Note 2 — Chapter XI · ON THE PROGRESS OF EVOLUTION.

I will here briefly give my reasons for the opinion that the so-called “complete metamorphosis” of Insects, in which these animals quit the egg as grubs or caterpillars, and afterwards become quiescent pupæ incapable of feeding, was not inherited from the primitive ancestor of all Insects, but acquired at a later period. The order Orthoptera, including the Pseudoneuroptera (Ephemera, Libellula, etc.) appears to approach nearest to the primitive form of Insects. In favour of this view we have:— 1. The structure of their buccal organs, especially the formation of the labium, “which retains, either perfectly or approximately, the original form of a second pair of maxillæ” (Gerstäcker). 2. The segmentation of the abdomen; “like the labium, the abdomen also very generally retains its original segmentation, which is shown in the development of eleven segments” (Gerstäcker). The Orthoptera with eleven segments in the abdomen, agree perfectly in the number of their body-segments with the Prawn-larva represented in Fig. 33, or indeed, with the higher Crustacea (Podophthalma and Edriophthalma) in general, in which the historically youngest last thoracic segment (see Chapter 12), which is sometimes late-developed, or destitute of appendages, or even deficient, is still wanting. 3. That, as in the Crustacea, the sexual orifice and anus are placed upon different segments; “whilst the former is situated in the ninth segment, the latter occurs in the eleventh” (Gerstäcker). 4. Their palæontological occurrence; “in a fossil state the Orthoptera make their appearance the earliest of all Insects, namely as early as the Carboniferous formation, in which they exceed all others in number” (Gerstäcker). 5. The absence of uniformity of habit at the present day in an order so small when compared with the Coleoptera, Hymenoptera, etc. For this also is usually a phenomenon characteristic of very ancient groups of forms which have already overstepped the climax of their development, and is explicable by extinction in mass. A Beetle or a Butterfly is to be recognised as such at the first glance, but only a thorough investigation can demonstrate the mutual relationships of Termes, Blatta, Mantis, Forficula, Ephemera, Libellula, etc. I may refer to a corresponding remarkable example from the vegetable world: amongst Ferns the genera Aneimia, Schizæa and Lygodium, belonging to the group Schizæaceæ which is very poor in species, differ much more from each other than any two forms of the group Polypodiaceæ which numbers its thousands of species. If, from all this, it seems right to regard the Orthoptera as the order of Insects approaching most nearly to the common primitive form, we must also expect that their mode of development will agree better with that of the primitive form, than, for example, that of the Lepidoptera, in the same way that some of the Prawns (Penéus) approaching most closely the primitive form of the Decapoda, have most truly preserved their original mode of development. Now, the majority of the Orthoptera quit the egg in a form which is distinguished from that of the adult Insect almost solely by the want of wings; these larvæ then soon acquire rudiments of wings, which appear more strongly developed after every moult. Even this perfectly gradual transition from the youngest larva to the sexually mature Insect, preserves in a far higher degree the picture of an original mode of development, than does the so-called complete metamorphosis of the Coleoptera, Lepidoptera, or Diptera, with its abruptly separated larva-, pupa- and imago-states. The most ancient Insects would probably have most resembled these wingless larvæ of the existing Orthoptera. The circumstance that there are still numerous wingless species among the Orthoptera, and that some of these (Blattidæ) are so like certain Crustacea (Isopods) in habit that both are indicated by the same name (“Baratta”) by the people in this country, can scarcely be regarded as of any importance. The contrary supposition that the oldest Insects possessed a “complete metamorphosis,” and that the “incomplete metamorphosis” of the Orthoptera and Hemiptera is only of later origin, is met by serious difficulties. If all the classes of Arthropoda (Crustacea, Insecta, Myriopoda and Arachnida) are indeed all branches of a common stem (and of this there can scarcely be a doubt), it is evident that the water-inhabiting and water-breathing Crustacea must be regarded as the original stem from which the other terrestrial classes, with their tracheal respiration, have branched off. But nowhere among the Crustacea is there a mode of development comparable to the “complete metamorphosis” of the Insecta, nowhere among the young or adult Crustacea are there forms which might resemble the maggots of the Diptera or Hymenoptera, the larvæ of the Coleoptera, or the caterpillars of the Lepidoptera, still less any bearing even a distant resemblance to the quiescent pupæ of these animals. The pupæ, indeed, cannot at all be regarded as members of an original developmental series, the individual stages of which represent permanent ancestral states, for an animal like the mouthless and footless pupa of the Silkworm, enclosed by a thick cocoon, can never have formed the final, sexually mature state of an Arthropod. In the development of the Insecta we never see new segments added to those already present in the youngest larvæ, but we do see segments which were distinct in the larva afterwards become fused together or disappear. Considering the parallelism which prevails throughout organic nature between palæontological and embryonic development, it is therefore improbable that the oldest Insects should have possessed fewer segments than some of their descendants. But the larva of the Coleoptera, Lepidoptera, etc., never have more than nine abdominal segments, it is therefore not probable that they represent the original young form of the oldest Insects, and that the Orthoptera, with an abdomen of eleven segments, should have been subsequently developed from them. Taking into consideration on the one hand these difficulties, and on the other the arguments which indicate the Orthoptera as the order most nearly approaching the primitive form, it is my opinion that the “incomplete metamorphosis” of the Orthoptera is the primitive one, inherited from the original parents of all Insects, and the “complete metamorphosis” of the Coleoptera, Diptera, etc., a subsequently acquired one.