[LETTER XXVIII.]
DEFINITION OF THE TERM INSECT.
What is an insect? This may seem a strange question after such copious details as have been given in my former Letters of their history and economy, in which it appears to have been taken for granted that you can answer this question. Yet in the scientific road which you are now about to enter, to be able to define these creatures technically is an important first step which calls for attention. You know already that a butterfly is an insect—that a fly, a beetle, a grasshopper, a bug, a bee, a louse, and flea, are insects—that a spider also and centipede go under that name; and this knowledge, which every child likewise possesses, was sufficient for comprehending the subjects upon which I have hitherto written. But now that we are about to take a nearer view of them—to investigate their anatomical and physiological characters more closely—these vague and popular ideas are insufficient. In common language, not only the tribes above mentioned, but most small animals—as worms, slugs, leeches, and many similar creatures, are known by the name of insects. Such latitude, however, cannot be admitted in a scientific view of the subject, in which the class of insects is distinguished from these animals just as strictly as beasts from birds, and birds from reptiles and amphibia, and these again from fishes. Not, indeed, that the just limits of the class have always been clearly understood and marked out. Even when our correspondence first commenced, animals were regarded as belonging to it, which since their internal organization has been more fully explained, are properly separated from it. But it is now agreed on all hands, that an earthworm, a leech, or a slug, is not an insect; and a Naturalist seems almost as much inclined to smile at those who confound them, as Captain Cook at the islanders who confessed their entire ignorance of the nature of cows and horses, but gave him to understand that they knew his sheep and goats to be birds.
You will better comprehend the subsequent definition of the term Insect, after attending to a slight sketch of the chief classifications of the animal kingdom, more especially of the creatures in question, that have been proposed. That of Aristotle stands first. He divides animals into two grand sections, corresponding with the Vertebrata and Invertebrata of modern Zoologists: those, namely, that have blood, and those that have it not[1]:—by this it appears that he only regarded red blood as real blood; and probably did not suspect that there was a true circulation in his Mollusca and other white-blooded animals. His Enaima, or animals that have blood, he divides into Quadrupeds, Birds, Fishes, Cetacea, and Apods or reptiles; though he includes the latter, where they have four legs, amongst the quadrupeds[2]; and his Anaima, or animals without blood, into Malachia, Malacostraca, Ostracoderma, and Entoma. The first of these, the Malachia, he defines as animals that are externally fleshy and internally solid, like the Enaima; and he gives the Sepia as the type of this class, which answers to the Cephalopoda of the moderns. The next, the Malacostraca, synonymous with the Crustacea of Cuvier and Lamarck, are those, he says, which have their solid part without and the fleshy within, and whose shell will not break, but splits, upon collision[3]. The Ostracoderma, corresponding with the Testacea of Linné, he also defines as having their fleshy substance within, and the solid without; but whose shell, as to its fracture, reverses the character of the Malacostraca. He defines his last class Entoma, in Latin Insecta, with which we are principally concerned, as animals whose body is distinguished by incisures, either on its upper or under side, or on both, and has no solid or fleshy substance separate, but something intermediate, their body being equally hard both within and without[4]. This definition would include the Annelida and most other Vermes of Linné, except the Testacea, which accordingly were considered as insects by those Zoologists that intervened between Aristotle and the latter author. The Stagyrite, however, in another place, has expressly excluded all apods[5]. From other passages in his works, it appears that he regarded the Vermes, &c. either as larvæ, or as produced spontaneously and not ex ovo[6].
This definition of an insect, though partly founded on misconception, as well as his primary division of animals in general, is by no means contemptible. If you look at a bee or a fly, you will observe at first sight that its body is insected, being divided as it were into three principal pieces—head, trunk, and abdomen[7]; and if you examine it more narrowly, you will find that the two last of these parts, especially the abdomen, are further subdivided. And this character of insection, or division into segments, more or less present in almost every insect[8], is not to be found (with the exception of the Crustacea, which Aristotle distinguishes by the nature of their integument and its contents) in any of the other classes into which he divided animals without blood. It was on account of this most obvious of their characters, that these little creatures were in Greek named Entoma, and in Latin Insecta; and from the former word, as you know, our favourite science takes the name of Entomology.
