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 Larvæ. | Aptera. | Arachnida. | Crustacea. |
| 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.