Chapter XV: Letter XLVII: System of Insects (1)

Having considered insects as to their History, Anatomy and Physiology, we must next enter a new and ample field, in which, like most of our predecessors, we shall often be perplexed and bewildered by the infinite variety of devious paths which traverse it, and by the mazy labyrinths in which the more we wander the less ground we seem to gain.--You will easily perceive I am speaking of the _System of Insects_. System is a subject which has engaged the attention of Naturalists from the time of Aristotle to the present day; and even now that it has been so much and so ably discussed, they are far from being agreed concerning it. In our own country a clue has, however, of late been furnished, which upon the whole seems better calculated to enable us to thread the intricate labyrinth of nature, than any thing previously excogitated.

There are two words relating to this subject concerning which Naturalists seem not to have very precise ideas--_Method_ and _System_. They have often been confounded and used indifferently to signify the same thing. Thus we hear of a Natural Method and a Natural System. Linné seems to have regarded the _former_ of these terms as representing the actual disposition of objects in nature[1126], while by _System_ he understands their classification and arrangement by Naturalists[1127]. But if we consider their real meaning,--a _Method_ should signify an _Artificial_, and a _System_ a _Natural_ arrangement of objects[1128]. As many systematists, however, have aimed at giving a _natural_ arrangement, though with various success,--some, as the French school, (to which we are principally indebted for the progress already made,) approximating nearer to the true idea than others,--and none having a _perfect_ conception of it, of which probably in our present state, our minds, from its intricacy, are incapable,--it might perhaps be as well to call every arrangement whose object is confessedly artificial, a _Method_; and that which aims at the plan of nature, a _System_. Under this view system-makers would be divided into two classes,--the _Methodists_ and _Systematists_.

The system of nature, which we are now to consider, may be viewed under a double aspect; for with regard to all created objects there is a _System_ of _Distribution_, and a _System_ of _Correlation_, which appear to be quite independent of each other. The former will best fall under our notice when we are treating of the _Geography_ of insects: I shall therefore now confine myself to the latter.

When the ALMIGHTY CREATOR willed to bring into existence this mundane system, he formed it according to a preconcerted plan, with all its parts beautifully linked together and mutually corresponding. All things were ordered in _measure_, and _number_, and _weight_[1129]. There was nothing deficient, nothing superfluous; but the whole in the strictest sense "was very good[1130]," and calculated in the highest degree to answer the purpose of its GREAT AUTHOR. I call it a system of _Correlation_, because there is discernible in it, in the first place, a concatenation of its parts, by which, as to their forms and uses, objects are linked together in groups by a chain of affinities; so that we pass from one to the other by gentle gradations, without having to overleap any _wide_ interval. We see also a gradual ascent from low to high, from less to more excellent. And this leads us to another kind of relationship between natural objects, by which, though placed in distinct groups or in a different series, they in some sort represent and symbolize each other. Examples of this relationship by analogy are to be found in every kingdom of nature, and often form an ascending series from the lowest to the highest; for, as we shall see hereafter, these resemblances appear to maintain a certain correspondence with each other as to their relative situations; so that, for instance, in the animal kingdom they ascend step by step, without being linked by affinity or having any real juxtaposition, from the lowest groups, towards man, who stands alone at the head, or in the centre of all.--I shall say something on each of these kinds of relationship.

I. The relation of _affinity_ may be considered as to its _series_ and _groups_. A series, of course, consists of parts either _concatenated_ like a chain, or placed _separately_ at small intervals from each other. It may run either in a right line, or deviate from it in various ways. It appears to be the opinion of most modern Physiologists, that the series of affinities in nature is a _concatenated_ or continuous series; and that though an _hiatus_ is here and there observable, this has been caused either by the annihilation of some original group or species in consequence of some great convulsion of nature, or that the objects required to fill it up are still in existence but have not yet been discovered[1131]: and this opinion is founded on a _dictum_ of Linné, _Natura ... saltus non facit_[1132]. If this dictum be liberally interpreted, according to the evident meaning of the word _saltus_, few will be disposed to object to it; since both observation and analogy combine to prove that there must be a regular approximation of things to each other in the works of God; and that could we see the whole according to his original plan, we should find no _violent_ interval to break up that approximation: but if it be contended, that in this plan there is no difference in the juxtaposition of the nearest groups or individuals, and never any interval between them, I think we are going further than either observation or analogy will warrant. Were this really and strictly the case, it seems to follow that every group or individual species must on one side borrow half its characters from the _preceding_ group or species, and on the other impart half to the _succeeding_[1133]. But one of the most evident laws of creation is _variety_; and if we survey all the works of the MOST HIGH, we shall no where discover that kind of order and symmetry that this strict interpretation implies. The general march of nature therefore seems to say, that there must be _varying_ though not _violent_ intervals in the series of beings: or in other words, some _conterminous_ species or groups have more characters in common than others.

It was the opinion of Bonnet (in this field himself a host) and many other Naturalists, that the series of beings was not only continuous, but _undeviating_, ascending in a _direct_ line from the lowest to the highest[1134]. Others, finding that this theory could not be made to accord with the actual state of things in nature, thought that a scale of the kingdoms of nature must represent a _map_ or _net_[1135]; thus abandoning a continuous series: and Lamarck, as was before observed[1136], for the solution of the difficulty, arranged Invertebrate animals in a double subramose one. Mr. W. S. MacLeay and (without consultation nearly at the same time) Professor Agardh, Mr. Fries, &c. have given to the learned world an opinion which approximates more nearly to what we see in nature: viz. That the arrangement of objects is indeed in a continuous series, but which in its progress forms various convolutions, each of which may be represented by a _circle_, or a series that returns into itself[1137]. According to this opinion,--which seems the most consistent of any yet advanced, and which reconciles facts which upon no other plan can be reconciled,--the series of beings is involved in the highest degree, rolling wheel within wheel _ad infinitum_, and revolving, if I may so speak, round its centre and summit--_man_[1138]: who, though not including in himself all that distinguishes them, is still the great Archetype in which they terminate, and from which they degrade on all sides.

