Chapter XXI: 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_.

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.

It has been observed with respect to insectiferous amber, that the greater part of the insects found in it exist no longer in the countries that produce that amber, and that in every different locality the insects found in it are different. Thus the amber of Sicily contains various species of _Coleoptera_ not to be met with in other ambers, while that of the Baltic is rich in _Diptera_ and _Neuroptera_[1592]. It is further observed, that the insects inclosed in the amber of Prussia, and those figured by Sendelius in his _Historia Succinorum_, all belong to genera at this time found in Europe[1593]. Insects of the following genera are recorded as having been found in this singular substance: _Platypus_, _Elater_, _Atractocerus_; _Gryllus_, _Mantis_; larvæ of _Lepidoptera_; _Trichoptera_; _Ephemera_, _Perla_, _Termes_; _Formica_; _Tipula_, _Bibio_, _Empis_; _Scolopendra_; and various _Arachnida_[1594]. In a piece of amber in my collection I find _Evania_, _Formica_, _Chironomus_, and some _Arachnida_.

Fossil insects have also been found in other substances. Parkinson figures larvæ of _Libellulina_ found in limestone[1595]; some _Melolonthæ_ in slate; a _Polistes_ in schistus; _Carabi_ and _Necrobia_ in vegetable debris: but some of these rather belong to a comparatively modern formation[1596].

* * * * *

I observed in the outset of our correspondence, that we were entering an august temple, exhibiting in its inmost sanctuary the symbols of the Divine Presence[1597]. In proportion as we have penetrated, glory from that Shechinah has more and more shone forth: and whether we have considered the uses of insects, their ways and instincts, their forms and structure, and their arrangement in a wondrous and complex system, the WISDOM, POWER and GOODNESS of their and our CREATOR have every where been marvellously conspicuous, and calculated to awaken in us every devotional feeling. If, indeed, we admire and study these little creatures, or any other department of nature, without reference to their CREATOR, and collect and love them merely for _themselves_, we shall be in some sense idolaters, and, like the ancient world, put the _works_ of GOD in his place. But if, while we admire them and store them up and study them, we see in them his glory reflected, and in the _creature_ love the CREATOR, the study of them, in conjunction with that of the written Word, will be highly beneficial to us, and at the same time that it ministers to our temporal enjoyment will promote our eternal interests.

Taking this view, I cannot better close our correspondence on the subject that has so long occupied us, than in the pious words of one of our most admired poets:

"Happy if full of days--but happier far,
If, ere we yet discern life's evening star,
Sick of the service of a world that feeds
Its patient drudges with dry chaff and weeds,
We can escape from custom's idiot sway,
To serve the Sovereign we were born t' obey.
Then sweet to muse upon his skill display'd
(Infinite skill) in all that he has made!
To trace, in Nature's most minute design,
The signature and stamp of pow'r divine,
Contrivance intricate, express'd with ease,
Where unassisted sight no beauty sees,
The shapely limb and lubricated joint,
Within the small dimensions of a point,
Muscle and nerve miraculously spun,
His mighty work, who speaks and it is done,
Th' Invisible in things scarce seen reveal'd,
To whom an atom is an ample field:
To wonder at a thousand insect forms,
These hatch'd, and those resuscitated worms,
New life ordain'd and brighter scenes to share,
Once prone on earth, now buoyant upon air,
Whose shape would make them, had they bulk and size,
More hideous foes than fancy can devise;
With helmet-heads and dragon-scales adorn'd,
The mighty myriads, now securely scorn'd,
Would mock the majesty of man's high birth,
Despise his bulwarks, and unpeople earth:
Then with a glance of fancy to survey,
Far as the faculty can stretch away,
Ten thousand rivers pour'd at his command
From urns that never fail through every land;
These like a deluge with impetuous force,
Those winding modestly a silent course;
The cloud-surmounting alps, the fruitful vales;
Seas on which every nation spreads her sails;
The sun, a world whence other worlds drink light;
The crescent moon, the diadem of night;
Stars countless, each in his appointed place,
Fast anchor'd in the deep abyss of space:--
At such a sight to catch the poet's flame,
And with a rapture like his own exclaim,
These are thy glorious works, thou source of good!
How dimly seen, how faintly understood!
Thine, and upheld by thy paternal care,
This universal frame, thus wondrous fair;
Thy power divine, and bounty beyond thought,
Adored and praised in all that thou hast wrought.
Absorb'd in that immensity I see,
I shrink abas'd, and yet aspire to thee;
Instruct me, guide me to that heavenly day,
Thy words, more clearly than thy works, display,
That, while thy truths my grosser thoughts refine,
I may resemble thee, and call thee mine.[1598]"

FOOTNOTES:

[1582] Compare what is said VOL. I. p. 47--.

[1583] VOL. III. p. 28--. See above, p. 377--.

[1584] See above, p. 546.

[1585] In _Elater_, Fabricius describes 137 species; in _Melolontha_, 149; in one section of _Rhynchænus_, 161; of _Curculio_, 183; and in his _Papiliones Heliconii_, 300.

[1586] Thus he places _Chlænius holosericæus_ and _nigricornis_, which might pass for varieties, far asunder; and _Dromius agilis_ is even put in a different section from _D. quadrimaculatus_, _truncatellus_, &c.

[1587] The continuance of this important privilege, by the lamented death of the learned President, is now rendered uncertain; but I trust we may anticipate, that by the liberality of the members of the Linnean Society, and if necessary of the public, this invaluable treasure, by being fixed in the Metropolis, will be more than ever accessible to the British Naturalist.

[1588] It may not be amiss to mention a few:--_Sphæridium dytiscoides_ is a _Hydrophilus_ related to _H. fuscipes_. _S. glabratum_ is heteromerous, probably one of the _Helopii_ Latr. _Carabus retusus_ and _Maderæ_ both belong to _Calosoma_. _Cistela angustata_ is a true _Choleva_. See _Linn. Trans._ xi. 138.--S.

[1589] See above, p. 406.

[1590] VOL. III. p. 304.

[1591] For dissections the one recommended above, p. 201, may be used. Sometimes a watchmaker's eye-glass, which also sets the hands at liberty, will be found useful.

[1592] _N. Dict. d'Hist. Nat._ xxxii. 264.

[1593] _Ibid._ xvi. 281.

[1594] _Ibid._

[1595] _Organic Remains_ iii. _t._ xvii. _f._ 2.

[1596] _Ibid._ 281--.

[1597] VOL. I. p. 20.

[1598] Cowper's _Retirement_.