quite smooth and level[713]. The thorax of the female in several species of _Colymbetes_ of the same tribe, as _C. Hybneri_ and _transversalis_, on each side has several tortuous impressed lines or scratches, like net-work, which are not to be discovered in the male. _Hyphydrus gibbus_ Latr., which differs solely from _H. ovalis_ (_Dytiscus ovalis_ Illig.) in being thickly covered with minute impressed puncta, is, from the observation of the Rev. R. Sheppard, the other sex of this last, with which he has taken it coupled; and it is by no means improbable that _Hydroporus picipes_ (_Dytiscus punctatus_ Marsh.) and _H. lineatus_,--between which, as Gyllenhal has justly observed, the same difference only exists,--are in like manner sexual varieties. With respect to _pubescence_, I have not much to say. Another aquatic beetle, _Acilius sulcatus_ Leach, has not only its elytra sulcated, but the furrows of these, and a transverse one of the thorax, are thickly set with hair; while the male is smooth, and quite naked. Particular care seems to have been taken by the Creator, that when all the above inhabitants of the water are paired, the male should be able to fix himself so firmly, by means of his remarkable anterior tarsi, (which I shall afterwards describe,) and these asperities, &c. in the upper surface of his mate, as not to be displaced by the fluctuations of that element, the reluctance of the coy female, or any other slighter cause.

In a moth called the ghost (_Hepialus Humuli_), the posterior tibia of the male is densely bearded, but not of the female[714].--Some _Hymenoptera_, as _Ammophila_ Kirb. and _Stigmus_ Jurine, have the upper lip of the male clothed with silver pile, while that of the female is not so ornamented. The legs of some bees are distinguished in the sexes by a difference in their _clothing_. That observable in those of the hive-bee has been before noticed[715]. In _Andrena_ of Latreille[716] the posterior tibia of the female is covered externally with a dense brush of hairs, for collecting the pollen; and the posterior legs at their base have a curled lock of hair--which are not to be found in the male[717]. In _Dasypoda_, _Melecta_, _Anthophora_, _Centris_, _Epicharis_, &c. of the same author, the first joint of the tarsus of the female, and in _Xylocopa_ almost the whole tarsus, is also similarly signalized from that of the other sex. In _Bombus_, as in the hive-bee, the posterior tibiæ of the females and neuters are furnished with a basket of hairs for carrying their pollen paste, which you will in vain look for in the male[718]. The latter, however, in some species of this tribe are distinguished from the former by the longer hairs of their legs, but not in the posterior ones. Thus, in _Anthophora retusa_ the first joints of the intermediate tarsus are bearded internally with a thin fringe of long hairs, and the first externally with a triangular one of short ones at the apex: but what is most remarkable, the last or unguicular joint, which in almost every other bee is naked, is on both sides fringed with long hairs[719]. In that remarkable genus _Acanthopus_ Illig., of which the male only is known, the first and last joint of the intermediate tarsus have a dense external brush of stiff hairs, which probably is also a sexual character[720]. Another sexual kind of clothing is exhibited by the females of those bees that have their labrum or upper-lip inflexed (_Megachile_ Latr.)[721]. Their abdomen is covered underneath with a brush of stiff hairs, involved in which they carry the pollen they collect. In the males of some of this tribe, as of _M. Willughbiella_, the first four joints of the anterior tarsus on their inner side have a long dense fringe of incurved hairs[722]: a circumstance also to be found in the same sex of _Xylocopa latipes_, in which the claw-joint also is bearded[723]. In _Andrena_ Latr. the last dorsal segment of the abdomen of the same sex is fringed, while that of the male is naked[724]. In the humble-bees (_Bombus_), the mandibles of the male are bearded with curled hairs, while those of the females and neuters are without them. Some bees, as _Andrena_ and _Halictus_ Latr., have the anus of the female bearded, and that of the male naked: in some _Bombyces_ the reverse takes place.