It is by this convolving series that the various _groups_ into which the kingdoms of nature seem resolvable are formed. We are instructed by the highest authority that every thing was created "after its kind;" and the common sense of mankind in all ages has imposed classic, generic, and other names implying sections, as well as specific ones, upon natural objects: and though many modern Physiologists have asserted that species form the only _absolute_ division in nature; yet as all seem to allow that there are _groups_, and many that these are represented by a circle or group returning into itself[1139], the most absolute division in nature, we will not contend for a term[1140]. We now come to consider these groups themselves, and may notice them under various denominations.

It is customary to consider all the substances of which our globe consists as divided into _three_ kingdoms,--the _Mineral_, _Vegetable_, and _Animal_; but strictly speaking the _primary_ division is into organized and inorganized matter; the former resolving itself into the two kingdoms last mentioned. These, like England and Scotland of old, have their "Land Debateable;" occupied by those _Productions moyennes_, (to use a term of Bonnet's[1141],) which are as it were partly animal and partly vegetable. From this territory common to both, the two kingdoms are extended in a nearly parallel direction till they reach their extreme limits, without any incursion from either side upon their mutual boundaries, but each showing its kindred with the other by certain resemblances observable between _opposite_ points; so that valley corresponds with valley, mountain with mountain, river with river, sea with sea[1142]; not, however, so as to form an exact counterpart, but only in some general features. But to leave metaphor;--as the vegetable kingdom is distinguished from the mineral by its organization and life, by its circulation of sap, and by its powers of reproduction by seed or otherwise; so is the animal from the vegetable by its powers of volition and locomotion[1143], by its nervous systems and organs of sensation, and the senses to which they minister, by its muscular irritability, and by its instinctive endowments.

Having made these observations with regard to the primary division of natural objects in general,--what I have further to say will be confined to the _animal_ kingdom, and ultimately to the branch of which we are treating.

i. Lamarck divided the animal kingdom into two _provinces_, or _subkingdoms_ as they are now called; the one consisting of all those animals whose skeleton is _internal_ and built upon a vertebral column, which are denominated _Vertebrates_; and the second, of those whose skeleton or its representative is for the most part _external_, including the muscles,--these are called _Invertebrates_[1144]. Though this distinction is so marked as in general to form a most striking characteristic, yet when these two provinces approach each other, it begins to disappear. Thus the vertebral column, forming one piece with the shell[1145], becomes almost _external_ in the Chelonian reptiles, or tortoises and turtles, and almost disappears in the cyclostomous fishes; and there is the beginning of an _internal_ one in the _Cephalopoda_, or cuttle-fish belonging to the Invertebrates. Dr. Virey, assuming the nervous system as his basis, long since divided the animal kingdom, without assigning names to them, into _three_ subkingdoms[1146]; M. Cuvier has _four_--_Vertebrata_; _Mollusca_; _Articulata_; _Radiata_[1147]: and Mr. MacLeay, finding _five_ variations of that system, divides animals into _five_ provinces or subkingdoms, of which I formerly gave you some account[1148];--viz. _Vertebrata_, in which the nervous system has only one principal centre; _Annulosa_, in which it is ganglionic, with the ganglions arranged in a series, with a double spinal chord; _Mollusca_, in which it is ganglionic, with the ganglions dispersed irregularly but connected by nervous threads; _Radiata_, in which it is _filamentous_, with the nervous threads radiating from the mouth; and _Acrita_, in which this system is _molecular_[1149]. And to this division of the kingdom, as founded on a satisfactory basis, I should recommend you to adhere: still however we may speak of vertebrate and invertebrate animals, as forming the _primary_ subdivision of them, taken from a striking character and obvious to every one who sees them.

If you inquire into the rank of each of these subkingdoms, of course you will assign the principal station to the _Vertebrates_, which are the most perfectly organized, to which _man_ belongs, and over which he immediately presides. If we form the scale according to the nervous system of each province, that in which the organ of sensation and intellect is most concentrated will stand first; and in proportion as this organ is multiplied and dispersed will be the station of the rest, which will place them in the order in which I have mentioned them; and the _Annulosa_, to which insects belong, will precede the _Mollusca_, which Cuvier and Lamarck had placed before them on account of their system of circulation. But when we reflect that a _heart_ and _circulation_ occur in some of the conglomerate _Polypi_[1150], animals that approach the _vegetable_ kingdom; that some of the acephalous _Mollusca_ have no visible organs of sense, except that of taste, whose substance is little better than a homogeneous gelatinous pulp, and who seem from their inert nature to have very slight powers of voluntary motion[1151], we shall be convinced that a heart and circulation alone, unaccompanied by a more concentrated nervous system and more perfect structure, cannot place an animal above those which in every other respect so obviously excel them. With regard to _insects_ particularly, we may further ask--Who that considers how man employs his powers and organs even in his most degraded state, or that contemplates the wonderful works that he is enabled to accomplish when his faculties receive their due cultivation and direction, can avoid regarding him as superior to the rest of the animal creation? And what unsophisticated mind, not entangled in the trammels of system, when it surveys the industry, the various proceedings, and almost miraculous works that have been laid before you, the waxen palaces of the bee,--the paper cottages of the wasp and hornet,--the crowded metropolis of the white ants,--the arts, the manufactures, and stratagems of other insects,--the associations and labours for the common good of those that are gregarious;--will not at once conclude that they must be a superior race to the slug, the snail, and others, which live only to eat and propagate their kind?

Or who, that considers the wonderful structure of the animals whose cause I advocate,--the analogy that exists between their organs of manducation, of motion, and of sensation, and between various other parts of it[1152], with those of the higher animals,--the acuteness of their senses, their wonderful strength of muscle[1153], and powers of locomotion[1154],--but will think them superior to the headless and almost inanimate oyster or muscle, or the conglomerate _Alcyonia_, though they have a heart and circulation?

Who again, that observes that in proportion as pedate animals approach to the human type, their motions are accomplished by fewer organs,--that man walks _ore sublimi_ upon _two_ legs; the majority of quadrupeds upon _four_; insects upon _six_: the _Arachnida_ apparently upon _eight_; most _Crustacea_ upon _ten_; and the _Myriapods_ and others upon _many_,--but will thence conclude that insects must precede the _Arachnida_ and _Crustacea_?

Who, once more, that reflects that if any of the superior animals are deprived of a limb it can never be reproduced, and that in insects the same circumstance occurs; while spiders and _Crustacea_ if they lose a leg have the power of reproducing it, and the _Mollusca_ if they are decapitated can gain a new head,--will consent to their being placed after any of these animals[1155]?

