The importance of insects to us both as sources of good or evil, I shall endeavour to prove at large hereafter; but for the present, taking this for granted, it necessarily follows that the study of them must also be important. For when we suffer from them, if we do not know the cause, how are we to apply a remedy that may diminish or prevent their ravages? Ignorance in this respect often occasions us to mistake our enemies for our friends, and our friends for our enemies; so that when we think to do good we only do harm, destroying the innocent and letting the guilty escape. Many such instances have occurred. You know the orange-coloured fly of the wheat, and have read the account of the damage done by this little insect to that important grain; you are aware also that it is given in charge to three little parasites to keep it within due limits; yet at first it was the general opinion of unscientific men, that these destroyers of our enemy were its parents, and the original source of all the mischief[53]. Middleton, in his "Agriculture of Middlesex," speaking of the Plant-louse that is so injurious to the bean, tells us that the lady-birds are supposed either to generate or to feed upon them[54]. Had he been an entomologist, he would have been in no doubt whether they were beneficial or injurious: on the contrary, he would have recommended that they should be encouraged as friends to man, since no insects are greater devourers of the Aphides. The confounding of the apple Aphis (A. lanigera, Myzoxyla?[55]) that has done such extensive injury to our orchards, with others, has led to proceedings still more injurious. This is one of those species from the skin of which transpires a white cottony secretion. Some of the proprietors of orchards about Evesham, observing an insect which secreted a similar substance upon the poplar, imagined that from this tree the creature which they had found so noxious was generated; and in consequence of this mistaken notion cut down all their poplars[56]. The same indistinct ideas might have induced them to fell all their larches and beeches, since they also are infested by Aphides which transpire a similar substance. Had these persons possessed any entomological knowledge, they would have examined and compared the insects before they had formed their opinions, and being convinced that the poplar and apple Aphis are distinct species, would have saved their trees.
But could an entomological observer even ascertain the species of any noxious insect, still in many cases, without further information, he may fall short of his purpose of prevention. Thus we are told that in Germany the gardeners and country people, with great industry, gather whole baskets full of the caterpillar of the destructive cabbage moth (Mamestra Brassicæ), and then bury them, which, as Roesel well observes[57], is just as if we should endeavour to kill a crab by covering it with water; for, many of them being full grown and ready to pass into their next state, which they do underground, instead of destroying them by this manœuvre, their appearing again the following year in greater numbers is actually facilitated. Yet this plan applied to our common cabbage caterpillar, which does not go underground, would succeed. So that some knowledge of the manners of an insect is often requisite to enable us to check its ravages effectually. With respect to noxious caterpillars in general, agriculturists and gardeners are not usually aware that the best mode of preventing their attacks is to destroy the female fly before she has laid her eggs, to do which the moth proceeding from each must be first ascertained. But if their research were carried still further, so as to enable them to distinguish the pupa and discover its haunts, and it would not be at all difficult to detect that of the greatest pest of our gardens, the cabbage butterfly, the work might be still more effectually accomplished. Some larvæ are polyphagous, or feed upon a variety of plants; amongst others that of the yellow-tail moth (Arctia chrysorhœa); yet gardeners think they have done enough if they destroy the web-like nests which so often deform our fruit-trees, without suspecting that new armies of assailants will wander from those on other plants which they have suffered to remain. Thus will thousands be produced in the following season, which, had they known how to distinguish them, might have been extirpated. Another instance occurred to me last year, when walking with a gentleman in his estate at a village in Yorkshire. Our attention was attracted by several circular patches of dead grass, each having a stick with rags suspended to it, placed in the centre. I at once discerned that the larva of the cock-chafer had eaten the roots of the grass, which being pulled up by the rooks that devour this mischievous grub, these birds had been mistaken by the tenant for the cause of the evil, and the rags were placed to frighten away his best friends. On inquiry why he had set up these sticks, he replied, "He could n't beer to see'd nasty craws pull up all'd gess, and sae he'd set'd bairns to hing up some aud clouts to flay 'em away. Gin he'd letten 'em alean they'd sean hev reated up all'd close." Nor could I convince him by all that I could say, that the rooks were not the cause of the evil. Even philosophers sometimes fall into gross mistakes from this species of ignorance. Dr. Darwin has observed, that destroying the beautiful but injurious woodpeckers is the only alternative for preventing the injury they do to our forest-trees by boring into them[58]; not being aware that they bore only those trees which insects have previously attacked, and that they diminish very considerably the number of such as are prejudicial to our forests.