iii. With regard to the general _shape_ of their body, the male and female usually resemble each other: but there are some exceptions to this rule. The male of the hive-bee is much thicker and more clumsy than either the female or the worker; but in _Halictus_ Latr. the males are nearly cylindrical in shape, and very narrow; while the other sex are oblong or ovate, especially their abdomen: and in _Andrena_ Latr. the former are much slenderer than the females, and of a more lanceolate shape. But a still more striking difference in this respect between the sexes is exhibited by some species of the genus _Ptinus_ F., in which the male is long and slender, and the female short and thick. This, in more than one instance, has occasioned them to be mistaken for distinct insects: thus, _P. Lichenum_ and _P. similis_, _P. ovatus_ and _P. testaceus_, of Mr. Marsham, are mere sexual varieties. But the most entire abalienation of shape at present known, is that which distinguishes the male from the female _Coccus_; these are so completely dissimilar as scarcely to have any part in common. In _Bombyx vestita_ F., and others of the same family, while the males are of the ordinary conformation of the order, the females are without even the slightest rudiments of wings; they have no antennæ, the legs are extremely short, not longer than those of the caterpillar; and the body is entirely destitute of scales, so that they altogether assume the exact appearance of hexapod larvæ[725]. A conformation nearly similar takes place in the female of _Tinea Lichenella_; but in this the feet are longer, and the anus is furnished with a long retractile ovipositor[726].

iv. In many cases, the structure of particular _parts_ and _organs_ of the body differs in the sexes. As the facts connected with this part of our present subject are extremely numerous and various, it will be convenient to subdivide it, and consider the sexual characters that distinguish--the _Head_, _Trunk_, and _Abdomen_ of insects, and their several appendages.

1. The _Head_. This part in some females is considerably larger than it is in the male. This is the case with the ants, and several other _Hymenoptera_; while in some _Andrenæ_, as _A. hæmorrhoidalis_, and _Staphylinidæ_, as _St. olens_, that of the male is the largest. But in none is the difference more conspicuous than in the stag-beetle (_Lucanus_); in which genus the male not only exceeds the female in the length of his mandibles, but also greatly in the size and dimensions of his head. In the _Apion_ genus, the rostrum of the female is generally longer and slenderer than that of her mate; and in _Brentus_, the rostrum of one sex (probably the male) is long and filiform, while in the other it is thick and short. This is particularly visible in _B. dispar_ and _maxillosus_[727], &c.

