Anus terminating in a mucro, and sometimes in a filiform jointed tail without a sting at the end.
v. Having considered the Orders into which Insecta and Arachnida may be divided, I am next to give you some account of the groups into which each is further resolvable. To draw out, however, a complete scheme of these would be deviating from my province, and extend this letter to an enormous length. Indeed, to give the natural primary and subordinate sections of every Order, would require a knowledge of the subject to which no Entomologist has yet attained. I shall therefore only say something general upon them, and refer you to an example of each kind of group.
Previously to the groups themselves their nomenclature claims our attention. M. Latreille in his last arrangement of Annulose animals has divided his Orders into Sections; Families; Tribes; and Genera: his tribes he has often further subdivided into lesser sections, represented by capital and small letters, &c.[1262]. Mr. MacLeay, discarding the term section, has Tribes; Races (Stirps); Families; Genera, and Subgenera[1263]. But as in descending from the Order to the lowest term, or the species, a series of groups gradually diminishing in value, which require a greater number of denominations than have yet been employed by Entomologists, often occur, I think we may with benefit to the science add to the list. I would therefore propose the following primary and subordinate divisions of an Order: 1. Suborder; 2. Section; 3. Subsection; 4. Tribe; 5. Subtribe; 6. Stirps; 7. Family; 8. Genus; 9. Subgenus. I would further propose that each of these successive groups should have a name always terminating alike, so that the value of the group when spoken of might always be known by the termination:—thus if a subclass end in ata, a suborder might end in ita; a section in ana, a subsection in ena; a tribe in ina, a subtribe in ona; a stirps in una; and a family in idæ; the genera being left free.
With regard to their characters, we are not to place our groups upon Procrustes' bed, and lop or torture them to accommodate them to every standard we may have fixed for them: assuming one set of characters for suborders, another for tribes, and so for every other group; for the value of characters varies,—those that in some cases are common to an Order, in others indicate only sections, or tribes, or genera and species, or sometimes even sexes. What is constant in one group is not so in another, and vice versâ; so that it is a vain labour to search for a universal character. If it is our wish really to trace the labyrinth of nature, we can only accomplish it by a careful perusal and examination of her various groups. It is singular how much and how far various Entomologists, and those of the very highest class, have been misled by a kind of favouritism to give too universal a currency to certain characters for which they have conceived a predilection. Some have been the champions of the antennæ; others of the trophi; others again of the wings; and others of the metamorphosis. These are all characters which within certain limits lead us right, and are an index to a natural group; but if we follow them further, we leave the system of nature, and are perplexed in the mazes of a method[1264].
Let us now see whether we can pitch upon any suborder which will afford an example of every group that we have lately named. Mr. MacLeay, from a consideration of the larvæ of that Order, has divided the Coleoptera into five primary groups that may be denominated Suborders. Whether these are all natural groups has not yet been made sufficiently evident. It answers my present purpose, however, to assume it as proved. I select therefore his Chilopodimorpha for my suborder, altering the name as above proposed to Chilopodimorphita: for my Section I take the Predaceous beetles, or Adephaga of M. Clairville, distinguished by having the upper lobe of their maxillæ biarticulate and palpiform;—these I would denominate Adephagana, or devourers. They consist of two groups forming two subsections, the one terrestrial and the other aquatic; which I would name, following Mr. MacLeay, Geodephagena and Hydrodephagena. These two subsections are each resolvable into two Tribes constituted by Linné's four genera Cicindela and Carabus; Dytiscus and Gyrinus. The first tribe, remarkable for the swiftness of their flight, I would name Eupterina, or fliers; the second, equally noted for running, Eutrechina, or runners; the third Eunechina, or swimmers; and the fourth Gyronechina, or swimmers in a circle. The second of these groups, the Eutrechina, are resolvable into two other groups or Subtribes; one distinguished by having the cubit or anterior tibia notched, (which, from their being in general not very brilliant in colour, I would call Amaurona, or obscure); the other having the cubit without a notch, (which, from the brilliancy of many of them, I would name Lamprona, or splendid). These subtribes are both further resolvable into two or more races (Stirpes). I select that to which the crepitant Eutrechina belong, containing those which from their usually truncated elytra MM. Latreille and Dejean have named Truncatipennes[1265]: these, to shorten the name, I call Truncipennuna. This brings us down to the lowest group formed out of genera and subgenera: or the family, which from its principal genus is named Brachinidæ, and which leads us to the genus Brachinus, and the subgenus Aptini. Thus we get the following scale, expressing every division of an Order, till we arrive at its lowest term, or the species that compose it.
Suborder
Chilopodimorphita McL.
Section
Adephagana Clairv.