Lastly, who that recollects that the _Mollusca_ are hermaphrodites, like most plants, bearing both male and female organs in the same body,--but will allow that insects, in which the sexes are separate as in the Vertebrates, must be more perfect, and of a higher grade[1156]?

ii. We now come to the _Classes_ into which the _Annulosa_ are divided. This term appears first to have been employed by Tournefort, and was adopted by Linné[1157]. As the nervous system of animals furnishes the most prominent distinction of a subkingdom, so the _circulation_ of their fluids, and their _respiration_ necessarily connected with it, seems best to point out the _classes_ into which it may next be resolved. But having fully explained my ideas on this subject in a former letter, I need not here repeat what I then said[1158].

iii. As we have subkingdoms, so we may also have _subclasses_, or such large divisions of a class--not founded upon internal organization or any of the primary vital functions, but upon different modes of taking their food, or such other _secondary_ characters--as include more than one _Order_. To this description Clairville's _Mandibulata_ and _Haustellata_ appear to me to belong, which I think are by no means entitled to the rank of Classes; for whoever compares these two tribes together will at the first glance be convinced, by the numerous characters they possess in common, notwithstanding the different mode in which they take their food, that they form one connected primary group. This circumstance, therefore, only furnishes a clue for their further subdivision into two secondary groups, separated by distinctions certainly of a lower value than those which separate the _Crustacea_ and _Arachnida_ from _Insecta_. This is further confirmed by the variations that take place in their mode of feeding in their different states; some from masticators becoming suctorious (_Lepidoptera_), and others from being suctorious becoming masticators (_Myrmeleon_, _Dytiscus_, &c.),--which shows that this character does not enter the essential idea of the animal.

iv. Next to Classes and Subclasses we are to consider those groups of insects that are denominated _Orders_. The characters of these at first were taken principally from the instruments of flight or the absence of them; and the name appropriated to each Order by Linné, after Aristotle, had reference to this circumstance. But this alone does not afford characters sufficiently discriminating: for though to an accurate observer a difference in these organs appears to be characteristic of most of the Orders, yet in some it is not easily detected or defined. In the _Neuroptera_ there are as many different types of wings as there are of tribes or suborders. So that it seems not possible so to construct the definition of every Order, as to take its character from the organs of flight alone. Linné was sensible of this, and was compelled to have recourse to subsidiary characters in the majority of his: his observation therefore with regard to _Genera_,--that the character does not give the genus, but the genus the character[1159],--applies equally to _Orders_; and the characters included in the definition of an Order, should be the result of a careful examination of its component groups.

On a former occasion I named to you the Orders into which it appeared to me the Class _Insecta_ might be divided[1160]; they were these. _Coleoptera_; _Strepsiptera_; _Dermaptera_; _Orthoptera_; _Hemiptera_; _Trichoptera_; _Lepidoptera_; _Neuroptera_; _Hymenoptera_; _Diptera_: _Aphaniptera_; _Aptera_. I then briefly explained them merely for the sake of illustration, and that you might know what description of insects were meant when these Orders were mentioned in my letters, without intending to affirm that I had arranged them in a natural series, or that all of them were perfectly natural. I shall now consider them separately, and conclude with giving my sentiments as to which should be placed first.

* _ORDERS in which the ordinary Trophi all occur, or the_
Mouth _is_ perfect[1161]. (_Mandibulata._)

1. COLEOPTERA[1162] (_Eleutherata_ F.). Aristotle may be called the founder of this Order, since he both named and defined it[1163]. Both his name and definition were adopted by Linné; and the former (with the exception of Fabricius and his school) by all succeeding Entomologists. To his definition _Wings in a sheath_[1164], other characters have been added; as the folding of the wings, and the straight suture by which the elytra are united[1165]. Aristotle's character, though to be found in the great majority of the Order, is not universal, since there are some beetles that have neither wings nor sheath, as the female glow-worm; and many that though they have the sheath have no wings, as _Meloe_, many _Carabi_, &c. To the transverse folding of the wings there are also exceptions; as in _Buprestis_, _Molorchus_, &c. The straight suture by which one elytrum exactly coincides with the other without lapping over, fails in _Meloe_: so that no one of these characters can be called universal in the Order; but as an exception or two does not invalidate a rule, and these are sufficiently universal for the purpose of pointing it out, they may be retained. Perhaps it will be an improvement to add the kind of the _metamorphosis_, which, as far as known, prevails universally.

DEF. _Metamorphosis_ incomplete[1166].

_Legs_ inosculating, posterior coxæ usually transverse.

_Elytra_ corneous, or coriaceous, without veins, united
by a straight suture, so as mostly to cover the wings
completely[1167].

_Wings_ longitudinally and transversely folded[1168]:
_neuration_ simple[1169].

2. STREPSIPTERA[1170] (_Rhiphiptera_ Latr.) The characters of this Order were first given in the _Linnean Transactions_, and it has been adopted by Latreille, who however, without sufficient reason, has changed the name originally imposed to _Rhiphiptera_[1171]. Rossi, who was the first that discovered an insect of this Order, concluded that because it was parasitic it must be _Hymenopterous_; and it is certainly more nearly related to that Order than to the _Diptera_, amongst which M. Lamarck has arranged it, and with which it has no character in common, except having two wings. This is one of those Orders, consisting of few genera and species, which, from their connecting two circles, Mr. MacLeay has called _osculant_, who places it between the _Hymenoptera_ and _Coleoptera_[1172].

DEF. _Metamorphosis_ subincomplete[1173]?

_Pseudelytra_ twisted, attached to the anterior leg[1174].

_Wings_ not covered by the elytra, longitudinally folded,
forming nearly the quadrant of a circle[1175]: _neuration_
simple.

_Anus_ styliferous[1176].

3. DERMAPTERA[1177] (_Ulonota_ F. _Orthoptera_ Oliv.). This is another osculant Order, evidently connecting the _Coleoptera_ with the _Orthoptera_. The elytra are of a coriaceous substance, have a straight suture, and are not veined, and the wings are folded longitudinally as well as transversely,--circumstances which connect it with the former Order,--while the shape of its wings, its oral organs, and its metamorphosis, show its affinity to the latter. It was established at the same time and in the same work with the preceding Order, in pursuance of a suggestion of Dr. Leach, and consists solely of the Linnean genus _Forficula_.