From these facts it is sufficiently evident that entomological knowledge is necessary both to prevent fatal mistakes, and to enable us to check with effect the ravages of insects. But ignorance in this respect is not only unfit to remedy the evil; on the contrary, it may often be regarded as its cause. A large proportion of the most noxious insects in every country are not indigenous, but have been imported. It was thus that the moth (Galleria Mellonella) so destructive in bee-hives, and the asparagus beetle (Lema Asparagi) were made denizens of Sweden[59]. The insect that has destroyed all the peach-trees in St. Helena was imported from the Cape: and at home (not to mention bugs and cock-roaches) the great pest of our orchards, before mentioned, the apple Aphis, there is good reason to believe, was introduced with some foreign apple-trees. Now, extensive as is our commerce, it is next to impossible, by any precautions, to prevent the importation of these noxious agents. A cargo, or even a sample, of peas from North America might present us with that ravager of pulse, the pea-beetle (Bruchus Pisi); or the famed Hessian fly, which some years ago caused such trepidation in our cabinet, might be conveyed here in a ship-load of wheat. Leeuwenhoek's wolf (Tinea granella) might visit us, in a similar conveyance, from Holland or France. But though introduced, were Entomology a more general pursuit, their presence would soon be detected, and the evil at once nipt in the bud; whereas in a country where this science was not at all or little cultivated, they would most probably have increased to such an extent before they attracted notice, that every effort to extirpate them would be ineffectual.
It is needless to insist upon the importance of the study of insects, as calculated to throw light upon some of the obscurest points of general physiology; nor would it be difficult, though the task might be invidious, to point out how grossly incorrect and deficient are many of the speculations of our most eminent philosophers, solely from their ignorance of this important branch of Natural History. How little qualified would that physiologist be to reason conclusively upon the mysterious subject of generation, who should be ignorant of the wonderful and unlooked-for fact, brought to light by the investigations of an entomologist, that one sexual intercourse is sufficient to fertilize the eggs of numerous generations of Aphides! And how defective would be all our reasonings on the powers of nutrition and secretion, had we yet to learn that in insects both are in action unaccompanied by the circulating system and glands of larger animals!
In another point of view entomological information is very useful. A great deal of unnecessary mischief is produced, and unnecessary uneasiness occasioned, by what are called vulgar errors, and that superstitious reliance upon charms, which prevents us from having recourse to remedies that are really efficacious. Thus, for instance, eating figs and sweet things has been supposed to generate lice[60]. Nine larvæ of the moth of the wild teasel inclosed in a reed or goose quill have been reckoned a remedy for ague[61]. Matthiolus gravely affirms that every oak-gall contains either a fly, a spider, or a worm; and that the first foretells war, the second pestilence, and the third famine[62]. In Sweden the peasants look upon the grub of the cock-chafer as furnishing an unfailing prognostic whether the ensuing winter will be mild or severe; if the animal have a blueish hue (a circumstance which arises from its being replete with food) they affirm it will be mild, but on the contrary if it be white the weather will be severe: and they carry this so far as to foretell, that if the anterior part be white and the posterior blue, the cold will be most severe at the beginning of the winter. Hence they call this grub Bemärkelse-mask, or prognostic worm[63]. A similar augury as to the harvest is drawn by the Danish peasants from the mites which infest the common dung beetle (Geotrupes stercorarius), called in Danish Skarnbosse or Torbist. If there are many of these mites between the fore feet, they believe that there will be an early harvest, but a late one if they abound between the hind feet[64]. The appearance of the death's head hawk-moth (Acherontia Atropos) has in some countries produced the most violent alarm and trepidation amongst the people, who, because it emits a plaintive sound, and is marked with what looks like a death's head upon its back, regarded it as the messenger of pestilence and death[65]. We learn from Linné that a similar superstition, built upon the black hue and strange aspect of that beetle, prevails in Sweden with respect to Blaps mortisaga, L.[66]; and in Barbadoes, according to Hughes, the ignorant deem the appearance of a certain grasshopper in their houses as a sure presage of illness to some of the family[67].
