The assumption of the pupa state is the concomitant of an ecdysis, and so also is the appearance of the imago; but the commencement of each of these two stages precedes the ecdysis, which is merely the outward mark of the physiological processes. The ecdysis by which the pupa is revealed occurs after the completion of growth and when great changes in the internal organs have occurred and are still taking place; the ecdysis by which the imago appears comes after development has been quite or nearly completed.
Although the existence of a pupa is to the eye the most striking of the differences between Insects with perfect and those with imperfect metamorphosis, yet there is reason for supposing that the pupa and the pupal period are really of less importance than they at first sight appear to be. In Fig. 85 we showed how great is the difference in appearance between the pupa and the imago. The condition that precedes the appearance of the pupa is, however, really the period of the most important change. In Fig. 89 we represent the larva and pupa of a bee; it will be seen that the difference between the two forms is very great, while the further change that will be required to complete the perfect Insect is but slight. When the last skin of the larva of a bee or of a beetle is thrown off, it is, in fact, the imago that is revealed; the form thus displayed, though colourless and soft, is that of the perfect Insect; what remains to be done is a little shrinking of some parts and expansion of others, the development of the colour, the hardening of certain parts. The colour appears quite gradually and in a regular course, the eyes being usually the first parts to darken. After the coloration is more or less perfected—according to the species—a delicate pellicle is shed or rubbed off, and the bee or beetle assumes its final form, though usually it does not become active till after a farther period of repose.
Fig. 89.—Larva and pupa of a bee, Xylocopa violacea: A, larva; B, pupa, ventral aspect; C, pupa, dorsal aspect. (After Lucas.)
CHAPTER VI
CLASSIFICATION—THE NINE ORDERS OF INSECTS—THEIR CHARACTERS—PACKARD'S ARRANGEMENT—BRAUER'S CLASSIFICATION—CLASSIFICATIONS BASED ON METAMORPHOSIS—SUPER-ORDERS—THE SUBDIVISIONS OF ORDERS.
Classification.
We have already alluded to the fact that Insects are the most numerous in species and individuals of all land animals: it is estimated that about 250,000 species have been already described and have had scientific names given to them, and it is considered that this is probably only about one-tenth of those that really exist. The classification in a comprehensible manner of such an enormous number of forms is, it will be readily understood, a matter of great difficulty. Several methods or schemes have since the time of Linnaeus been devised for the purpose, but we shall not trouble the reader to consider them, because most of them have fallen into disuse and have only a historical interest. Even at present there exists, however, considerable diversity of opinion on the question of classification, due in part to the fact that some naturalists take the structure of the perfect or adult Insect as the basis of their arrangement, while others prefer to treat the steps or processes by which the structure is attained, as being of primary importance. To consider the relative values of these two methods would be beyond our scope, but as in practice a knowledge of the structures themselves must precede an inquiry as to the phases of development by which the structures are reached; and as this latter kind of knowledge has been obtained in the case of a comparatively small portion of the known forms,—the embryology and metamorphosis having been investigated in but few Insects,—it is clear that a classification on the basis of structure is the only one that can be at present of practical value. We shall therefore for the purposes of this work make use of an old and simple system, taking as of primary importance the nature of the organs of flight, and of the appendages for the introduction of food to the body by the perfect Insect. We do not attempt to disguise the fact that this method is open to most serious objections, but we believe that it is nevertheless at present the most simple and useful one, and is likely to remain such, at any rate as long as knowledge of development is in process of attainment.
Orders.
The great groups of Insects are called Orders, and of these we recognise nine, viz. (1) Aptera, (2) Orthoptera, (3) Neuroptera, (4) Hymenoptera, (5) Coleoptera, (6) Lepidoptera, (7) Diptera, (8) Thysanoptera, (9) Hemiptera. These names are framed to represent the nature of the wings; and there is some advantage in having the Orders named in a uniform and descriptive manner. The system we adopt differs but little from that proposed by Linnaeus.[[107]] The great Swedish naturalist did not, however, recognise the Orders Orthoptera and Thysanoptera; and his order Aptera was very different from ours.