Chapter II: Letter XXVIII: Definition of the Term Insect (1)

What is an insect? This may seem a strange question after such copious details as have been given in my former Letters of their history and economy, in which it appears to have been taken for granted that you can _answer_ this question. Yet in the scientific road which you are now about to enter, to be able to define these creatures technically is an important first step which calls for attention. You know already that a butterfly is an insect--that a fly, a beetle, a grasshopper, a bug, a bee, a louse, and flea, are insects--that a spider also and centipede go under that name; and this knowledge, which every child likewise possesses, was sufficient for comprehending the subjects upon which I have hitherto written. But now that we are about to take a nearer view of them--to investigate their anatomical and physiological characters more closely--these vague and popular ideas are insufficient. In common language, not only the tribes above mentioned, but most _small_ animals--as worms, slugs, leeches, and many similar creatures, are known by the name of _insects_. Such latitude, however, cannot be admitted in a scientific view of the subject, in which the class of insects is distinguished from these animals just as strictly as beasts from birds, and birds from reptiles and amphibia, and these again from fishes. Not, indeed, that the just limits of the class have always been clearly understood and marked out. Even when our correspondence first commenced, animals were regarded as belonging to it, which since their internal organization has been more fully explained, are properly separated from it. But it is now agreed on all hands, that an earthworm, a leech, or a slug, is not an insect; and a Naturalist seems almost as much inclined to smile at those who confound them, as Captain Cook at the islanders who confessed their entire ignorance of the nature of cows and horses, but gave him to understand that they knew his sheep and goats to be birds.

You will better comprehend the subsequent definition of the term _Insect_, after attending to a slight sketch of the chief classifications of the animal kingdom, more especially of the creatures in question, that have been proposed. That of Aristotle stands first. He divides animals into two grand sections, corresponding with the _Vertebrata_ and _Invertebrata_ of modern Zoologists: those, namely, that have blood, and those that have it not[1]:--by this it appears that he only regarded _red_ blood as real blood; and probably did not suspect that there was a true circulation in his _Mollusca_ and other white-blooded animals. His _Enaima_, or animals that have blood, he divides into _Quadrupeds_, _Birds_, _Fishes_, _Cetacea_, and _Apods_ or reptiles; though he includes the latter, where they have four legs, amongst the quadrupeds[2]; and his _Anaima_, or animals without blood, into _Malachia_, _Malacostraca_, _Ostracoderma_, and _Entoma_. The first of these, the _Malachia_, he defines as animals that are externally fleshy and internally solid, like the _Enaima_; and he gives the _Sepia_ as the type of this class, which answers to the _Cephalopoda_ of the moderns. The next, the _Malacostraca_, synonymous with the _Crustacea_ of Cuvier and Lamarck, are those, he says, which have their solid part without and the fleshy within, and whose shell will not break, but splits, upon collision[3]. The _Ostracoderma_, corresponding with the _Testacea_ of Linné, he also defines as having their fleshy substance within, and the solid without; but whose shell, as to its fracture, reverses the character of the _Malacostraca_. He defines his last class _Entoma_, in Latin _Insecta_, with which we are principally concerned, as animals whose body is distinguished by _incisures_, either on its upper or under side, or on both, and has no solid or fleshy substance separate, but something intermediate, their body being equally hard both within and without[4]. This definition would include the _Annelida_ and most other _Vermes_ of Linné, except the _Testacea_, which accordingly were considered as insects by those Zoologists that intervened between Aristotle and the latter author. The Stagyrite, however, in another place, has expressly excluded all _apods_[5]. From other passages in his works, it appears that he regarded the _Vermes_, &c. either as _larvæ_, or as produced spontaneously and not _ex ovo_[6].

This definition of an insect, though partly founded on misconception, as well as his primary division of animals in general, is by no means contemptible. If you look at a bee or a fly, you will observe at first sight that its body is _insected_, being divided as it were into three principal pieces--head, trunk, and abdomen[7]; and if you examine it more narrowly, you will find that the two last of these parts, especially the abdomen, are further subdivided. And this character of _insection_, or division into segments, more or less present in almost every insect[8], is not to be found (with the exception of the _Crustacea_, which Aristotle distinguishes by the nature of their integument and its contents) in any of the other classes into which he divided animals without blood. It was on account of this most obvious of their characters, that these little creatures were in Greek named _Entoma_, and in Latin _Insecta_; and from the former word, as you know, our favourite science takes the name of _Entomology_.

Pliny adhering to the definition of Aristotle, as far as it relates to the _insection_ of the animals we are speaking of, expressly includes _Apods_, as well as _Aptera_, amongst them[9]; and in this was followed, without any attempt at improvement, by all the entomological writers that intervened between him and the great Aristotle of the moderns, Linné.

This illustrious naturalist, aware of the incorrectness of the primary divisions of the animal kingdom founded upon the presence or absence of blood, establishes his system upon the structure of the heart, and upon the temperature and colour of the circulating fluid. He divided animals into two great _sections_ or _sub-kingdoms_, each comprising two _classes_. His _first_ section included those having a heart with _two_ ventricles, _two_ auricles, and _warm_ and red blood, viz. the _Mammalia_ or beasts, and the _Aves_ or birds. His _second_, those having a heart with _one_ ventricle, _one_ auricle, and _cold_ and red blood, namely, the classes _Amphibia_, which included reptiles, serpents, &c. and _Pisces_ or fish. His _third_, those having a heart with _one_ ventricle and _no_ auricle, and _cold white_ sanies in the place of blood, namely, his classes _Insecta_ et _Vermes_, including the _Invertebrate_ animals of Lamarck. Thus the first of Aristotle's great divisions he increased by the addition of a new and very distinct class, the _Amphibia_, by which some ground was gained in the science; but as much was lost by his compressing the four classes of which the last consisted into two, by which the natural classes of _Cephalopoda_ and _Crustacea_ merged under _Insecta_ and _Vermes_. Linné was not aware of the extraordinary fact, that the _Cephalopoda_ have three hearts; and that though the _Crustacea_ and _Arachnida_ have a circulation, _Insects_ have none, or he would never have taken this retrograde step.