One would not think that the excrements of insects could be objects of terror, yet so it has been. Many species of Lepidoptera, when they emerge from the pupa state, discharge from their anus a reddish fluid, which, in some instances, where their numbers have been considerable, has produced the appearance of a shower of blood; and by this natural fact, all those bloody showers, recorded by historians as preternatural, and regarded where they happened as fearful prognostics of impending evils, are stripped of their terrors, and reduced to the class of events that happen in the common course of nature. That insects are the cause of these showers is no recent discovery; for Sleidan relates that in the year 1553 a vast multitude of butterflies swarmed through a great part of Germany, and sprinkled plants, leaves, buildings, clothes and men, with bloody drops, as if it had rained blood[68]. But the most interesting account of an event of this kind is given by Reaumur, from whom we learn that in the beginning of July 1608 the suburbs of Aix, and a considerable extent of country round it, were covered with what appeared to be a shower of blood. We may conceive the amazement and stupor of the populace upon such a discovery, the alarm of the citizens, the grave reasonings of the learned. All agreed however in attributing this appearance to the powers of darkness, and in regarding it as the prognostic and precursor of some direful misfortune about to befall them. Fear and prejudice would have taken deep root upon this occasion, and might have produced fatal effects upon some weak minds, had not M. Peiresc, a celebrated philosopher of that place, paid attention to insects. A chrysalis which he preserved in his cabinet, let him into the secret of this mysterious shower. Hearing a fluttering, which informed him his insect was arrived at its perfect state, he opened the box in which he kept it. The animal flew out and left behind it a red spot. He compared this with the spots of the bloody shower, and found they were alike. At the same time he observed there was a prodigious quantity of butterflies flying about, and that the drops of the miraculous rain were not to be found upon the tiles, nor even upon the upper surface of the stones, but chiefly in cavities and places where rain could not easily come. Thus did this judicious observer dispel the ignorant fears and terror which a natural phenomenon had caused[69].
The same author relates an instance of the gardener of a gentleman being thrown into a horrible fright by digging up some of the curious cases, which I shall hereafter describe to you, of the leaf-cutter bees, and which he conceived to be the effect of witchcraft portending some terrible misfortune. By the advice of the priest of the parish he even took a journey from Rouen to Paris, to show them to his master: but he, happily having more sense than the man, carried them to M. Nollet, an eminent naturalist, who having seen similar productions was aware of the cause, and opening one of the cases, while the gardener stood aghast at his temerity, pointed out the grub that it contained, and thus sent him back with a light heart, relieved from all his apprehensions[70].
Every one has heard of the death-watch, and knows of the superstitious notion of the vulgar, that in whatever house its drum is heard one of the family will die before the end of the year. These terrors, in particular instances, where they lay hold of weak minds, especially of sick or hypochondriac persons, may cause the event that is supposed to be prognosticated. A small degree of entomological knowledge would relieve them from all their fears, and teach them that this heart-sickening tick is caused by a small beetle (Anobium tessellatum) which lives in timber, and is merely a call to its companion. Attention to Entomology may therefore be rendered very useful in this view, since nothing certainly is more desirable than to deliver the human mind from the dominion of superstitious fears, and false notions, which having considerable influence on the conduct of mankind are the cause of no small portion of evil.