"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.