Indeed Linné's definition of an _Insect_ is, in many most material points, inapplicable, not only to the _Crustacea_, but to many other animals included under that denomination. This will appear evident from a very slight examination. Thus it runs: "_Polypod animalcula, breathing by lateral spiracles, armed every where with an osseous skin, whose head is furnished with movable sensitive antennæ_[10]." Now of this definition only the first member can be applied to the whole class which it is meant to designate; for the entire genus _Cancer_ L., which, with some others, forms the class _Crustacea_ of the moderns, does not respire by _spiracles_ at all, but by _gills_; and the same in some degree may be said of spiders, scorpions, &c. With the last member of the definition Linné himself must have been aware that a large number of what he conceived to be insects were at variance, as mites, spiders, and many other of his apterous tribes: though from some very recent observations of M. Latreille[11], there seems some ground for thinking, that in these the antennæ are represented by the mandibles, palpi, &c.[12], and to the soft flexible, coriaceous or membranous skin of a vast number of insects, the term _cutis ossea_ is by no means applicable.

Evident as these incongruities are, when the Herculean task which Linné imposed upon himself, and the vastness and variety of his labours, are considered, they become very venial. Indeed, unless he had divided his class _Insecta_ into two or more, it was impossible to define it intelligibly to ordinary readers, otherwise than nearly in the terms which he actually employed; and these characters, restricted and amended by qualifying clauses, are still those to which recurrence must be had in a popular definition of the class, when separated as it ought to be from the _Crustacea_ and _Arachnida_.

Pennant, Brisson, and other zoologists, who, attending to _nature_ rather than _system_, saw the impropriety of uniting a crab or a lobster in the same class with a bee or a beetle, long since assigned the _Crustacea_ their ancient distinct rank. "But these changes," as Latreille observes[13], "being only founded upon _external_ characters, might be deemed arbitrary; and to fix our opinion, it was necessary to have recourse to a decisive authority--the _internal_ and comparative organization of these animals." It results from the observations of the most profound comparative anatomist of our age, M. Cuvier, that the _Crustacea_ and _Arachnida_ differ from insects properly so called, and particularly from those that are furnished with wings, in having a complete system of circulation, a different mode of respiration, and that they have a more perfect organization. Influenced by these motives, both Cuvier and Lamarck have considered them as forming two classes separate from insects. Treviranus, led by considerations founded on the organs of circulation, of respiration, and of generation, is of opinion that spiders and scorpions ought to form one class with the _Crustacea_: he observes, however, that the nervous system of all three is very dissimilar; and that in an arrangement founded on this circumstance, the organs of motion, and the external shape, even spiders and scorpions must be placed in different classes[14].

It is to be observed with regard to the _Arachnida_ of the French school, that the class as laid down by them includes several animals that have no circulation, and breathe by _tracheæ_, of which description are the mites (_Acarus_ L.), and the harvest-men (_Phalangium_ L.) &c.; and therefore it has been divided into two orders, _Pulmonaria_ and _Tracheana_; but if the definition from the internal organization be adhered to, the latter should either remain with the class _Insecta_, or form a new one by themselves. Yet the animals that compose the _Trachean_ order of _Arachnida_, their external form considered, are certainly much more nearly related to the spiders and scorpions than to any members of the class _Insecta_ at present known. This circumstance, perhaps, may seem to throw some doubt upon the modern system of classification.

I must further observe, that the assertion of Treviranus, which appears to intimate that the respiration of the _pulmonary Arachnida_ is the same with that of the _Crustacea_, is not quite correct, since in the latter the _branchiæ_ or gills are _external_, and in the former _internal_, the air entering by spiracles before it acts upon them[15].

It may not be amiss in this place to lay before you the principal points in which the _Crustacea_ and _Arachnida_ agree with _Insecta_, and also those in which they differ.

The _Crustacea_ agree with _Insecta_ in having a body divided into segments, furnished with jointed legs, compound eyes, and antennæ. Their nervous system also is not materially different, and they are both oviparous. They differ from them in having the greater insections of the body less strongly marked; in the greater number of legs on the trunk, the anterior ones performing the office of maxillæ: in their eyes usually on a moveable footstalk: their palpigerous mandibles; and their four antennæ at least in the great majority. But the principal difference consists in the internal organization and the fountains of vitality; for the _Crustacea_ have a double circulation, the fountain of which is a heart in the middle of their thorax[16]. They have too a kind of gizzard and liver, at least the _Decapods_[17], and their respiration is by gills. Genuine insects terminate their existence after they have laid their eggs[18]; but the _Crustacea_ live longer, and lay more than once.

The _Arachnida_ will be found to differ from insects more widely than even the _Crustacea_. They agree in their jointed legs and palpi; immoveable eyes; and in being covered with a coriaceous or corneous integument: but they differ in having a system of circulation; gills instead of tracheæ; their organs of generation double; and the females lay more than once in their lives. Their head also is not distinct from the trunk as in insects; they have no compound eyes; and their antennæ, if we admit the opinion on this head of MM. Latreille and Treviranus, that they have representatives of these organs, differ totally in structure, situation, and use, from those of the great body of insects. In the _Araneidæ_ or Spiders, their body seems to have no segments or incisure but that which separates the abdomen from the trunk; and in the _Scorpionidæ_ they are observable only in the abdomen. Other particulars might be enumerated in which these two classes differ from insects; but these will be sufficient to convince you that Aristotle and MM. Cuvier and Lamarck were justified in separating them.

The two last-mentioned authors made further improvements in Zoology. The latter, from the consideration of the general structure of animals, perceiving that Aristotle's _Enaima_ were distinguished from his _Anaima_, by being built as it were upon a vertebral column, very judiciously changed the denomination, which was indeed improper, of "_The Philosopher's_" two sub-kingdoms, into that of _Vertebrata_ or animals that have a vertebral column, and _Invertebrata_ or those that have no vertebral column. These he distributes into three primary divisions according to their supposed degrees of _intelligence_--Thus:

* _Apathetic Animals._ 1. INFUSORIA.
2. POLYPI.
3. RADIATA.
4. VERMES.

** _Sensitive Animals._ (_Epizoaria._)
5. INSECTA.
6. ARACHNIDA.
7. CRUSTACEA.
8. ANNELIDA.
9. CIRRHIPEDA.
10. MOLLUSCA.

*** _Intelligent Animals._ 11. PISCES.
12. REPTILIA.
13. AVES.
14. MAMMALIA.[19]

Profiting by the light afforded by the Aristotelian system, this eminent zoologist improved, we see, upon that of Linné, by resolving his _Insecta_ into three classes, and his _Vermes_ into seven, interposing the Linnean _Insecta_ between the four first and three last, in which he was not so happy, since as to _sense_ insects should certainly occupy the place he has here assigned to the _Mollusca_.