Pliny adhering to the definition of Aristotle, as far as it relates to the insection of the animals we are speaking of, expressly includes Apods, as well as Aptera, amongst them[9]; and in this was followed, without any attempt at improvement, by all the entomological writers that intervened between him and the great Aristotle of the moderns, Linné.
This illustrious naturalist, aware of the incorrectness of the primary divisions of the animal kingdom founded upon the presence or absence of blood, establishes his system upon the structure of the heart, and upon the temperature and colour of the circulating fluid. He divided animals into two great sections or sub-kingdoms, each comprising two classes. His first section included those having a heart with two ventricles, two auricles, and warm and red blood, viz. the Mammalia or beasts, and the Aves or birds. His second, those having a heart with one ventricle, one auricle, and cold and red blood, namely, the classes Amphibia, which included reptiles, serpents, &c. and Pisces or fish. His third, those having a heart with one ventricle and no auricle, and cold white sanies in the place of blood, namely, his classes Insecta et Vermes, including the Invertebrate animals of Lamarck. Thus the first of Aristotle's great divisions he increased by the addition of a new and very distinct class, the Amphibia, by which some ground was gained in the science; but as much was lost by his compressing the four classes of which the last consisted into two, by which the natural classes of Cephalopoda and Crustacea merged under Insecta and Vermes. Linné was not aware of the extraordinary fact, that the Cephalopoda have three hearts; and that though the Crustacea and Arachnida have a circulation, Insects have none, or he would never have taken this retrograde step.
Indeed Linné's definition of an Insect is, in many most material points, inapplicable, not only to the Crustacea, but to many other animals included under that denomination. This will appear evident from a very slight examination. Thus it runs: "Polypod animalcula, breathing by lateral spiracles, armed every where with an osseous skin, whose head is furnished with movable sensitive antennæ[10]." Now of this definition only the first member can be applied to the whole class which it is meant to designate; for the entire genus Cancer L., which, with some others, forms the class Crustacea of the moderns, does not respire by spiracles at all, but by gills; and the same in some degree may be said of spiders, scorpions, &c. With the last member of the definition Linné himself must have been aware that a large number of what he conceived to be insects were at variance, as mites, spiders, and many other of his apterous tribes: though from some very recent observations of M. Latreille[11], there seems some ground for thinking, that in these the antennæ are represented by the mandibles, palpi, &c.[12], and to the soft flexible, coriaceous or membranous skin of a vast number of insects, the term cutis ossea is by no means applicable.
Evident as these incongruities are, when the Herculean task which Linné imposed upon himself, and the vastness and variety of his labours, are considered, they become very venial. Indeed, unless he had divided his class Insecta into two or more, it was impossible to define it intelligibly to ordinary readers, otherwise than nearly in the terms which he actually employed; and these characters, restricted and amended by qualifying clauses, are still those to which recurrence must be had in a popular definition of the class, when separated as it ought to be from the Crustacea and Arachnida.
Pennant, Brisson, and other zoologists, who, attending to nature rather than system, saw the impropriety of uniting a crab or a lobster in the same class with a bee or a beetle, long since assigned the Crustacea their ancient distinct rank. "But these changes," as Latreille observes[13], "being only founded upon external characters, might be deemed arbitrary; and to fix our opinion, it was necessary to have recourse to a decisive authority—the internal and comparative organization of these animals." It results from the observations of the most profound comparative anatomist of our age, M. Cuvier, that the Crustacea and Arachnida differ from insects properly so called, and particularly from those that are furnished with wings, in having a complete system of circulation, a different mode of respiration, and that they have a more perfect organization. Influenced by these motives, both Cuvier and Lamarck have considered them as forming two classes separate from insects. Treviranus, led by considerations founded on the organs of circulation, of respiration, and of generation, is of opinion that spiders and scorpions ought to form one class with the Crustacea: he observes, however, that the nervous system of all three is very dissimilar; and that in an arrangement founded on this circumstance, the organs of motion, and the external shape, even spiders and scorpions must be placed in different classes[14].