Part 2

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.