DEF. _Metamorphosis_ semicomplete.

_Elytra_ coriaceous, without veins, united by a straight
suture, so as partly to cover the wings.

_Wings_ longitudinally and transversely folded, each forming
nearly the quadrant of a circle: _neuration_ radiating[1178].

_Anus_ forcipate.

4. ORTHOPTERA[1179] (_Ulonota_ F.). This Order, which Linné at first regarded as belonging to the _Coleoptera_[1180], and afterwards improperly added to the suctorious _Hemiptera_, was very judiciously separated from both by De Geer, under the name of _Dermaptera_, a name not improper, and which ought to have been retained. Its present name was, I believe, assigned to it by Olivier; and as this is generally in use, I shall not attempt to disturb it. Dr. Leach divided the Order into two, separating the _Blattina_ from it, under the name of _Dictyoptera_[1181]. He was led to this by the tegmina decussating or lapping obliquely over each other, whereas in the rest the horizontal portion of one tegmen lies longitudinally over that of the other; he also probably took their depressed body into consideration;--these circumstances, however, rather indicate a _tribe_ or suborder; and as such Mr. MacLeay regards it.

DEF. _Metamorphosis_ semicomplete.

_Legs_ suspended.

_Tegmina_ generally pergameneous[1182], reticulated with
nervures, more or less incumbent, covering the wings.

_Wings_ longitudinally folded, ample: _neuration_ reticulated.

5. NEUROPTERA[1183] (_Synistata_, _Odonota_ F.). Of all the Linnean Orders this appears to consist of the most discordant tribes; so that it seems next to impossible to construct a definition that will include them all, unless indeed we admit M. Latreille's idea, adopted by Mr. MacLeay[1184], that a varied metamorphosis is its essential character; or, to speak more largely, variety itself seems the characteristic of the insects composing it, in every state; and there is scarcely a common distinctive character in their perfect state, upon detecting which in any individual you may exclaim--This is a Neuropterous insect. The only one that I have been enabled to seize is, that their _scapulæ_ and _parapleuræ_ are parallel and placed obliquely[1185]. Whether, with all this puzzling variation and dissonance between the different tribes of which it is now composed, this Order can be considered as a natural group, in the present state of our knowledge it would be rash to decide. I shall observe, however, that the _Libellulina_,--whether we regard their metamorphosis and the singular character before described that distinguishes their larva and pupa[1186], their oral instruments[1187], the remarkable position of their legs[1188], their general form, the wonderful and peculiar machinery by which their wings are moved[1189], and other circumstances of their internal anatomy,--if any are to be regarded as forming a separate Order, are the first entitled to that distinction. At present, with our friend Mr. MacLeay, I shall consider it as not further divisible, and as consisting of five principal forms. I must not omit to observe, that in the _Ephemerina_ the parts of the mouth, except the labrum and palpi, appear to be mere rudiments[1190].

DEF. _Metamorphosis_ varying. _Larva_ a hexapod.

_Wings_ four in most, and reticulated with numerous areolets.

_Prothorax_ distinct.

_Scapulæ_ and _Parapleuræ_ parallel and oblique.

_Tail_ of the female without a terebrant, or pungent
multivalve ovipositor[1191].

6. HYMENOPTERA[1192] (_Piezata_ F.). Mr. MacLeay considers _Sirex_ L. as being osculant between the Order we are now entering upon and the _Trichoptera_, and _Tenthredo_, L. as belonging to the latter. He appears to ground this opinion chiefly upon a consideration of their larvæ and a slight difference in their ovipositor. As the Order, as settled by Linné, has always been deemed one of the most natural ones, and all the great Entomologists of the present æra have agreed with him in thinking it so; it seems to me that to prove them mistaken in this opinion, the question should have been discussed at more length, and that it requires arguments of more weight than any Mr. MacLeay has at present produced to set it aside. He appears in general to lay great stress upon an agreement in larvæ and the kind of metamorphosis; and I am ready to acknowledge that it forms a strong _presumption_ in favour of any hypothesis of affinity between certain tribes. But when it is had recourse to as fundamental and infallible, I think it is pushed far beyond what it will bear, or is warrantable. I may be wrong; but in my apprehension, a striking agreement in their general structure in the _perfect_ state, which is the acme of their nature, affords a much more satisfactory reason for keeping two tribes together, than any difference observable in their larvæ or metamorphosis, for separating them. Let any one compare the structure of these two tribes with the _Trichoptera_ on one side, and the _Hymenoptera_ on the other, and it will require but a glance to convince him of their greater affinity to the latter; and the simple inspection only of Jurine's plates of the wings of _Hymenoptera_ is calculated to produce the same effect. With regard to their _larvæ_, the resemblance between the case-worms and the pseudo-caterpillars of the saw-flies seems to me very distant, and the numerous prolegs of the latter have scarcely a legitimate representative in the former. The larvæ of the genus _Lyda_ lose the prolegs intirely, and in one species, which much resembles the vermiform larvæ of _Hymenoptera_, the real legs are so extremely short as to be scarcely discernible[1193]; so that it requires no great stretch of faith to believe that saw-flies or _Sirices_ may exist in whose larvæ the legs disappear[1194]. But it is this very tribe, whose larvæ thus approach to those of the other _Hymenoptera_, in which Mr. MacLeay finds the greatest external resemblance to the _Trichoptera_[1195]. In fact the difference between the saw-flies and _Siricidæ_, and the remainder of the _Hymenoptera_, amounts to little more than what takes place in the _Diptera_ Order between the _Tipulidæ_, _Asilidæ_, _Muscidæ_, &c., in which also the _metamorphosis_ differs.

