It will have occurred to you, in making out your genera or lowest groups, that some consist of a vastly greater number of species than others. It seems advisable therefore, when you apply yourself seriously to ascertain what described ones your cabinet contains, to begin with those genera which appear to be poor in them; for here your labour will be comparatively light, from the small number you will have to examine; and you will become practised in the employment before you are called upon to attack those that overflow. Had Fabricius and other describers of species taken the trouble to subdivide the larger groups, as might easily have been done, into more genera and subgenera, the student would have been spared a most discouraging labour. To be obliged to compare a single individual with the descriptions of from 100 to 300 species[1585], to ascertain its name, seems enough to make you start aside with horror from the employment, and be content that your species should remain unnamed, rather than expose yourself to such a waste of time and patience. But to lessen your alarm and encourage you to proceed, I must observe to you, though in a few instances it may be necessary to advert to the description of every single species in a section, yet that this is seldom requisite; and where it is, there are many helps to diminish the labour and abridge the process. A large number of insects are characterized by their colour; and it is the practice of all good describers to begin their definition of the species with that which predominates, and then to enumerate the variations from it. Thus, if an insect be all black except the thorax, antennæ, and legs, you will find it thus characterized, "Black: with thorax, antennæ, and legs ferruginous"; and so on. Hence, having noticed the predominant colour of your unknown species, in many genera you may compare it with the descriptions contained in a whole page at a single glance, and only read the further descriptions when the colour agrees. A practised Entomologist will thus investigate his insects with a rapidity which to an unlearned bystander would seem impossible. Though I have instanced colour as being the character most commonly employed in describing species of insects, you will readily conceive that in some tribes other characters afford more prominent distinctions. Thus in the Dynastidæ and many other Petalocerous beetles, the principal specific character is derived from the horns or tubercles that arm the head and thorax: in Lucanus from the mandibulæ; and in Prionus from the marginal teeth of the thorax. If the insect, then, you want to name belongs to any of these genera, having observed its peculiar characters in this respect, you may ascertain in a very few minutes whether any already described exhibit the same. This facility of investigation can be better acquired by practice than precept, and cannot be attained all at once. The above hints, however, may be of some use; and cannot fail to be so, if you always endeavour to make yourself acquainted by a previous careful examination with the characters of every new insect you acquire,—whether those of form, colour, or sculpture,—before you attempt to discover its name in Fabricius or any other author.

When you have made such proficiency in the study as to be familiar with a few species of each section of an extensive genus, the labour of investigation will sometimes be greatly facilitated by attending to that conformity between the proportions, general aspect, and figure of a known and an unknown insect, which Naturalists express by the name of habit, and which, though easily perceived by a practised eye, is described with such difficulty. Scientific Entomologists in their descriptions have usually taken care to place near to each other, species agreeing in habit. When therefore you know the name of one species, and find another of the same general habit, you may commonly take it for granted that if described at all by your author, it will be placed near that already known to you. Thus, supposing you are acquainted with that common weevil Cionus Scrophulariæ, and find its near relation C. Blattariæ; instead of comparing it one by one with the 161 species which compose the Longirostres femoribus dentatis of the Fabrician genus Rhynchænus in the Systema Eleutheratorum, you would at once turn to the former, very near which you would without further trouble discover it. Fortunate, would it be, could the Entomologist always depend on thus finding descriptions of allied species in the neighbourhood of each other; but unhappily the most distinguished authors have sometimes violated this important rule, so that we cannot always be certain that any given species is not elsewhere described than in its right place. Fabricius in many instances often removes widely asunder insects not merely related, but which are in reality scarcely more than varieties of the same species[1586]. In fact, the attention of this celebrated author was so distracted by the immensity of the materials he had to arrange, by the distance of the cabinets, in many cases, from each other, the new species of which he undertook to describe, and the rapidity with which they necessarily passed under his eye, that he seems never to have attained any nice perception of the affinities of insects.

You must not conclude, however, that the investigation of a new insect is even to an adept always a work of ease and dispatch. Often, when seemingly ascertained by the rapid process above indicated, a further inquiry will be requisite; the more detailed description must be read, and figures consulted, before its name can be indisputably determined. In addition to the difficulty arising from the insufficient characters frequently given by Fabricius and the older authors, obstacles arising from their errors not seldom intervene. Thus they have sometimes selected for a specific character,—as in the case of Megachile centuncularis, Nomada ruficornis, and various other insects,—what really only indicates a family. At other times sexual characters common to many,—as in Eucera longicornis, Locusta perspicillata, &c.,—have been had recourse to. In these cases, in order satisfactorily to ascertain your species, you must further consult the synonyms and habitat given by the original describer, especially the figures he has referred to. When all these fail, as they sometimes will, the dernier resort is a reference to the cabinet containing the original specimen from which the description was drawn. British Entomologists possess an invaluable privilege, which their continental brethren may well envy them, in having the most liberal access, indulged to them by the learned President of the Linnean Society, to Linné's collection of insects, from which a large proportion of the species he described may be ascertained[1587]. Several of the cabinets, especially the Banksian,—now the property of the Linnean Society,—from which Fabricius described his insects, may also still be consulted; and thus many mistakes rectified, which would otherwise greatly mislead[1588].