In the work from which I have taken this statement of Lamarck's system, that acute writer has given a sketch of another method of arrangement, in which he has made the first deviation from the beaten track of an unbroken and unbranching series. In the Supplement to the first volume, he has distributed the _Invertebrata_ in a double subramose series--one consisting of _articulate_, and the other of _inarticulate_ animals[20].

Upon Lamarck's system, most of the modern ones, with some variation, are founded. There is one, however, by a learned countryman of ours, that is more unique, _sui generis_, and I may add profound, than any that has yet appeared. I am speaking of that, you will perceive, of which our friend Mr. Wm. MacLeay has given a detailed statement in his _Horæ Entomologicæ_. In this he goes even far beyond what Lamarck has attempted in the above sketch, and substantiates his claim to be considered as one of those original thinkers, _rari nantes in gurgite vasto_, that do not appear every day. The following are the principal bases of his system.

1. That all natural groups, whether kingdoms or any subdivision of them, return into themselves; a distribution which he expresses by circles.

2. That each of these circles is formed precisely of _five_ groups, each of which is resolvable into five other smaller groups, and so on till you reach the extreme term of such division.

3. That proximate circles or larger groups are connected by the intervention of lesser groups, which he denominates _osculant_.

4. That there are relations of analogy between the corresponding points of contiguous circles.

This system he has represented by tables of circles inscribed with the five primary divisions of each group. His first table exhibits a general view of organized matter as distributed in the animal and vegetable kingdoms--Thus:

Our learned author here divides the animal kingdom into what may be denominated five sub-kingdoms or provinces, in three of which (with the exception of the _Crustacea_ and _Arachnida_ belonging to his _Annulosa_) no circulation of blood is visible, but which obtains in the rest. These he names--

1. ACRITA, consisting of the _Infusory Animals_, the _Polypi_, the _Corallines_, the _Tæniæ_, and the least organized of the _Intestinal Worms_.

2. RADIATA, including the _Jelly-fish_, _Star-fish_, _Echini_, and some others.

3. ANNULOSA, consisting of _Insecta_, _Arachnida_, and _Crustacea_.

4. VERTEBRATA, consisting of _Beasts_, _Birds_, _Reptiles_, _Amphibia_, and _Fishes_.

5. MOLLUSCA, including the numerous tribes of _shell-fish_, _land-shells_, _slugs_, &c., which, from their mucous or gelatinous substance, from their nervous system and the imperfection of their senses, return again to the _Acrita_, though connected with the _Vertebrata_ by having a heart and circulation.

His next set of circles shows the sub-division of these five sub-kingdoms into classes--Thus:

In this scheme the _osculant_ classes are those placed between the circles. In the _Mollusca_ circle two classes are still wanting to complete the quinary arrangement of that sub-kingdom. I am not sufficiently conversant with the details of the animal kingdom at large to hazard any decided opinion upon Mr. MacLeay's whole system, or to ascertain whether all these classes are sufficiently distinct[21]. My sentiments with regard to those of the _Annulosa_ I shall state to you hereafter.

Upon a future occasion I shall consider more at large the station to which insects seem entitled in a system of invertebrate animals, which will not accord exactly with that assigned by MM. Cuvier and Lamarck. But I am now in a field in which I have no intention to expatiate further, than as it is connected with the subject of the present letter. I shall therefore confine myself in what I have more to say to the definitions of _Insecta_ that have been given by modern authors, beginning with that of the zoologist last mentioned. Insects form a part of his _second_ group, which he terms _sensitive_ animals (_animaux sensibles_), which group he thus defines: "_They are sentient, but obtain from their sensations only perceptions of objects--a kind of simple ideas which they cannot combine to obtain complex ones._ Charact. _No vertebral column; a brain, and most commonly an elongated medullary mass; some distinct senses; the organs of movement attached under the skin: form symmetrical, by parts, in pairs_[22]." This division of animals, from the kind and degree of sense and intelligence that they possess, seems rather fanciful than founded in nature, since many insects show a greater portion of them than many vertebrate animals. Compare in this respect a _bee_ with a _tortoise_[23]. Lamarck divides his group of _animaux sensibles_ into two sections, namely, _Articulated_ animals, exhibiting segments or articulations in all or some of their parts; and _Inarticulatcd_ animals, exhibiting neither segments nor articulations in any of their parts. _Insecta_, _Arachnida_, and _Crustacea_, belong to the first of these sections, which he defines as "_those whose body is divided into segments, and which are furnished with jointed legs bent at the articulations_[24]." INSECTA he defines--"_Articulate animals, undergoing various metamorphoses, or acquiring new kinds of parts--having, in their perfect state, six feet, two antennæ, two compound eyes, and a corneous skin. The majority acquiring wings. Respiration by spiracles_ (stigmates), _and two vascular opposite chords, divided by plexus, and constituting aeriferous tracheæ, which extend every where. A small brain at the anterior extremity of a longitudinal knotty marrow, with nerves. No system of circulation, no conglomerate glands. Generation oviparous: two distinct sexes. A single sexual union in the whole course of life_[25]." ARACHNIDA he defines--"_Oviparous animals, having at all times jointed legs, undergoing no metamorphosis, and never acquiring new kinds of parts. Respiration tracheal or branchial: the openings for the entrance of the air spiraculiform (stigmatiformes). A heart and circulation beginning in many. The majority couple often in the course of life_[26]." I shall next add his definition of CRUSTACEA: "_Oviparous, articulated, apterous animals, with a crustaceous integument more or less solid, having jointed legs; eyes either pedunculate or sessile, and most commonly four antennæ, with a maxilliferous mouth seldom rostriform; maxillæ in many pairs placed one over the other; scarcely any under-lip; no spiraculiform openings for respiration; five or seven pair of legs; a longitudinal knotty marrow terminated anteriorly by a small brain. A heart and vessels for circulation. Respiration branchial with external branchiæ, sometimes hid under the sides of the shell of the thorax, or shut in prominent parts; sometimes uncovered, and in general adhering to particular legs or to the tail. Each sex usually double_[27]."