Another argument upon which Mr. MacLeay seems to lay some stress, is taken from the number of parts into which the _ovipositor_ of the saw-flies is resolvable, which he finds to consist of _four_ pieces; while in what he considers as the genuine _Hymenoptera_, it is formed only of _three_[1196]: but in fact, in these last there are _two_ spiculæ, answering to the two saws of _Tenthredo_, so that the vagina in which these move may be considered as a _double_ sheath: only, as these were to be pushed out at the _same_ time, and the others _alternately_, it was necessary that in the latter each sheath should be separate, to admit of this motion; but as to its composition, the weapon in both is essentially the same. At any rate this structure could furnish a reason only for the formation of a separate group in the _same_ Order, but none for the transfer of such group to _another_, which had no such instrument at all; since, as we have seen, the _Trichoptera_ extrude their eggs at once in a mass[1197]. I do not mean, however, that it should be inferred from what I have here said, that there is no _tendency_ in the saw-flies towards a Trichopterous type, for in them nature seems pointing that way, but the distance is too great, and the number of types of form necessary to fill up the interval too many, to warrant in my opinion their removal from the one Order to the other.

DEF. _Metamorphosis_ incomplete[1198].

_Trophi_ in most not used for mastication[1199].

_Wings_ four: _neuration_ generally areolate[1200].

_Prothorax_ obsolete, giving place to an ample collar.

_Tarsi_ pentamerous.

_Ovipositor_ 5-6-valved, the vagina darting forth two
retroserrulate spiculæ.

** _ORDERS in which all the ordinary Trophi do_ not _occur,
or the_ Mouth _is_ imperfect[1201] (_Haustellata_).

7. HEMIPTERA[1202] (_Ryngota_ F.). Linné at first confined this Order to those insects which have a _promuscis_, which he denominated a _rostrum_[1203]; but afterwards, convinced that the _Orthoptera_ of the moderns could not be associated properly with the _Coleoptera_; instead of forming them into a distinct Order, as nature would have dictated--perhaps to avoid the multiplication of Orders and without altering his definition--with equal infelicity he added them to this. Subsequent Entomologists, who saw the impropriety of masticating insects thus herding with suctorious ones, restricted the Order to its old limits; but Latreille very judiciously altered its arrangement, and divided it into two Sections, separating those whose hemelytra terminate in membrane, from those in which they are mostly tegmina, or of a substance intermediate between that of the elytra of _Coleoptera_ and that of the wings of the Tetrapterous Orders. He denominated the first of these sections, or rather suborders, _Heteroptera_, and the last _Homoptera_[1204]. Dr. Leach, observing that very considerable differences take place both in the economy and structure of Heteropterous and Homopterous insects, followed De Geer in considering them as separate Orders, which he has called _Hemiptera_ and _Omoptera_, and in which he has been followed by Mr. MacLeay; who, however, with his usual accuracy and judgment, has restored the aspirate to the latter name[1205]. Their agreement in having a _promuscis_, or instrument of suction, with a jointed sheath, at present induces me to hesitate as to the propriety of their separation, and to consider them as forming _secondary_ rather than _primary_ sections of the Class. That you may be enabled to judge for yourself upon this subject, I will state the principal features in which they differ. In the first place, the Heteropterous section usually sucks the juices of _animals_, and the Homopterous, those of _plants_; in the former, the _Hemelytra_, besides their different substance, as well as the wings, cross each other; while in the latter, the organs of flight are deflexed, and do not lap over each other at all. The antennæ also of the one are often long, and do not terminate in a _bristle_; while in the other, with few exceptions, they are very short and setigerous. In the _Heteroptera_ the body is depressed and flat, in the _Homoptera_ convex and thick. In the former, the scutellum is one of the principal features of the trunk; in the latter, not at all remarkable[1206]. Other differences in the structure, both of head, trunk, and abdomen, might be pointed out; but these you will chiefly find noticed in my letters on the External Anatomy of Insects, where I treated of those parts. I shall here, therefore, only further mention the ovipositor also as forming a most striking distinction[1207].

DEF. _Metamorphosis_ semicomplete in almost all.

_Mouth_ promuscidate[1208].

_Wings_ covered by _Hemelytra_ or _Tegmina_[1209].

_Tarsi_ mostly trimerous, rarely dimerous or monomerous[1210].

8. TRICHOPTERA[1211] Kirby (_Synistata_ F. _Neuroptera_ Latr.). MM. Latreille and MacLeay are of opinion that _Semblis_ F. and _Phryganea_ L. ought to be associated in the _same_ group; and the latter gentleman has backed his opinion by some apparently cogent arguments[1212]: there are others, however, that seem to me more cogent, for considering them as belonging to _different_ Orders. Whoever examines the several tribes into which Mr. MacLeay has divided the _Neuroptera_, will observe in all of them a distinct _prothorax_, a circumstance which they possess in common with those Orders that use their mandibles for _mastication_; whereas in those that do _not_ use them for mastication, as the _Hymenoptera_, or that take their food by suction, this part is replaced by a mostly narrow collar, forming a part of the alitrunk[1213]. The existence then of the _prothorax_ in the _Perlidæ_, and of the _collar_ in the _Trichoptera_, affords no slight presumptive evidence that they belong to different Orders. Another circumstance that weighs much with me is, that the type of the neuration of the wings in _Perla_ is taken from the _Neuroptera_, in the _Trichoptera_ from the _Lepidoptera_; the same observation extends to the legs of both[1214], and likewise to the abdomen. Even in their oral organs, as far at least as relates to their mandibles, those of _Perla_, though membranaceous--a circumstance occurring even in _Coleoptera_--are of a Neuropterous type; while the angular termination of the cheeks in the _Phryganeæ_ approaches to the Lepidopterous mandibular rudiments. The principal argument on which Mr. MacLeay's opinion seems to rest, is, that the larvæ of both are aquatic, and clothe themselves in cases formed of various materials: but though this circumstance shows that they approximate in the system, it does not prove that they belong to the same order, since the general habit and appearance of the two animals when arrived at perfection contravenes it. The larvæ of _Myrmeleon_ and of _Leptis Vermileo_ form pitfalls of sand for their prey, and when they become pupæ, cover themselves with it[1215]; but this in them does not even prove an affinity, but only an analogy. The larva of _Perla_ is carnivorous[1216], that of _Phryganea_ mostly herbivorous[1217]: so that they are not precisely similar in their habits. Whether they resemble each other _altogether_, in their form, does not clearly appear. The above reasons will, I trust, justify me for considering them _at present_ as belonging to different Orders; but if further discoveries should confirm the opinion Mr. MacLeay espouses, I shall have no hesitation in yielding to it.

DEF. _Metamorphosis_ incomplete[1218].

_Mouth_ emandibulate.

_Prothorax_ replaced by a collar.