Subsection
Geodephagena McL.
Tribe
Eutrechina
Subtribe
Amaurona
Stirps
Truncipennuna Latr.
Family
Brachinidæ
Genus
Brachinus
Subgenus
Aptini.
In the construction of this scale I have endeavoured to steer clear of being led by any system, but, with the exception of the Suborder, which I assume, to resolve it into natural groups gradually decreasing in value, or tending to the lowest term, which appear all of them to have been considered as such by preceding Entomologists. The four Tribes into which the two subsections Geodephagena and Hydradephagena appear resolvable, are not only distinguished by the characters of the perfect insect, but likewise by those of their larvæ, which are constructed on four distinct types; those of the Gyronechina being the most perfectly Chilopodimorphous of the whole, and those of the Eunechina the least so[1266]. The former appear rather to form an osculant tribe, or one without the circle, than one within it; and to be going off towards another section, including Hydrophilus, Sphæridium, &c. I must observe, that between Dytiscus and Hydrophilus there is a striking agreement both in their form and habits in the larvæ[1266], and even in several characters in the perfect insect; so as in many respects to generate a doubt whether they ought not to enter the same circle and to follow each other. Yet the change of habits in the latter, which from a carnivorous larva becomes a herbivorous beetle; the consequent change of structure in their oral organs, their antennæ, and other striking differences; and the evident intervention of the Gyronechina and some other osculant tribes between the two, forbid their union in one and the same circle.
vi. I need not say more on those larger groups of an Order which conduct us to what are denominated its genera; but upon these last it will not be a waste of your time to enlarge a little. In the last edition of the Systema Naturæ, and in its appendixes, Linné has described 2840 species of Insecta and Arachnida, which he divided into 83 genera, allowing upon an average nearly 35 species to each genus. From the paucity of the materials, therefore, of which his system was constructed, there was no loud call upon him for numerous genera. But now more than thirty times that number are said to have found a place in the cabinets of collectors[1267], and there is good reason for thinking that perhaps half that are in existence are as yet undiscovered;—this makes it a matter of absolute necessity to subdivide the Linnean genera, which in fact, with regard to the majority of them, were the primary groups of his Orders, rather than an approximation to the ultimate. But this principle may be carried too far: for it is the nature of man to pass from one extreme to the other: and this seems to me to be the case when it is proposed to make genera the extreme term of subdivision before you arrive at species. But it is argued by a very acute Zoologist, that simplicity, perspicuity, and room for necessary variations are best preserved by distinguishing these subdivisions each by an appropriate name[1268]:—Granted. But still it is only a choice of evils. It would require probably more than 10,000 names to designate them, were every extreme group distinguished by a name: but if Mr. MacLeay's admirable pattern exhibited in his genus Phanæus[1269] were followed, it would not call for more than 2000—could the trifling difficulty occasioned sometimes by the discovery of a new group, be set against the advantage of having only 2000 names to commit to memory instead of 10,000[1270]? But if, after all, it is judged best to name subgenera, M. Savigny's excellent plan of distinguishing them by a plural termination would diminish the weight of the above objection, and might be used with advantage.
When the component parts of any minor group differ from another,—for the most part in important characters, indicating some tangible difference in their habits and economy, and confirmed by peculiarities in their larvæ; and these differences run through the whole, except that as usual they grow weaker as it is passing off to another; especially where they are striking in the centre or type of the group,—this is always a legitimate genus: but where the characters assumed are very slight, and nothing peculiar in its habits, economy or larva, warrant such distinction, it ought not to be conferred.
vii. I must next say a word concerning species and varieties. A species is a natural object whose differences from those most nearly related to it had their origin when it came from the hands of its Creator; while those that characterize a variety, have been produced since that event. As we do not know the value and weight of the momenta by which climate, food, and other supposed fortuitous circumstances operate upon animal forms, we cannot point out any certain diagnostic by which in all cases a species may be distinguished from a variety;—for those characters that in some are constant, in others vary. In general, where there is no difference in form, appendages and organs, sculpture, proportions and larvæ,—colour alone, especially in insects inhabiting the same district, only indicates a casual variety. Thus Aphodius luridus has sometimes pale elytra with the striæ black (Scarabæus nigro-sulcatus Marsh.): at others it has black spots between the striæ, as in the type: in a third variety the elytra are black at the base and pale at the apex (Sc. varius Marsh.); and lastly, in a fourth they are intirely black (Sc. gagates Marsh.);—yet all these in every other respect precisely correspond. But the converse of this will scarcely hold good; for doubtless minor differences of structure are sometimes produced by a different food and climate: which may probably account for some variations observable in the individuals apparently of the same species obtained from different countries.