I have given Lamarck's definitions of these three classes, all considered as _Insecta_ by Linné, that by comparing them together you may be better enabled to appreciate the system of this author. On looking over the characters of the _Arachnida_ as here given, you will see at once that it consists of heterogeneous animals--for in fact he includes in this class not only the _Trachean Arachnida_ of Latreille, but the _Ametabolia_ of Dr. Leach, or the _Hexapod_ Aptera, and the _Myriapoda_.

I shall next copy for you Latreille's latest definition of _Insecta_ and _Arachnida_.

"INSECTA: _A single dorsal vessel representing the heart: two trunks of tracheæ running the whole length of the body, and opening externally by numerous spiracles; two antennæ; very often upper appendages for flight, indicating the metamorphosis to which the animal is subject when young; legs most commonly reduced to six._ ARACHNIDA: _Distinguished from_ Crustacea _by having their respiratory organs always internal, opening on the sides of the abdomen or thorax to receive the respirable fluid. Sometimes these organs perform the office of lungs, and then the circulation takes place by means of a dorsal vessel, which sends forth arterial, and receives venose branches. Sometimes they are tracheæ or air-vessels, which, as in the class_ Insecta, _replace those of circulation. These have only the vestige of a heart, or a dorsal vessel alternately contracting and sending forth no branch. The absence of antennæ, the reunion of the head with the thorax, a simple trachea but ramified and almost radiating, serve to distinguish these last_ Arachnida, _or the most imperfect of insects, which respire only by tracheæ_[28]." Under this head he observes--"Of all these characters, the most easy to seize and the most certain would doubtless be, if there were no mistake in it, that of the absence of antennæ; but later and comparative researches, confirmed by analogy, have convinced me, that these organs, under particular modifications it is true, and which have misled the attention of naturalists, do exist[29]:" and he supposes, from the situation and direction of the _mandibles_ of the _Arachnida_, corresponding with that of the _intermediate_ pair of _antennæ_ in _Crustacea_, that they really represent the latter organs. If this supposition be admitted, their use is wholly changed; the palpi, in fact, executing the functions of antennæ, which probably induced Treviranus to call them _Fühlhörner_ (_Feelinghorns_). Perhaps these last may be regarded as in some sort representing the _external_ antennæ of the _Crustacea_? With regard to _Insecta_, their antennæ seem to disappear in the _Pupiparæ_ Latr., or the genus _Hippobosca_ L.

The above definitions of the _Arachnida_ by these two celebrated authors, appear to me the reverse of satisfactory. When we are told of animals included in it, that some breathe by gills and others by tracheæ, that some have a heart and circulation and others not, we are immediately struck by the incongruity, and are led to suspect that animals differing so widely in the fountains of life ought not to be associated in the same class. A learned zoologist of our own country, Dr. Leach, seems to have made a nearer approach to a classification in accordance with the internal organization, by excluding from _Arachnida_ the _Acari_ and _Myriapoda_.

Sub-kingdom ANNULATA Cuv.

* Gills for respiration. _Classes._
_Legs sixteen:_ Antennæ two or four 1 CRUSTACEA.

** Sacs for respiration.
_Legs twelve:_ Antennæ none 3 ARACHNÖIDEA.

*** Tracheæ for respiration.
a. No Antennæ.
4 ACARI.
b. Two Antennæ.
_Six thoracic legs:_ Abdomen also bearing legs 2 MYRIAPODA.
_Six thoracic legs:_ No abdominal legs 5 INSECTA[30].

Mr. MacLeay, on whose system I shall now say a few words, divides his sub-kingdom _Annulosa_ into five classes, namely, _Crustacea_, _Ametabola_, _Mandibulata_, _Haustellata_, _Arachnida_. From the _Crustacea_ he goes by the genus _Porcellio_ Latr. to _Iulus_[31], which begins his _Ametabola_: these he connects with the _Mandibulata_, by _Nirmus_, which he thinks approaches some of the corticarious _Coleoptera_[32]. This class he appears to leave by the _Trichoptera_ Kirby, and so enters his _Haustellata_ by the _Lepidoptera_[33], and leaves it again by the _Diptera_ by means of the _Pupiparæ_ Latr., especially _Nycteribia_, connecting this class with the _Arachnida_, which he enters by the Hexapod _Acari_ L.[34], and these last he appears to leave by the _Araneidæ_, and to enter the _Crustacea_ by the Decapods[35]: thus making good his circle of classes, or a series of Annulose animals returning into itself. Mr. MacLeay's whole system upon paper appears very harmonious and consistent, and bears a most seducing aspect of verisimilitude; but it has not yet been so thoroughly weighed, discussed, and sifted, as to justify our adopting it _in toto_ at present: should it, however, upon an impartial and thorough investigation, come forth from the furnace as gold, and be found to correspond with the actual state of things in nature, my objections, which rest only upon some parts of his arrangement of _Annulosa_, would soon vanish. Some of those objections I will state here, and some will come in better when I treat of the Systems of Entomology. My first objection is, that his _Ametabola_, _Mandibulata_, and _Haustellata_, approach much nearer to each other than they do to the other two classes of his circle, or than even these last to each other; so that under this view it should primarily consist of _three_ greater groups, resolvable, it may be, into five smaller ones. My next objection is, that he has also considered the _Trachean_ and _Pulmonary_ Arachnida as forming _one_ class. Whether an animal breathes by gills or tracheæ, or has a circulation or not, is surely as strong a reason for considering those so distinguished as belonging to different classes, as the taking of their food by suction or by manducation is, for separating others to the full as much or more nearly related as to their external structure. But of this more hereafter. I cannot help, as a last objection, lamenting that our learned author has rejected from his system a term consecrated from the most remote antiquity, and which, even admitting his arrangement, might have been substituted for _Annulosa_, a name borrowed by Scaliger from Albertus Magnus, neither of whom, in Entomology, is an authority to weigh against Aristotle, from whom we derive the term _Insecta_, in Greek Εντομα.

As Fabricius did not alter Linné's _class_ Insecta, but merely broke up his _orders_ into new ones, which he named classes, I shall give you a detail of the alterations he introduced into the science in a future letter.