One of the most striking distinctions of the males in this part of their body, are those threatening horns, usually hollow, with which the heads of many of the male lamellicorn insects and some others are armed, and which give them some resemblance to many of the larger quadrupeds. Many are _unicorns_, and have their head armed with only a _single_ horn; which in some, as in _Oryctes_ Illig., _Dynastes Endymion_[728], &c. is very short; in others, very long, as in _Dynastes Enema_, _Pan_, _Elephas_[729]. In one, again, it is thick and robust; as in the clumsy _Dynastes Actæon_[730]: in another very slender, as in _Onthophagus spinifer_[731]. With respect to its direction in _Elephastomus proboscideus_ MacLeay, it is horizontal[732] and straight; in _Phaleria cornuta_ horizontal and broken, or the apex turning outwards and forming an angle with the base[733]; in _Dynastes Hercules_ horizontal, and recurved at the apex[734]; in _D. Actæon_, _Elephas_, and _Typhon_, recurving from the base. In _Geotrupes dispar_ it is recurved, so that its point exactly coincides with that of the porrected thoracic horn, with which it forms a kind of forceps[735]. In _Copris lunaris_ F. and _Diaperis horrida_, the horn is nearly upright[736]. In _Onthophagus Xiphias_ it is dilated at the base, and reclining upon the thorax; and at the apex attenuated, and bending forwards, or nodding. In _Passalus cornutus_ it rises a little, and then bends wholly forwards. In _Dynastes Milon_, a most remarkable beetle, it slopes backwards in a waving line[737]; and in _Onthophagus spinifer_ it is recurved and reclining.--In speaking of the direction of the horn, you must recollect that it will vary in proportion as the head varies from a horizontal position: so that an upright horn will become inclined or reclined, as the head bends forwards or backwards; but I speak of it as it appears when the head is horizontal. Again, it varies in its teeth or branches. In _Dynastes Hercules_ it is armed with several teeth. In _D. Elephas_ and _Actæon_ it has only one large one at its upper base[738]. In _D. Milon_ it is serrated above. In _D. Alcides_, _Tityus_, _Ægeon_, _Copris lunaris_, &c. the horn is unarmed and simple at the apex. In _D. Oromedon_, _Gedeon_, _Enema_, _Actæon_ and congeners, it is bifid. In some the horn is at first a broad lamina or ridge, which terminates in two branches, as in _Onthophagus Vacca_. In this the branches are straight; but in another undescribed species in my cabinet (_O. Aries_ Kirby, MS.) they are first bent inwards, and then at the apex a little recurved: and in _D. dichotomus_ it is divided into two short branches, each of which is bifid[739]. Other males emulate the bull, the he-goat, or the stag, in having a _pair_ of horns on their head. In _Onthophagus Taurus_, these arms in their curvature exactly resemble those of the first of these animals[740]. In _Goliathus pulverulentus_, the straight, robust, diverging, sharp horns are not unlike those of some of the goat or gazel tribe. I have a beautiful little specimen in my cabinet, (I believe collected by Mr. Abbott of Georgia,) in which the horns have a lateral tooth, or short branch, like those of a stag; and which I have therefore named _O. cervicornis_. In _O. Vacca_, _Camelus_, &c. the horns are very short, and nearly perpendicular. In the male of _Copris Midas_, the two longer perpendicular horns have a deep cavity between them, which, together with its black colour, give it a most demoniac aspect; so that you would think it more aptly representative of a Beelzebub or _Beelzebul_ than a Midas[741], or than _Phanæus Beelzebul_ MacL. A similar cavity is between the occipital horns of _Diaperis hæmorrhoidalis_ Payk. Some species of _Rynchænus_, as _R. Taurus_, have a pair of long horns upon the rostrum of the male, the rudiments only of which are to be traced in the female[742]. Other species go beyond any known quadrupeds in the number of horns that arm their heads. Thus _Ditomus calydonius_ Bonelli, belonging to _Carabus_ L., has _three_ equal horns[743]. The same number distinguishes _Onthophagus Bonasus_; but the intermediate one is very short. In _Goliathus Polyphemus_ the middle horn, on the contrary, is much longer and thicker than the lateral ones, and forked at the apex; so that it looks as if it had four of these weapons[744]. A little _Diaperis_ (_D. viridipennis_ F.), a native of _Carolina_, has four horns upon the head of the male; namely, two long ones on the occiput, and two short dentiform ones on the nose. In a species nearly related to this, sent me by Professor Peck from New England, there is a cavity between the two occipital horns. The same number distinguishes _Onthophagus quadricornis_ (_Copris_ F.). The _situation_ also of the horns varies: In some it is in the _middle_ of the head, as _Oryctes nasicornis_, _Copris lunaris_, &c.: in others, as in _Onthophagus nuchicornis_, _Xiphias_, &c. it is a process of the _occiput_ or hind-head; and in _O. Oryx_ F. the two horns proceed from the _anterior_ part of the head. In the other sex, in insects the head of whose males is armed with horns, they are usually replaced by mere tubercles, or very short elevations, as you may see in the female of _Copris lunaris_; or by transverse ridges, as in the _Onthophagi_: or else the head is without arms, and quite smooth, as in _Diaperis_, _Phaleria_, &c. What may be the use of these extraordinary appendages, as well as those on the thorax, and in some cases on the abdomen, (which I shall mention afterwards), to the males, has not yet been ascertained. Whether the individuals of this sex are more exposed to the attack of birds and other enemies, in consequence of being more on the wing than the females, and are therefore thus provided with numerous projecting points for defence, is a question worth considering[745]. It is the only probable conjecture on the _cui bono_ of these arms that I can at present make. Under this head I ought to notice the remarkable membranous process of an obovate shape, which like an umbrella covers the head of _Acheta umbraculata_ F.[746] Whether the sharp curved horns which arm this part in another _Acheta_ figured by Stoll[747], in an incumbent posture, with their point towards the mouth, are a sexual distinction, we are not informed,--probably they are.

The _organs_ of the head also present many sexual distinctions. The upper lip (_labrum_) in _Halictus_ Latr., a tribe of wild bees, in the female is furnished with an inflexed appendage, which is not discoverable in that of the male[748]; and the shape of this lip in _Sphecodes_ Latr. differs in the sexes[749]. Perhaps the horn or tubercle observable on this part of some female _Nomadæ_ F.[750] may be wanting in the male.

The under-lip (_labium_)--taken in a restricted sense for that central part from which emerge the labial palpi, and which is often considered as the mentum,--does not offer any striking variations in the sexes. One, however, is of importance, as it helps to prove which are the true female _Lucani_. In the male the labium is emarginate, in the female it is intire. This may be seen both in _L. Cervus_ and _femoratus_, and probably in other species. The sculpture also is different, the lip being smooth in the former and covered with excavated _puncta_ in the latter. The tongue (_lingua_ or _ligula_) of the sexes is usually the same; except in the hive-bee, in which that of the neuters is longer than that of the male and female.