_Wings_ four, upper pair mostly hairy, lower ample, folded:
_neuration_ branching.

_Anus_ without setæ. _Eggs_ extruded in a gelatinous
mass[1219].

9. LEPIDOPTERA[1220] (_Glossata_ F.). Concerning this Order, no difference of opinion exists amongst Entomologists. Besides the scales that cover their wings, they are distinguished by the peculiar instrument of suction formerly described: neither of these characters, however, is perfectly universal; some of the Order (_Nudaria_) having no scales upon their wings, and others being without any _antlia_ (_Aglossa_). Other peculiar characters are to be found in them; for instance, the _patagia_, or tippets, that adorn their evanescent thorax[1221], and the _tegulæ_, or base-covers, of a shape quite dissimilar to those of _Hymenoptera_, which cover and defend the base of their wings[1222]. As in the last Order, their legs are located all together with scarcely any space intervening between them; and they often agree also in their spurs.

DEF. _Metamorphosis_ obtected[1223].

_Mouth_ antliate[1224].

_Prothorax_ very short, covered by a pair of tippets.

_Wings_ four, covered partially or generally with minute
scales: _neuration_ branching, often with a central areolet.

10. DIPTERA[1225] (_Antliata_ F.). This Order likewise appears indebted for its name to the philosopher of Stagyra, who distinguishes the members of it from their counter-parts--the _Hymenoptera_--by their having an _oral_, while these have an _anal_ sting[1226]: and we may add, that while the last, on account of their wonderful economy and the benefits which by them PROVIDENCE confers upon mankind, have been justly regarded as the _princes_ of the winged insect world,--the former, when we consider the filthy and disgusting habits of their grubs, and the annoyance, both from their numbers and incessant assaults, of them, in their fly-state, may very properly be considered as its _canaille_. Almost all the tribes of _Hymenoptera_, from the saw-flies to the ants, have their representatives in this Order. Though the number of wings is its prominent feature, yet there are two-winged insects in other Orders, as some _Ephemeræ_: and the _Eproboscidea_ of Latreille seem rather a kind of winged _Aptera_, if we consider their _trophi_, than real _Diptera_; or they may form an osculant group, partly winged and partly apterous, between the two. I have before remarked, that though, apparently, the insects of this Order have only _two_ wings, yet the under or secondary wings of the other Orders have in them their representative[1227]. Their poisers also, I formerly observed to you, are probably more connected with their respiration than with their flight[1228].

DEF. _Metamorphosis_ incomplete, or coarctate.

_Mouth_ proboscidate[1229].

_Prothorax_ replaced by a collar. _Sutures_ of the trunk
mostly spurious[1230].

_Wings_ two, with winglets attached to them: _neuration_
various[1231]. _Poisers._

_Tarsi_ pentamerous.

_Ovipositor_ various[1232].

11. APHANIPTERA[1233] (_Aptera_ L. Lamarck. _Rhyngota_ F. _Suctoria_ Latr.) This is an osculant Order, and is distinguished from the other _Aptera_ L. in undergoing a regular metamorphosis. The larva is vermiform, the pupa incomplete, and inclosed in a cocoon. Probably the common flea and the chigoe would form distinct genera. The number of species of fleas is greater than has been supposed. I have been informed that Dr. Leach is acquainted with fourteen British species alone. Besides their metamorphosis, they are distinguished from the _Aptera_ by the number of segments into which their body is divided, and by their pentamerous tarsi. Something like elytra and a scutellum appear to distinguish these insects.

DEF. _Metamorphosis_ incomplete.

_Body_ apterous, compressed.

_Mouth_ rostrulate[1234].

_Tarsi_ pentamerous.

We are now come to those insects which, though they change their skin in their progress to their state of perfection, and some of them, as we have seen[1235], gain additional segments and pairs of legs, yet none of them acquire wings or wing-cases: these I have considered as forming one Order, under the denomination of

12. APTERA[1236] (_Synistata_, _Antliata_, _Unogata_, _Mitosata_ F.). I do not give this as a _natural_ Order. Our knowledge, however, of the internal organization of its groups, is not at present sufficiently matured to warrant the formation of them into new _Classes_[1237]: till that is more fully ascertained, it seems to me therefore best to consider these groups as forming three _Suborders_: the _first_ consisting of the _Hexapods_; the _second_ of the _Octopods_; and the _third_ of the _Polypods_. It will be better, I think, instead of giving a general character of the Order,--which principally consists in the insects composing it being _Apterous_, or never acquiring organs of flight,--to define each of these groups.

_Hexapods_ (_Ametabolia_ Leach, _Ametabola_ M^cL.). _Six_ legs may be regarded as the natural number in _all_ the insect tribes[1238]: but our business now is with those _Aptera_ whose body consists of _three_ greater segments, and which in none of their states have ever more or less than _six_ legs, and consist of the three Linnean genera _Pediculus_, _Lepisma_, and _Podura_ (_Thysanura_ and _Anoplura_). Some of the mites (_Acarus_ L.) are hexapods, but their body has no distinction of head, trunk, and abdomen. The metamorphosis of most female _Blattæ_, and of some other _Orthoptera_ that are apterous, cannot be regarded as materially different from that of the Hexapods. Amongst the _Anoplura_,--the _Pediculi_, or lice, are suctorious, and the _Nirmi_, or bird-lice, masticators,--a circumstance which in them does not appear to indicate even a different Order, and proves that undue stress ought not to be laid, independently of general characters, on the mode in which insects take their food.

DEF. _Metamorphosis_ complete.

_Body_ consisting of three principal segments.

_Mouth_ perfect, or rostellate[1239].

_Antennæ_ distinct.

_Legs_ six, in every state.

_Octopods._ This suborder consists of the _Trachean Arachnida_ of Latreille, excluding the _Pycnogonida_; of the _Acaridea_, _Sironidea_, _Phalangidea_, and part of the _Scorpionidea_ of Mr. MacLeay, and, with some exceptions, of the Linnean genera _Acarus_ and _Phalangium_. This last tribe (for with Linné, I include _Chelifer_ and _Obsidium_ in the _Phalangidea_,) on one side approaches _Scorpio_ by _Thelyphonus_, and on the other the _Aranidea_ by _Gonyleptes_; or, according to Mr. MacLeay, the transit is to both by _Galeodes_[1240]. But as there is reason for thinking that this last belongs to the _Pulmonary Arachnida_[1241], and forms a peculiar type in that Class, I consider the transit from the one to the other as above stated. The folded abdomen of _Gonyleptes_ seems much to correspond with that of the _cancriform_ spiders (_Carkinodes cancriformis_, &c.).