Having stated what my predecessors have done in classification, I shall next proceed to lay before you my own sentiments as to--_What is an insect._ Since our correspondence commenced, the _Arachnida_, principally on account of their internal organization, have been excluded from bearing that name, carrying with them, as we have seen, several tribes, which as yet have not been discovered to differ materially in that respect from the present _Insecta_: for the sake, therefore, of convenience and consistency, that I may, as far as the case will admit, adhere to the Horatian maxim

---- Servetur ad imum
Qualis ab incepto processerit et sibi constet,

I shall regard as _Insects_ all those _Annulosa_ that respire by tracheæ[36] and have no circulation, considering the _Trachean Arachnida_ and the _Myriapoda_ for the present as sub-classes, the one bordering upon the _Arachnida_, and the other upon the _Crustacea_. Some of these I am ready to own seem separated by an interval sufficiently wide from the _Hexapods_, which may be regarded as more peculiarly entitled to the denomination of _Insects_. The most striking differences will be found in the coalition of the head with the trunk in some (_Phalangidæ_), and the disappearance of the annulose form of the body in others (_Acarus_ L.), so that the legs only are jointed[37]. Yet an approach to such structure may be traced in some Hexapods; for instance, the coalition of the head and trunk in _Melophagus_, Latr., and that of the trunk and abdomen in _Sminthurus_, Latr.[38] The _Myriapoda_ exhibit other remarkable differences; though their head and trunk are distinct, the former antenniferous, and their body annulose, the abdomen as well as the trunk is furnished with legs, sometimes amounting to hundreds; but even to this a tendency has been observed in some Hexapods[39]. If you examine a specimen of _Machilis polypoda_, an insect related to the common sugar-louse (_Lepisma saccharina_), you will find that the abdomen is furnished with a double series of elastic appendages, which, being instruments of motion, may be regarded as representing legs. It is worthy of notice, that the _Myriapoda_ when first disclosed from the egg have never more than _six_ legs[40], and keep acquiring additional pairs of them and additional segments to their abdomen as they change their skins: and it is equally remarkable, that many Hexapods are subject to a law in some degree the very reverse of this, having many abdominal legs in their first state, and losing them all in their last. The union of the head with the trunk in the _Trachean Arachnida_ has been regarded as almost an unanswerable argument, in spite of their different internal organization, for including them in the same class with the _Pulmonary Arachnida_; but the case of _Galeodes_, which, though furnished with gills, (as an eminent Russian Entomologist Dr. G. Fischer is reported to have discovered,) implying also a circulation, and evidently belonging to the last-mentioned class, has nevertheless a distinct thorax consisting of more than one piece, to which are affixed only six legs[41], proves that even this circumstance possesses no weight when set against the organization. If it was a difference in this respect, that proved the _Crustacea_ classically distinct from _Insecta_--that likewise was the principal reason for the separation also of the _Arachnida_--it seems to follow that it ought also to furnish an argument equally cogent for considering the _Trachean Arachnida_, as well as the _Myriapoda_, distinct from the _Pulmonary_.

Another difference between the tribes in question is that of their _metamorphosis_; and this appears to have had great weight with Lamarck, inducing him to include in his _Arachnida_, not only the _Tracheans_ and _Myriapods_, but even the apterous _Hexapods_, except _Pulex_, or the _Anoplura_ and _Thysanura_ of modern authors. But the metamorphosis alone, unless supported by the internal organization, will I think scarcely be deemed a sufficient reason for separating from each other tribes agreeing in that respect, and placing them with others with which they disagree. The metamorphosis in some of the Hexapods (_Lepidoptera_) consists in the loss of legs, the acquisition of wings, a great change in the oral organs and in the general form; in others (some _Coleoptera_), in the acquisition only of wings and a change of shape, the oral organs remaining much the same; in others again (_Curculio_ L.), in the acquisition of six legs and wings and a change of form; in the flea, in the acquisition of six legs and a change of form only; in the _Orthoptera_, _Hemiptera_, &c. in the mere acquisition of wings; in the _Libellulidæ_, in the loss of the mask that covers the mouth and the acquisition of wings; in the _Diptera_, in the acquisition of six legs, wings, a change of the oral organs and of the form; in some of the _Octopods_ (_Acarus_ L.), in the acquisition of a pair of legs; and in others (_Phalangium_ and _Aranea_ L.), solely in a modification of them as to their proportions; in the _Myriapods_, the alteration that takes place in this respect is considerable; a large number of pairs of legs is acquired and many additional abdominal segments, and the proportion which the abdomen bears to the whole insect is quite altered. In all these cases there is a change more or less, either partial or general, of the original shape or organs of the animal; and with regard to their metamorphosis, there is a greater difference between a young and adult _Iulus_ than between a young and adult _grasshopper_ or _bug_: so that if the metamorphosis, _per se_, be assumed as a principal regulator of the class, the grasshopper or bug have as little claim to belong to it as the _Iulus_.

M. Lamarck lays considerable stress upon another character--That _Insecta_ engender only once in the course of their lives, and _Arachnida_ more than once. But this, if examined, will be found to be confined chiefly to the _Pulmonary Arachnida_, the _Tracheans_ following the law of _Insecta_ in this respect[42].

You may perhaps object that the bringing of the Trachean _Arachnida_ and the _Myriapoda_ into the class _Insecta_ will render the approximation of them to a natural arrangement more difficult, since it will be impossible at the same time to connect the _Myriapods_ with the _Crustacea_, and the _Trachean_ with the genuine _Arachnida_. I admit the validity of your objection, but by no arrangement of insects in a simple series can we attain this object: the difficulty, however, may perhaps be obviated in this way. The distribution of organized matter, to adopt Mr. Wm. MacLeay's metaphor[43], begins in a dichotomy, constituting the animal and vegetable branches of the great tree of nature, and from these two great branches, by means of infinite ramifications, the whole system is formed, and, what is remarkable, these branches unite again so as to represent a series returning into itself, a discovery due to the patient investigation and acumen of our learned friend just mentioned. Now, in considering the _Aptera_ order, we find at first setting out from the Hexapods, a dichotomy, where the _Anoplura_ Leach branch off on the one side, and the _Thysanura_ Latr. on the other--the former, by means of the _Pediculidæ_, taking their food by suction, particularly _Phthirus_ Leach, or the Morpion (in which the segments of the trunk and abdomen become indistinct[44]) approach the _Octopods_ by the hexapod _Acari_ L.--the latter by _Machilis polypoda_ tending towards the _Myriapods_. In the Octopod branch a further dichotomy takes place, from which you proceed on one side to the _Araneidæ_ in the _Arachnida_, by _Phalangium_, &c.; and in the other by _Chelifer_, &c. to _Scorpio_. Again, the _Myriapod_ branch also divides, going by the _Iulidæ_ to one branch of the _Isopod Crustacea_, and by the _Scolopendridæ_ to another.