Though sometimes the limits that separate good species appear at first very slight, and require a practised eye to catch them, yet it occasionally happens that considerable apparent differences may safely be disregarded. The colour of insects,—to which unhappily for want of better characters we are so generally forced to have recourse,—though usually constant, is in some species very variable[1589]. This is the case sometimes with whole colours. Thus Carabus arvensis, Pœcilus cupreus, &c., are sometimes of a copper colour; at others, resemble brass; at others, they are green or blue, and even black. The colour of spots also often varies. In some individuals of Pentatoma oleracea they are pale, and in others red. The number and shape of spots are also often inconstant. Many of the species of Coccinella so abound in these variations, that nothing short of the most careful examination can enable you to distinguish the species from the variety. Insects vary also in size: but as this is never assumed as a specific character, it will not occasion you much trouble. Where the difference in this respect between two specimens is very great, the presumption is that they are specifically distinct. Differences in sculpture and proportion do not always indicate different species; this being sometimes, as we have seen above, only a sexual character[1590]. Authors also in their descriptions, in this respect sometimes mislead the young student. When Linné calls the thorax of Aphodius erraticus smooth (lævis) he would not expect to find it covered with impressed puncta, and with a longitudinal posterior impressed line. Likewise in describing Chlænius vestitus and nigricornis, Fabricius passes without notice their punctate surface, so different from that of other Harpalidæ. Errors of this kind however, it is but fair to observe, are chiefly to be attributed to the circumstance that both Linné and Fabricius rarely employed a microscope in making descriptions; though no one now attempts this, except where insects are large, without such an aid.

If you ask, How am I to acquire this delicacy of tact which is to decide when the terms of a specific character are to be rigidly adhered to, and when taken with a certain latitude? I answer, In the same way in which a connoisseur attains the faculty of discerning the works of different masters in painting;—by such careful study of your author as will make you master of his style. Thus you will soon perceive in what cases expressions are to be taken literally and strictly, or with some allowance and abatement.

There yet remains more distinctly to be adverted to, the assistance that may be derived in the investigation of insects from figures. Generally speaking, these should never be referred to in the first instance, but be regarded as a resource when the ordinary methods leave the subject of inquiry doubtful. Those who begin their entomological studies by turning over figures usually end them there, and never attain to that nameless tact in making out insects that can only be the result of patient study. Indeed figures, though often very useful, and sometimes indispensable, can scarcely ever exhibit those nice characters, particularly as to sculpture, that distinguish some insects. Our modern artists, indeed, are remedying this defect of the art, by giving in many cases the thorax or elytrum apart, with all its sculptural peculiarities: but this is not, and cannot be, done so as to represent every one. But though in general figures should be your last resort, I know not whether an exception to the rule may not be advisable with respect to the Lepidoptera, which are more difficult to be intelligibly described than any other order of insects; while a good figure exhibits to the eye all those markings and shades, that scarcely any description can place clearly before the mind.

When every attempt to investigate the name of your unknown species fails, and you have consequently reason to believe that it is undescribed, the best mode you can pursue for retaining that knowledge of its characters, which from your long investigation you must have acquired, is to note them down in your entomological journal, inserting it under its proper genus with a trivial name of your own. Such a journal you will find almost a sine qua non for containing a catalogue of your insects, and to register any observations concerning individuals you may have had an opportunity of making. With regard to this journal, I should recommend to you to get two blank books. One a duodecimo of 200 or 300 pages, to contain the mere catalogue of your insects, their habitat and localities, or the source from which you derived them. In this you should number the genera in Roman capitals, and the species under each by a figure; leaving considerable space at the end of each genus for the insertion of new species. The other book should be of an octavo size, containing 400 or 500 pages. Under the number of each genus and species you might describe and figure it, if undescribed; if described, note in what it varies from the description, and what characters are overlooked: and in general, insert such observations, with regard to its economy and habits, as you may have had an opportunity of making.—As to foreign insects, wherever you can, upon good authority, be particular in indicating the country and station of each specimen.

I need not say much to you concerning the microscopes you should use for the examination of insects, a common pocket one of three glasses of different powers will answer every ordinary purpose[1591].

We have treated hitherto of insects as we find them now inhabiting our globe: but I must not conclude our correspondence without taking some notice of those that are found in a fossil state. Fossil insects may be divided into those that are found in amber, and those that are found in other substances.