When your drawers are smoothly corked[1581] and neatly papered, first divide each transversely by a full black line; parallel with this, on each side, draw a line with red ink: then, for arranging your insects, draw pencil lines, which are easily obliterated, at right angles with the others, according to the general size of the insects that are to occupy them. Insects look better thus arranged in double columns, than if the pencil lines traversed the whole width of the drawers. In arranging them, you may either place them in a straight line between the pencil lines,—which I think is best,—or upon them. You will begin your columns from the red lines in the middle, and not from the sides of the drawer; thus the heads of those on one side of it will be in an opposite direction to those on the other. Where your pins are very fine and weak, you must make a hole first with a common lace-pin; otherwise, in forcing them into the cork, they will bend. In labelling your specimens, you should stick the appellation of the genus or subgenus with a pin before the species that belong to it. As to the species themselves, you may either number them 1, 2, 3, &c., sticking the pin they are upon through the number, and denoting them by a corresponding one in your catalogue; or you may at once write the trivial name, with the initial of the genus upon a label transfixed in the same manner. Lepidoptera cannot easily be arranged in columns. Perhaps if squares, corresponding with the size and number of the specimens of any given species you wish to preserve, were made with pencil, a label of the trivial name of the species, or a number being placed at its head, it would be as good a way as any other. But every one must be left to his own taste in these matters. Wherever you can, procure a specimen of each sex of an insect, and where important characters require it, let some of your Lepidopterous specimens exhibit the under side of the wings.
In arranging insects in your cabinet, if you wish to have it scientific, as much as the nature of the subject will admit, follow the series of affinities; but you may reserve a few drawers to place in contrast analogous forms. As your numbers of species increase you will have to alter your arrangement; but as pencil lines are easily rubbed out, this will occasion you less trouble than if they were drawn with ink. You should always be careful under each genus to leave space for new species.
As certain Acarina, Tineidæ, Ptinidæ, &c., prey upon dead insects, you will of course wish to know how they may be kept out of your drawers, or banished when detected there. Camphor is the general remedy recommended. The cavity closed by the rabbet of the glass frame affords a good receptacle for this necessary article: put some roughly powdered into each side, and be careful to renew it when evaporated. This will generally preserve your insects, as will be seen from the result of the following experiment.—Some insects in a chip box having become much infested by mites and Psocus pulsatorius, I placed under a wine-glass several of each along with roughly-powdered camphor: at the end of twenty-four hours the mites were alive; but at the end of forty-eight they were all apparently dead, and did not revive upon the removal of the camphor. The specimens of Psocus all appeared dead in an hour, and never revived. If the camphor be put only into one side of a drawer, and in a lump, though perhaps it may keep out mites, &c., it will not expel them.
[LETTER LI].
INVESTIGATION OF INSECTS.
An entomologist who aspires to more than the character of a mere amateur, will not be content with filling his cabinet with nameless objects for the sole amusement of the eye; but will also be anxious to acquire some knowledge of what he has collected, and to ascertain by what names, whether indicating their genus or species, they have been distinguished by scientific writers who have described insects either in general or those of particular districts. Thus only can he himself derive profit from any discoveries he may make, or contribute to the further progress of the science[1582].
But in order to accomplish this object effectually, you must remember and practise the Onslow motto—Festina lente:—you must not be too eager to name your species, but begin first with grouping your collection. The only way to acquire, in any degree, a correct knowledge of the Natural System, or of the general plan of the Creator, which is the primum and ultimum of true science, is by studying groups. The knowledge of species is indeed indispensable for the registry of facts and other practical purposes, but the knowledge of groups leads to a higher wisdom; and indeed it is through these that we best descend to the study of species.
I will suppose you have made yourself master of so much of the technical language, particularly the names and most important attributes of the principal organs of insects, as will suffice for understanding descriptions, or knowing these parts when you see them. I will also further suppose that what was formerly said on these subjects has been sufficiently studied, to enable you without much difficulty or hesitation to say whether any given object belongs to the Class Insecta or Arachnida, or to which of their respective Orders[1583]. You are therefore qualified to arrange your collection into its primary groups. But you have seen that many others intervene between the Order and the genus or species. As the genera of Linné are mostly primary groups of Orders, perhaps, setting aside such insects included in them by him as your eye and their apparent characters convince you have no claim to a place there, your next best step would be to make yourself thoroughly acquainted with them. When you have accurately marshalled and intimately studied these groups, you will probably have acquired an eye and a tact, experto crede, for grouping without book, and may proceed by analysis to resolve your whole collection, as nearly as possible, into as many as nature seems to indicate to you. In doing this you will doubtless at first fall into many errors; but these, practice and a closer examination will in time enable you to rectify. Having thus got your groups as near to nature as you can, you may now have recourse to those authors, particularly Fabricius and Latreille, who have subdivided the genera of Linné; and you will see which of your groups agree with theirs, detect your own errors, and often theirs, and be enabled to label each of your genera and higher groups, if already known, with its modern appellation. You are now qualified also to enter scientifically into the study of the characters that distinguish groups, and may proceed, wherever opportunity is afforded, to examine the trophi, which may often be displayed sufficiently by the means recommended in my last letter[1584]. In this way you may learn also to know your groups as well by character as by habit, and be qualified to trace the gradual progress of nature from form to form; and may look upon yourself as duly prepared to put the last hand to your labours, and proceed to the examination of species.