But there is another view of this subject before alluded to, which may be repeated here, and which seems to prove that the types of form in one natural group or class are reproduced in another; this appears to result from the following parallel series:

_Neuropterous_ _Aptera._ _Arachnida._ _Crustacea._
_Larvæ._

Psocus Hexapoda Galeodes Larunda.

Myrmeleon _Phalangium_ Aranea {Decapoda
{ brachyura.

Octopoda

Panorpa? _Chelifer_ Scorpio {Decapoda
{ macroura.
{ _Thalassina
{ Scorpio_
{ especially.

Ephemera Myriapoda ***** Isopoda.

No type representing the _Myriapoda_ has yet been discovered in the _Arachnida_ class; but I have little doubt of its existence. You will observe that the analogies between the larvæ of the _winged_ orders and the _Aptera_ were first noticed by Mr. W. MacLeay[45]. It is probable that these parallel series of representatives of each other might be increased, as well as the numbers in the respective columns.

What I have said will, I trust, sufficiently justify me for making at present no more material alterations in the classification I long since proposed to you[46]; I shall, therefore, now proceed to define the objects I consider as _Insecta_; but I shall first observe--that as Latreille considers the _branchiopod Crustacea_ or _Entomostraca_ of Müller as entitled to the denomination of _Crustaceo-Arachnida_[47]; so his _Trachean Arachnida_ might be called _Arachnida-Insecta_, and his _Myriapoda_, _Crustaceo-Insecta_.

_Sub-kingdom_--ANNULOSA[48].

_Class_--INSECTA.

_First Definition--From their_ external _Organization_.

BODY--divided into Head--Trunk--Abdomen.

HEAD.--Principal seat of the organs of sensation.

_Organs of sight._ Immoveable eyes, simple or compound, varying
in number.

_Organs of hearing_ uncertain, probably connected with the
antennæ.

_Organ of taste._ Ligula or palate within the mouth, accompanied
by the _organs of manducation_--a pair of mandibles and maxillæ
and an upper and lower lip, or their representatives.

_Organs of touch._ Principally two jointed antennæ or their
representatives, and four jointed feelers--two maxillary and
two labial.

TRUNK. Principal seat of the organs of motion.

_Organs of walking, running, or jumping._ Six or eight jointed
thoracic legs, in pairs.

_Organs of flight._ Four wings or their representatives, mostly
with branching nervures containing air-vessels; found in the
majority of the class.

_Organs_ (_external_) _of respiration._ A double set of lateral
spiracles, some for expiration.

ABDOMEN. Principal seat of the organs of generation.

_Organs of motion._ In the _Myriapods_ many pairs of acquired
legs; in the _Thysanura_ elastic ventral or caudal appendages.

_Organs of respiration._ A double series of lateral spiracles for
inspiration in the majority; in some only a single series, and
in others only a single pair.

_Organs of generation_ those common to the _Vertebrata_, but
retractile within the body, attended usually by various anal
appendages, particularly a forceps in the males, and an
ovipositor in the females.

_Second Definition--From their_ internal _Organization_.

SENSATION.

_Nervous System._ A small brain usually subbilobed, crowning a
knotty double medullary chord; nerves proceeding from the brain
and other ganglions to all parts of the body.

CIRCULATION.

_Heart_ replaced by a simple alternately contracting dorsal vessel
or pseudocardia, without arteries or veins, but filled with a
white cold sanies.

RESPIRATION.

_Lungs_ replaced by tracheæ, which receive the air from the
spiracles, and distribute it by bronchiæ infinitely ramified. #/

DIGESTION.

_Liver and biliary vessels_ in most replaced by from 2 to + 150
floating hepatic filaments opening into the space between the two
skins of the intestinal canal below the pylorus.

GENERATION.

_Internal organs. Males_--Vasa deferentia, and vesiculæ seminales,
and the other ordinary organs. _Females_--Ovary usually
bipartite, with palmate lobes; genital organs single and mostly
anal; one sexual union impregnates the female for her life.

_Development_. In their passage to their adult state, after they have left the egg, _insects_ undergo several simultaneous changes of their integument or successive moults, and the majority assume three distinct forms, with distinct organs, which appear as rudiments in their second state, and are completely developed in their last.

* * * * *

In defining the _Arachnida_ I shall only mention those particulars in which they differ from _Insectæ_ in their external anatomy.

_Class_--ARACHNIDA.

BODY.

HEAD and Trunk usually not separated by a suture.

_Eyes_. Two to eight, not lateral.

_Mandibles_ cheliform or unguiculate, representing the interior
pair of the antennæ of the _Crustacea_.

_Palpi_ pediform or cheliform.

_Trunk_. Legs eight or their representatives: tibiæ mostly
consisting of two joints.

ABDOMEN with from two to eight spiracles.

SENSATION.

_Nervous System_. A small bilobed brain crowning a double, knotty,
medullary chord; nerves proceeding from the brain and other
ganglions to all parts of the body.

CIRCULATION.

_Heart_ unilocular, inaurite, with a system of circulation by
arteries and veins; blood a cold white sanies.

RESPIRATION.

_Lungs_ replaced by internal gills receiving the air by spiracles.

DIGESTION.

_Liver_, consisting of conglomerate glands, and enveloping the
intestines[49]; hepatic ducts.

GENERATION.

_Genital organs_ double, ventral; more than one sexual union in the
course of life.

The external characters in this class are the same almost in every respect as those which distinguish the _Phalangidæ_, the whole difference consisting almost in the systems of circulation, respiration, and digestion. Perhaps some future anatomist may discover in the tribe just mentioned, that there is a nearer agreement between them and the _Arachnida_ in these systems than is at present suspected, which would prove them true _Arachnida_. I am inclined to think that _Phrynus_ and _Gonyleptes_, &c. breathe by branchial spiracles; but having no opportunity of examining living specimens, I dare not speak with any confidence on the subject.

* * * * *

Having thus given you a view of the most important diagnostics by which what we have all along called _Insects_ may scientifically be distinguished from other invertebrate animals, it may not be without use, if, under this head, I take a more _popular_ and familiar view of the subject, and say something upon those distinctions which may attract the attention of the more common observer.