DEF. _Metamorphosis_ complete.

_Body_ consisting of one or two principal segments.

_Mouth_ various[1242].

_Antennæ_ obsolete, or represented by mandibles.

_Legs_ mostly eight, but in a few six only[1243].

_Polypods._ This suborder consists of Dr. Leach's Class _Myriapoda_, or the _Chilognatha_ and _Chilopoda_ of Latreille, corresponding with the Linnean genera _Iulus_ and _Scolopendra_. Mr. MacLeay has arranged them in the same Class with the Hexapods, and connects them with the _Anoplura_ by means of certain intestinal worms of an indistinct annulose structure[1244] (_Entozoa Nematoidea_ Rud.), in which the sexes are diœcious, and some of which are furnished with lateral spinulæ,--thus, as he supposes, connected with the Polypods; and with the _Anoplura_ by others (_Epizoaria_ Lam.) in which appendages appear somewhat analogous to the legs of Hexapods, as in _Cecrops_ Leach, and which like them are parasitic animals[1245]. But the right of these worms to be considered as members of the same Class with the Hexapods and Polypods at present appears rather problematical, and requires further examination.

DEF. _Metamorphosis_ subcomplete[1246].

_Body_ consisting of numerous segments.

_Mouth_ perfect[1247].

_Eyes_ compound or aggregate.

_Antennæ_ distinct.

_Legs_ six on the trunk, many on the abdomen.

I must next say something on the Orders of the _Arachnida_. Every one, at first sight, sees that _spiders_ and _scorpions_ are separated by characters so strongly marked, that they look rather like animals belonging to different Classes than to the same: these form the two _primary_ Orders of the _Arachnida_, and they appear to be connected by two _secondary_ or osculant ones,--on the one side by _Galeodes_, and on the other by _Thelyphonus_ and _Phrynus_[1248]. This Class, although there is an appearance of eight legs, is, strictly speaking, of a _Hexapod_ type; for the anterior pair, ordinarily regarded as legs and performing their function, are really the analogues of the maxillary palpi of perfect insects. This will be evident to you if you examine any species of _Galeodes_. These animals, if we look at them cursorily, we should regard as _Decapods_; but when we trace the two anterior pairs of apparent legs to their insertion, we find that both proceed from the _head_, which in that genus is distinct from the trunk; while the three last pairs, which alone are furnished with claws, are planted, as legs usually are, in the latter part. The first pair represent the ordinary palpi of _Arachnida_, are analogous to the labial ones of Hexapods, and, as likewise in _Phrynus_ and _Thelyphonus_, are more robust than what are usually taken for the first pair of legs; but they differ in being considerably longer, and instead of terminating in a _chela_ are furnished with a retractile sucker[1249]. The second pair are more slender and shorter than the first; they correspond precisely with what are deemed the first pair of legs of _Octopods_ and _Arachnida_, and are clearly analogous to the maxillary palpi of perfect insects. Whether the base of the first pair of these palpi is in any respect analogous to the labium of insects, (as that of the second seems to be to their maxillæ,) I am not prepared to assert: it will therefore be most advisable to name these palpi _anterior_ and _posterior_: but as they evidently proceed from the _head_ in _Galeodes_, and in that genus are clearly analogous to those of the _Phrynidea_, (which in their turn as clearly represent those of the _Aranidea_,) it follows that in all they are organs of the part representing the _head_, and therefore not in a _primary_ sense _legs_; although in a _secondary_, as M. Savigny has proved, they may be so called[1250].

1. ARANEIDEA M^cL. (_Aranea_ L., _Araneidæ_ Latr.) The _Araneidea_, or spiders, seem resolvable into _two_ suborders,--the _Sedentaries_ and the _Wanderers_; thus forming, perhaps, what Mr. MacLeay would denominate the normal groups of a circle of _Arachnida_.

DEF. _Mandibles_ armed with a perforated claw.

_Head_ and _Trunk_ coalite.

_Palpi_ pediform, anterior pair without claws.

_Abdomen_ without segments or elongated tail.

_Spiracles_ two[1251].

_Anus_ furnished with an apparatus for spinning[1252].

2. SCORPIONIDEA M^cL. (_Scorpio_ L. Latr.)

DEF. _Mandibles_ chelate.

_Head_ and _Trunk_ coalite.

_Anterior Palpi_ chelate[1253].

_Posterior Palpi_ pediform.

_Pectens_ two[1254].

_Abdomen_ divided into segments and terminating in a jointed
tail, armed at the end with a sting[1255].

_Spiracles_ four pairs.

3. GALEODEA.

DEF. _Head_ distinct[1256].

_Eyes_ two.

_Mandibles_ chelate with dentated chelæ.

_Palpi_ pediform, the anterior pair thickest with a
retractile sucker.

_Trunk_ consisting of two principal segments, with a minute
supplementary posterior one[1257].

_Spiracles_ two placed in the trunk[1258].

_Pseudo-pectens_ two[1259].

_Abdomen_ divided into segments.

_Anus_ unarmed and without a spinning apparatus[1260].

4. PHRYNIDEA.

DEF. _Mandibles_ unguiculate.

_Anterior Palpi_ chelate or unguiculate[1261], very robust.

_Posterior Palpi_ pediform, very long and slender.

_Abdomen_ divided into segments.

_Spiracles_ two pairs.

_Anus_ terminating in a mucro, and sometimes in a filiform
jointed tail without a sting at the end.

v. Having considered the _Orders_ into which _Insecta_ and _Arachnida_ may be divided, I am next to give you some account of the _groups_ into which each is further resolvable. To draw out, however, a complete scheme of these would be deviating from my province, and extend this letter to an enormous length. Indeed, to give the _natural_ primary and subordinate sections of every Order, would require a knowledge of the subject to which no Entomologist has yet attained. I shall therefore only say something general upon them, and refer you to an example of each kind of group.