The upper-jaws (_mandibulæ_), however, often afford striking sexual characters. The enormous protended ones of the common stag-beetle (_Lucanus Cervus_) attract the attention of the most incurious observer; and these are now generally allowed to be of this description. Geoffroy and Mr. Marsham, indeed, have asserted that they have taken _in coitu_ those with long mandibles: but as these males are pugnacious, and attack each other with great fury, as Mr. Sheppard informs me, it is not improbable that these gentlemen may have mistaken a _battle_ for an _amour_: since not only have those with long mandibles been often taken united with those that have short ones[751], but the same difference obtains in the sexes of other species. This is particularly observable in _Lucanus femoratus_, of which I received from Brazil many specimens agreeing in every respect except in this, that one had short and the other very long mandibles. These organs vary in different specimens, as to the number of their teeth and branches. They are singularly robust in _L. Alces_[752]; but in none more threatening than in _L. Elephas_[753], in which they curve outwards and downwards. In Mr. W. MacLeay's genus _Pholidotus_, they are almost parallel to each other, and curve downwards; in _Lucanus nebulosus_ Kirby, they assume a contrary direction[754]; as they do likewise in _Lamprima_ Latr.[755] In _Lucanus Capreolus_ the points close over each other[756]. In _Lethrus_ F. in the female, but not the male, the mandible is armed below with a long incurved horn. In _Lucanus serricornis_ they form a complete forceps[757]. In _Siagonium quadricorne_ Kirby[758] the mandible is furnished at its base with an exterior horn, which is probably a sexual distinction. The male of _Synagris cornuta_, a kind of wasp, is still more conspicuous in this respect; for from the upper side of the base of its straight slender mandibles proceed a pair of crooked, decurved, tortuous, sharp horns, not only longer than the mandible, but than the head itself[759]. Many sexual differences are observable in the mandibles of the various tribes of bees (_Anthophila_ Latr.). Thus, in _Colletes_ Latr. the _male_ mandible is more distinctly bidentate at the apex than the female[760]: in _Sphecodes_ Latr. and others, the reverse of this takes place[761]. Where these organs in both sexes are toothed at the apex, they often vary in the number of teeth. Thus, the female of _Megachile centuncularis_ Latr. has four teeth at the apex of its mandible, while the male has only two[762]. In _M. Willughbiella_, though the mandibles of both sexes have four teeth, yet those of the male are sharp, and the two external ones the longest; while those of his mate are obtuse, and all nearly equal in length[763]. In _Anthidium manicatum_ Latr., the former has only three teeth, while the latter has five[764]. The differences in this respect in the hive-bee have been before noticed[765]; those of the humble-bees (_Bombus_ Latr.) are strikingly distinguished from each other; the female mandible being very stout and wide, constricted in the middle, and furrowed on its outer surface; and the male, on the contrary, very slender at the apex, dilated at the base, and without furrows[766].

Of all the organs of the head, none seem so little subject to _sexual_ variation as the under-jaws (_maxillæ_)[767]. I can bring forward only one striking instance of it, and some degree of doubt rests even upon that. In the genus _Nemognatha_ of Illiger, the maxillæ of the male are elongated, narrow, setiform, and often involute or spiral, like those of a bee or a butterfly. But that this is peculiar to the males is at present only surmised[768]. I possess several species of the genus, all of which are distinguished by long maxillæ; though in some they are as long as the body, and in others scarcely half that length. _Gnathium_ Kirby is similarly characterized[769].

The _maxillary palpi_ occasionally differ in the sexes. In _Cerocoma_ those of the female are filiform, while the two intermediate joints of those of the other sex are much thicker than the first and the last[770]. In _Hylæcetus_ and _Lymexylon_, those of the male are still more remarkable: they are pendent, the last joint very large, and laciniated so as to form a tuft[771]. The female ones grow gradually larger towards the end, but are not at all divided there[772]. The palpi of male spiders are of a very different structure from those of the other sex, terminating in a very complex incrassated piece, which has been supposed to contain the organ of generation; but this, according to Treviranus, is a mistaken idea--that organ being, as usual, to be found in the abdomen[773]. In the common gnat the palpi of the male are as long as the proboscis, consist of five joints, and at the end are tufted with hairs; while those of the female are scarcely one-fourth of its length, have only three joints, and are not tufted. Whether the _labial palpi_ in any genus differ in the sexes, I cannot affirm with certainty: I have not, however, observed any such variation in them.