The notion of diminutive size, particularly as compared with vertebrate animals, seems more frequently attached to the idea of an insect than any other; and this notion is generally correct, for one insect that is bigger than the least of the above animals, thousands and thousands are vastly smaller: but there exist some that are considerably larger, whether we take length or bulk into consideration, and this in almost every order. To prove this most effectually, and that you may have a synoptical view of the comparative size of the larger insects of the different orders and tribes, I now lay before you a table of the dimensions of such of the largest as I have had an opportunity of measuring, including particularly those giants that are natives of the British isles.

Explanation of Table:

Column A - Length Inches
B - Breadth Inches
C - Expansion of Wings Longitudinal Inches} See note in table
D - Expansion of Wings Transverse Inches }

_Order and | _Species._ | A | B | C | D |
Family._ | | | | | |
| | | | | |
COLEOPTERA. | | | | | |
| | | | | |
CICINDELIDÆ |_Manticora grandis_ |1-5/8 | 7/10| | |
| | | | | |
ANTHIADÆ |_Anthia sexguttata_ |1-7/8 | 7/10| | |
| | | | | |
CARABIDÆ |_Carabus scabrosus_ |1-3/4 | 8/10| | |
| | | | | |
DYTISCIDÆ |_Dytiscus latissimus_|1-1/2 |1 | | |
| | | | | |
STAPHYLINIDÆ |_Staphylinus olens_ |1-1/2 | 5/10| | |
| Br. | | | | |
| | | | | |
HYDROPHILIDÆ |_Hydrophilus piceus_ |1-3/4 | 7/8 | | |
| Br. | | | | |
| | | | | |
BUPRESTIDÆ |_Buprestis grandis_ |2-1/2 |1 | | |
| | | | | |
|--------- _bicolor_ |2-5/12| 3/4 | | |
| | | | | |
DYNASTIDÆ |_Dynastes Hercules_ |4-1/2 |2-1/2 | | |
| |Horns | | | |
| |Incl. | | | |
| | | | | |
|_Megasoma Actæon_ K. |4-1/4 |2-1/4 | | |
| |Horns | | | |
| |Incl. | | | |
| | | | | |
CETONIADÆ |_Goliathus giganteus_|3-7/10|1-9/10| | |
| | | | | |
LUCANIDÆ |_Lucanus Cervus_ Br. |2 | 7/10| | |
| |Mands.| | | |
| |Incl. | | | |
| | | | | |
TENEBRIONIDÆ |_Tenebrio grandis_? |1-7/10| 7/10| | |
| | | | | |
BRENTIDÆ |_Brentus_ N. S. Mus. |2-1/10| 2/12| | |
| MacLeay | | | | |
| | | | | |
CALANDRIDÆ |_Calandra_ N. S. Mus.|3-5/8 |1 | | |
| MacLeay | | | | |
| | | | | |
BRACHYCERIDÆ |_Brachycerus apterus_|1-5/8 | 7/8 | | |
| | | | | |
|_Brachycerus_ |2 | 9/10| | |
| _Toxicophagus_ | | | | |
| Burch. | | | | |
| | | | | |
PRIONIDÆ |_Prionus grandis_ |6-1/2 |2-3/8 | | |
| | | | | |
|---- _cervicornis_ |5-1/2 |1-5/8 | | |
| |Mands.| | | |
| |Incl. | | | |
| | | | | |
|Do. |4-1/4 | | | |
| |Mands.| | | |
| |Excl. | | | |
| | | | | |
|---- _coriarius_ Br. |1-6/10| 7/10| | |
| | | | | |
LAMIADÆ |_Lamia grandis_ |2-3/4 |1 | | |
| | |nearly| | |
| | | | | |
CHRYSOMELIDÆ |_Chrysomela_ N. S. |1-1/4 | 3/4 | | |
| MacLeay | | | | |
| | | | | |
HISPIDÆ |_Alurnus grossus_ |1-3/10| 6/10| | |
| | | | | |
EROTYLIDÆ |_Erotylus grandis_ |1-6/10| 6/10| | |
| |nearly| | | |
| | | | | |
| | | | | |
DERMAPTERA. | | | | | |
| | | | | |
|_Labidura gigantea_ |1-1/4 | | | |
| Br.? |nearly| | | |
| |Forcs.| | | |
| |Incl. | | | |
| | | | | |
|_Forficula_ N. S. N. |1-4/5 | 2/5 | | |
| Holl. | | Do. | | |
| | | | | |
| | | | | |
ORTHOPTERA. | | | | | |
| | | | | |
BLATTIDÆ |_Blatta gigantea_ |2-5/8 |1-1/2 | | |
| | | | | |
|---- N. S. MacLeay |3-3/10|1-1/2 | | |
| | | | | |
PHASMIDÆ |_Phasma grandis_ |7 | 7/8 | | |
| |about | | | |
| | | | | |
|---- _australensis_ |8-1/2 | 3/4 |7-1/2 |2-3/4 |
| K. | | | | |
| | | | | |
ACHETIDÆ |_Gryllotalpa |1-3/4 | | | |
| vulgaris_ Br. | | | | |
| | | | | |
LOCUSTIDÆ |_Locusta Dux_ |4-3/8 | |8-1/4 |2 |
| | | | | |
CONOCEPHALIDÆ|_Acrida viridissima_ |2-1/10| | | |
| Br. | | | | |
| | | | | |
| | | | | |
HEMIPTERA. | | | | | |
* | | | | | |
| | | | | |
FULGORIDÆ |_Fulgora laternaria_ |3 | |5-6/10|2 |
| |nearly| | | |
| | | | | |
CICADIADÆ |_Cicada_ N. S. N. |2-1/10| |5-1/2 | |
| Holland |Wings | | | |
| |Excl. | | | |
| | | | | |
** |Do. |2-8/10| | | |
| |Wings | | | |
| |Incl. | | | |
| | | | | |
NEPIDÆ |_Belostoma grandis_ |3 | |5-6/10|1-3/10|
| | | | | |
LYGEIDÆ |_Lygæus Pharaonis_ |2 |1 | | |
| | | | | |
| | | | | |
LEPIDOPTERA | | | | | |
* | | | | | |
| | | | | |
PAPILIONIDÆ |_Papilio Remus_ | | |8-3/4 |3-5/8 |
| | | | | |
|---- _Priamus_ |2-1/2 | |7-6/10|3 |
| | | | | |
|---- _Machaon_ Br. |1 | |3-1/2 |1-1/2 |
| | | |nearly|Tail |
| | | | |Excl. |
| | | | | |
NYMPHALIDÆ |_Morpho Teucer_ |1-7/10| |7-1/2 |4-1/2 |
| | | | | |
SPHINGIDÆ |_Sphinx_ N. S. Brazil|2-1/2 | 7/10|6-1/2 | |
** | | | | | |
| | | | | |
SPHINGIDÆ |_Sphinx Atropos_ Br. |2 | 5/8 |4-3/4 |1-1/2 |
*** | | | | | |
| | | | | |
BOMBYCIDÆ |_Attacus Atlas_ |1-3/4 | |8-3/4 |5-1/2 |
| | | | | |
|_Gastropacha |1-1/4 | |3-1/2 |1-1/4 |
| quercifolia_ Br. |nearly| | | |
| | | | | |
NOCTUIDÆ |_Erebus Strix_ |2 1/4 | |10-3/4|3-1/4 |
| | | | | |
| | | | | |
NEUROPTERA. | | | | | |
| | | | | |
AGRIONIDÆ |_Agrion lineare_ |5-1/2 | |5 | |
| | | | | |
ÆSHNIDÆ |_Anax Imperator_ |3-1/3 | | | |
| Leach--Br. | | | | |
| | | | | |
MYRMELEONIDÆ |_Myrmeleon |2 | |5 | |
| libelluloides_ | | | | |
| | | | | |
SEMBLIDÆ |_Corydalis cornuta_ |2-3/4 | |5-1/4 | |
| |Mands.| | | |
| |Incl. | | | |
| | | | | |
|Do. |2 | | | |
| |Mands.| | | |
| |Excl. | | | |
| | | | | |
| | | | | |
HYMENOPTERA. | | | | | |
| | | | | |
SCOLIADÆ |_Scolia cyanipennis_ |2 | | | |
| | | | | |
POMPILIDÆ |_Pompilus ocellatus_ |2-1/8 | |3-1/2 | |
| | | | | |
VESPIDÆ |_Cyclostoma grandis_ |2 | | | |
| N. S. K. China | | | | |
| | | | | |
XYLOCOPIDÆ |_Xylocopa Nigrita_ |1-5/8 | | | |
| | | | | |
| | | | | |
DIPTERA. | | | | | |
| | | | | |
TIPULIDÆ |_Tipula sinuata_ Br. |1-3/8 | |2-1/2 | |
| | | | | |
ASILIDÆ |New Gen. Brazil. |1-3/4 | | | |
| | | | | |
MUSCIDÆ |_Echinomyia grossa_ |3/4 | | | |
| Br. | | | | |
| | | | | |
TABANIDÆ |_Tabanus_ N. S. Mus. |1-1/8 | | | |
| Drury | | | | |
| | | | | |
|---- _bovinus_ Br. |1 | | | |
| |nearly| | | |
| | | | | |
| | | | | |
APTERA. | | | |_Expansion of|
* | | | | Legs_ |
| | | | Inches. |
| | | | |
SCOLOPENDRIDÆ|_Scolopendra_ N. S. |13 |7/8 | |
| Mus. MacLeay | | | |
** | | | | |
| | | | |
IULIDÆ |_Iulus_ N. S. Do. |5-1/2 |8/10 | |
| | | | |
PHALANGIDÆ |_Gonyleptes_ N. S. | 3/8 | | 6 |
| Brazil | | | |
| | | | |
_Class._ | | | | |
| | | | |
ARACHNIDA. | | | | |
| | | | |
SCORPIONIDÆ |_Scorpio_ N. S. |7 | | |
| MacLeay |nearly| | |
| |Chelæ | | |
| |Excl. | | |
| | | | |
ARANEIDÆ |_Mygale_ N. S. Do. |3 |1-1/2 | 11 |