Previously to the groups themselves their _nomenclature_ claims our attention. M. Latreille in his last arrangement of _Annulose_ animals has divided his Orders into _Sections_; _Families_; _Tribes_; and _Genera_: his tribes he has often further subdivided into lesser sections, represented by capital and small letters, &c.[1262]. Mr. MacLeay, discarding the term section, has _Tribes_; _Races_ (_Stirps_); _Families_; _Genera_, and _Subgenera_[1263]. But as in descending from the _Order_ to the lowest term, or the _species_, a series of groups gradually diminishing in value, which require a greater number of denominations than have yet been employed by Entomologists, often occur, I think we may with benefit to the science add to the list. I would therefore propose the following primary and subordinate divisions of an Order: 1. _Suborder_; 2. _Section_; 3. _Subsection_; 4. _Tribe_; 5. _Subtribe_; 6. _Stirps_; 7. _Family_; 8. _Genus_; 9. _Subgenus_. I would further propose that each of these successive groups should have a name always terminating alike, so that the value of the group when spoken of might always be known by the termination:--thus if a subclass end in _ata_, a suborder might end in _ita_; a section in _ana_, a subsection in _ena_; a tribe in _ina_, a subtribe in _ona_; a stirps in _una_; and a family in _idæ_; the genera being left free.

With regard to their _characters_, we are not to place our groups upon Procrustes' bed, and lop or torture them to accommodate them to every standard we may have fixed for them: assuming one set of characters for suborders, another for tribes, and so for every other group; for the value of characters varies,--those that in some cases are common to an _Order_, in others indicate only _sections_, or _tribes_, or _genera_ and _species_, or sometimes even _sexes_. What is constant in one group is not so in another, and _vice versâ_; so that it is a vain labour to search for a _universal_ character. If it is our wish really to trace the labyrinth of nature, we can only accomplish it by a careful perusal and examination of her various groups. It is singular how much and how far various Entomologists, and those of the very highest class, have been misled by a kind of _favouritism_ to give too universal a currency to certain characters for which they have conceived a predilection. Some have been the champions of the _antennæ_; others of the _trophi_; others again of the _wings_; and others of the _metamorphosis_. These are all characters which within certain limits lead us right, and are an index to a natural group; but if we follow them further, we leave the system of nature, and are perplexed in the mazes of a _method_[1264].

Let us now see whether we can pitch upon any suborder which will afford an example of every group that we have lately named. Mr. MacLeay, from a consideration of the larvæ of that Order, has divided the _Coleoptera_ into five primary groups that may be denominated _Suborders_. Whether these are all natural groups has not yet been made sufficiently evident. It answers my present purpose, however, to assume it as proved. I select therefore his _Chilopodimorpha_ for my suborder, altering the name as above proposed to _Chilopodimorph_it_a_: for my _Section_ I take the Predaceous beetles, or _Adephaga_ of M. Clairville, distinguished by having the upper lobe of their maxillæ biarticulate and palpiform;--these I would denominate _Adephag_an_a_, or devourers. They consist of two groups forming two subsections, the one _terrestrial_ and the other _aquatic_; which I would name, following Mr. MacLeay, _Geodephag_en_a_ and _Hydrodephag_en_a_. These two subsections are each resolvable into two _Tribes_ constituted by Linné's four genera _Cicindela_ and _Carabus_; _Dytiscus_ and _Gyrinus_. The first tribe, remarkable for the swiftness of their _flight_, I would name _Eupter_in_a_, or fliers; the second, equally noted for _running_, _Eutrech_in_a_, or runners; the third _Eunech_in_a_, or swimmers; and the fourth _Gyronech_in_a_, or swimmers in a circle. The second of these groups, the _Eutrech_in_a_, are resolvable into two other groups or _Subtribes_; one distinguished by having the cubit or anterior tibia _notched_, (which, from their being in general not very brilliant in colour, I would call _Amaur_on_a_, or obscure); the other having the cubit without a notch, (which, from the brilliancy of many of them, I would name _Lampr_on_a_, or splendid). These subtribes are both further resolvable into two or more _races_ (_Stirpes_). I select that to which the _crepitant_ _Eutrech_in_a_ belong, containing those which from their usually truncated elytra MM. Latreille and Dejean have named _Truncatipennes_[1265]: these, to shorten the name, I call _Truncipenn_un_a_. This brings us down to the lowest group formed out of genera and subgenera: or the _family_, which from its principal genus is named _Brachinidæ_, and which leads us to the _genus Brachinus_, and the _subgenus Aptini_. Thus we get the following scale, expressing every division of an Order, till we arrive at its lowest term, or the _species_ that compose it.

SUBORDER

_Chilopodimorph_it_a_ M^cL.

SECTION

_Adephag_an_a_ Clairv.

SUBSECTION

_Geodephag_en_a_ M^cL.

TRIBE

_Eutrech_in_a_

SUBTRIBE

_Amaur_on_a_

STIRPS

_Truncipenn_un_a_ Latr.

FAMILY

_Brachinidæ_

GENUS

_Brachinus_

SUBGENUS

_Aptini_.

In the construction of this scale I have endeavoured to steer clear of being led by any system, but, with the exception of the _Suborder_, which I assume, to resolve it into natural groups gradually decreasing in value, or tending to the lowest term, which appear all of them to have been considered as such by preceding Entomologists. The four _Tribes_ into which the two subsections _Geodephag_en_a_ and _Hydradephag_en_a_ appear resolvable, are not only distinguished by the characters of the perfect insect, but likewise by those of their larvæ, which are constructed on four distinct types; those of the _Gyronech_in_a_ being the most perfectly Chilopodimorphous of the whole, and those of the _Eunech_in_a_ the least so[1266]. The former appear rather to form an osculant tribe, or one without the circle, than one within it; and to be going off towards another section, including _Hydrophilus_, _Sphæridium_, &c. I must observe, that between _Dytiscus_ and _Hydrophilus_ there is a striking agreement both in their form and habits in the larvæ[1266], and even in several characters in the perfect insect; so as in many respects to generate a doubt whether they ought not to enter the same circle and to follow each other. Yet the change of habits in the latter, which from a carnivorous larva becomes a herbivorous beetle; the consequent change of structure in their oral organs, their antennæ, and other striking differences; and the evident intervention of the _Gyronech_in_a_ and some other osculant tribes between the two, forbid their union in one and the same circle.