I shall next mention some organs of the head, in which the difference between the sexes is often very striking and peculiar. You will readily conjecture that I am speaking of the _antennæ_. And here the advantage seems wholly on the side of the males: since in them these wonderful instruments of unknown sensations are not only more complex, but usually more elegant, than those of the other sex. You will pardon me, therefore, if I enlarge a little more than ordinary upon a subject so full of interest, and say something upon the differences observable between the sexes--in the shape, magnitude, and length, number of articulations, ramification and plumage, and individual joints of their antennæ.

With regard to their _shape_, variations are sometimes observable between the antennæ of the sexes; but this principally occurs in the _Hymenoptera_ order. For instance, those of _Chelostoma maxillosa_, a small bee that deposits its eggs in little holes in posts and rails, are clavate in the female and filiform in the male[774]--a circumstance that distinguishes in some degree those of _Sphecodes_, _Halictus_, and _Andrena_ of Latreille, three other genera of wild-bees[775]. In _Dinetus_ Jur. the male antennæ are moniliform at the base, and filiform at the apex; the female, on the contrary, are entirely filiform[776].

The antennæ of the sexes also sometimes differ in _magnitude_ and _length_. This is the case in the three genera of wild bees just mentioned; those of the female being thicker than those of the male, while these last are longer than the former. But in this tribe the males of the Fabrician genus _Eucera_ are most remarkable for their long antennæ[777]. With regard to the different length of these organs in the sexes, no insects are more distinguished than some species of the capricorn-beetles (_Cerambyx_ L.). In _Lamia Sutor_ the male antennæ are twice the length of the female; and in another Brazilian species in my cabinet, related to _L. annulata_ (_Stenocorus_ F.), they are thrice their length. Some of the _Anthribi_ F. approach the _Cerambycidæ_, not only in some other characters, but also in this circumstance:--thus the antennæ of _A. albinus_, a native of Britain, are vastly longer in the male than in the female; and in _A. cinereus_ (_Macrocephalus_ Oliv.)[778], which I suspect to be of the former sex, they are as long nearly as is usual in the tribe just named, called in France capricorn-beetles.

I may here observe, that sometimes in the sexes a difference is also to be found in the _direction_ or flexure of their antennæ. Thus in _Scolia_ F., _Pepsis_ F. &c., in the males the antennæ are nearly straight, but in the females convolute or subspiral. The reverse of this takes place in _Epipone spinipes_, a kind of wasp, and its affinities; and _Systropha_ Illig., a kind of bee: for in these the male antenna is convolute at the apex[779], and the female straight. In the various tribes of bees (_Anthophila_ Latr.), these organs in the latter are what is denominated _broken_, the main body of the antenna forming an angle with the first joints: but in the former this does not take place.

The antennas of the sexes do not always agree in the _number_ of joints. In the bees, and many other _Hymenoptera_, the male has one more joint than the female; as is the case also in _Œdemera notata_ (_Cantharis acuta_ Marsh.). In _Pteronus Laricis_, a kind of saw-fly, the latter has only sixteen joints in its antennæ, while the former has twenty-four[780]. In _Rhipicera marginata_, a beetle, the beautiful antennæ of the male consist of thirty-two joints, while the female has no more than eleven! In _Chelonus_ Jur. the male, on the contrary, has the smallest number of joints, namely sixteen; while the female has twenty-five[781].