From this table you see that several insects included in it exceed some of the smallest _Vertebrata_ in bulk. In the _Mammalia_, the _Sorex Araneus_, called by the common people here the Ranny, is not more than two inches long excluding the tail; and the _Mus messorius_, or harvest-mouse, peculiar to the southern counties of England, is still more diminutive: so that to these little animals, the larger _Dynastidæ_, _Goliathi_, and _Prioni_, &c., appear giants, and may compete with the mole in size. Even some of the beetles of our own country, as the great _Hydrophilus_, the stag-beetle, &c., are more bulky than the two first-named quadrupeds. Amongst the birds, many _Picæ_, _Passeres_, &c., yield to several insects in dimensions, and their wings when expanded do not extend so far as those of not a few _Lepidoptera_. The great owl-moth of Brazil (_Erebus Strix_) in this respect is a larger fowl than the quail. Those beautiful little creatures, the humming-birds (_Trochilus_ L.), the peculiar ornament and life of tropical gardens, which emulate the most splendid butterflies in the brilliancy of their plumage, are smaller than a considerable number of insects in almost every order, and even than some of those that are natives of Britain. Various reptiles also are much inferior in size to many of the insects of the above table. The smallest lizard of this country would be outweighed by the great British beetles lately mentioned, and the mole-cricket (_Gryllotalpa vulgaris_); and some of the serpent tribe are smaller than the larger _Scolopendræ_ and _Iuli_. Amongst the _fishes_ also, though some are so enormous in bulk, others in this respect yield the palm to several insects. The _minnow_ and the _stickleback_ that frequent our own pools and streams are considerably inferior in size to some of our water beetles.

In looking over the table, and comparing the different species that compose it with each other, you will perceive that the largest insects of the two sections of _Hemiptera_, of the _Lepidoptera_ as to their _body_ merely, of the _Hymenoptera_ and _Diptera_, in general size fall considerably short of those of the other orders; and that certain individuals of the _Orthoptera_ and _Aptera_ bear away the palm in this respect from all the rest. In the _Coleoptera_ the giants, with the exception of the _Goliathi_, are chiefly to be found amongst the timber devourers in the _Lamellicorn_ and _Capricorn_ tribes. Of orthopterous insects the _Phasmidæ_ present the most striking examples of _magnitude_; and in the _Neuroptera_, the _Agrionidæ_ of great _length_.