In nothing do the sexes differ more materially than in the _ramification_ of these organs, and their _plumage_. By attending to this, you may often detect the sexes in an instant; since the antennæ of the males in numerous instances are much more complex than those of the females. For what end the Creator has so distinguished them is not quite clear; but most probably this complex structure is for the purpose of receiving from the atmosphere information of the station of the female. A tendency to branching will be found in the antennæ of some males, in tribes where these organs are usually perfectly simple in both sexes. Thus, in the male of _Chelostoma maxillosa_,--mistaken for another species by Linné, which he names _Apis florisomnis_,--the intermediate joints on their inner side project into an angle[782]; and those of the same sex of the common hornet, by means of a central sinus, have two obtuse teeth on each. With regard to more direct ramifications, some male antennæ terminate in a fork, or two branches. This is the case with _Hylotoma furcata_ Latr., a saw-fly[783]; and the peacock-louse (_Nirmus Pavonis_ Herm.)[784]. Others, again, have three lateral branches, as in _Eulophus_ Geoffr. a little parasite, the male antennæ of which send forth a hairy external and rather long branch, from the base of the fourth, fifth, and sixth joints[785]. In _Elater flabellicornis_ L., the eight last joints are flabellate, or elongated and flat, resembling the sticks of a fan in the male[786]; in the female they are shorter, and more properly may take their denomination from the teeth of a comb. In _Lampyris Latreillii_ Kirby, the antennæ of the former are flabellate on both sides, while those of the latter are little more than serrate[787]. These organs are extremely beautiful in the males of the _Rhipiceræ_ of Latreille. In _R. marginata_ K. they consist of thirty-two joints, from thirty of which issues a branch, the first very short, but the rest gradually increasing in length as they approach the middle of the antenna; then gradually decreasing to the end, so as to represent an expanded fan[788]. But in none are they altogether so remarkable as in those moths that Linné denominates _Bombyces Attaci_, and some others. In these, in the males, these organs in their contour are lanceolate, and every joint is furnished with a couple of parallel equal branches on each side[789]. In the females these branches are shorter on the whole, and alternately one long and one short; but in some, as _Saturnia Pavonia_, there is only one short branch or tooth on each joint in this sex[790]. In _Bombyx regalis_ &c. only the first part of the antenna is so branched; and those of the female are setaceous and without branches. In _B. versicolor_, &c. there is only one branch from each side on every joint; those of the female being much shorter than those of the male. The latter sex of _Pteronus Laricis_ Jur., a saw-fly, afford an example of a different structure, the antennæ on one side sending forth a branch from every joint but the two first; but on the other side, the nine or ten last joints also are without a branch. The female antenna is serrated[791]. In another of this tribe, _Pterygopterus cinctus_ Klug, the male antenna resembles a single-toothed comb, being branched only on one side: that of the female, like the former instance, is serrated[792]. Whether the remarkable antennæ that distinguish the known individuals of the genus _Phengodes_ (_Lampyris plumosa_ F.) is a sexual character has not been ascertained; but it is not improbable that it may be, as in other _Lampyridæ_. A pair of delicate flexile and almost convolute plumose branches proceeds from the apex of each joint except the basal ones, which have something the air of cirri, and give a more than usual degree of lightness and elegance to these organs[793]. Other antennæ, especially in the _Diptera_ order, assume an appearance of _plumes_--not from the branches that proceed from them, but from the fine long hairs that beset and adorn them. These are universally indications of the male sex, those of the females being generally comparatively naked. If you take the common gnat, you will find that the antennæ of one individual are thickly fringed on each side, and tufted at the end with fine long hairs, while in the other only four or five placed at intervals in a whorl are to be perceived[794]. In _Chironomus_ Meig., a kind of _Tipula_ L., resembling a gnat, the male antennæ are beset on all sides with the finest hairs, and resemble a beautiful plume[795], while the females to the unarmed eye appear naked. Even in some _Hymenoptera_, the antennæ of the males are thus feathered, in a less degree: for instance, in _Hylotoma_ Latr.[796] Whether the tufts and fringes which ornament, in a remarkable manner, the antennæ of many _Cerambycidæ_[797], are sexual characters, is not certainly known.

We are now to consider other sexual differences in these organs, resulting from the size or configuration of one or more _individual joints_. To begin with the first joint, or scapus. In many of the _Hymenoptera_, particularly the _Anthophila_ Latr., this is elongated, and the remaining joints form an angle with it in the females: while in the other sex it is much shorter, and in the same line with the rest of the antennæ; and in _Hylæus dilatatus_ (_Melitta dilatata_ Kirby) the first joint in the male is dilated and shaped something like a patella[798]. In _Malachius bipustulatus_, &c. the sex just mentioned is peculiarly distinguished by a white excrescence on the first four joints of the organs in question, most conspicuous in the second and fourth. The antennæ of male _Cerocomæ_ are not very different[799]. Mr. Marsham has described a little _Haltica_ under the name of _Chrysomela nodicornis_, from a peculiarity of the same sex not to be found in the other. The fourth joint is very large and obtriangular; in the female it is merely longer than the rest. In _H. Brassicæ_ and _quadripustulata_ the fifth joint is larger and longer than all but the first in the male, in their females it is only longer. In some moths (_Herminia_ Latr., _Crambus_ F.) there is also a knot in the middle of the male antennæ[800]. In _Noterus_, a water-beetle, the six intermediate joints are thicker than the rest, beginning from the fourth, and the last but one ends internally in a truncated tooth. The fifth and two following joints in the male antennæ of _Meloe_ are larger than the rest, which distinguishes them, as well as a remarkable bend observable at that part[801].