Chapter XII: Introduction: Habits–classification–structure–chilognatha–chilopoda (6)

Our Fig. 88, taken from Graber, represents a longitudinal median section of a full-grown larva of _Musca_, in which the processes of metamorphosis are taking place. The position of some of the more important imaginal rudiments is shown by it: _b^1_, _b^2_, _b^3_, rudiments of the three pairs of legs of the imago; _an_, of antennae; between _an_ and _w_, rudiment of eye; _w_, of wings; _h_, of halteres; _f_, fat-body; _d_, middle of alimentary canal; _n_, ventral chain; _st_, stigma; 6, 7, sixth and seventh body segments.

PHYSIOLOGY OF METAMORPHOSIS.

Many years ago, Harvey perceived the probable existence of a physiological continuity between the earlier and later stages of the Insect's life. Modern investigation has shown that in the blowfly a remarkable analogy exists between the conditions of the pupa and the egg. The outer shell of the pupa corresponds to the chorion or egg-shell, and the delicate outer membrane of the pronymph to the oolemn or lining membrane of the egg; the creamy matter corresponds with the yolk, and the regenerative buds are analogous to the formative portions of the developing egg. The process of histolysis as carried out by the phagocytes of the later life appears also to find a parallel in the vitellophags of the embryonic life.[106] It appears probable that the physiological processes of the post-embryonic metamorphosis may be essentially a repetition—or an interrupted continuation—of those of the embryonic period.

The inquiry as to what are the determining causes of the metamorphic changes of the blowfly and other Insects has as yet but little advanced. Why does the larva grow up to a certain period with great rapidity, then cease its appropriating power and break up the parts that have been so rapidly and recently formed? And why do the imaginal buds remain quiescent till the other tissues are being disintegrated, and then, instead of sharing the general condition of disintegration, commence a career of development? To these questions no satisfactory answer has yet been given, though the remarkable studies, already referred to, of Bataillon on the later larval life {169}of the silkworm suggest the direction in which knowledge may be found, for they show that the physiological conditions of the later larval life are different from those of the earlier life, possibly as the direct result of the mere aggregation of matter, and the consequent different relations of the parts of the organism to atmospheric and aqueous conditions.

If we wish to understand metamorphosis, we must supplement the old opinion that ecdysis is merely an occurrence to facilitate expansion, by the more modern conception that it is also an important physiological process. That shedding the skin is done solely to permit of enlargement of size is a view rendered untenable by many considerations. The integument can increase and stretch to an enormous extent without the aid of moulting; witness the queen-termite, and the honey-bearers of the _Myrmecocystus_ ants. Many moults are made when increase of size does not demand them, and the shedding of the skin at the time of pupation is accompanied by a decrease in size. And if moulting be merely connected with increase of size, it is impossible to see why _Cloëon_ should require two dozen moults, while _Campodea_ can do with one, or why a collembolon should go on moulting during the period of life subsequent to the cessation of growth.

The attention of entomologists has been chiefly directed to the ecdyses connected with the disclosure of the pupal and imaginal instars. Various important transformations may, however, occur previous to this, and when they do so it is always in connexion with ecdyses. Caterpillars frequently assume a different appearance and change their habits or character at a particular ecdysis; and in Orthoptera each ecdysis is accompanied by a change of form of the thoracic segments; this change is very considerable at one of the intermediate ecdyses.

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 {170}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.

{171}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 {172}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.

These Orders may be briefly defined as follows,—the reader being asked to recall the fact that by a mandibulate mouth we understand one in which the mandibles, or the maxillæ, or both, are fitted for biting, crushing, or grasping food; while the term suctorial implies that some of the mouth parts are of a tubular form or are protrusible as a proboscis, which assists, or protects, a more minute and delicate sucking apparatus:—

1. _Aptera_ (ἀ without, πτερόν a wing). Wingless[108] Insects; mouth
mandibulate or very imperfectly suctorial. Metamorphosis very little.

2. _Orthoptera_ (ὀρθός straight, πτερόν a wing). Four wings are present,
the front pair being coriaceous (leather-like), usually smaller than the
other pair, which are of more delicate texture, and contract in repose
after the manner of a fan. Mouth mandibulate. Metamorphosis slight.

3. _Neuroptera_ (νεῦρον nerve, πτερόν a wing). Four wings of membranous
{173}consistency, frequently with much network; the front pair not much,
if at all, harder than the other pair, the latter with but little or no
fanlike action in closing. Mouth mandibulate. Metamorphosis variable, but
rarely slight.

4. _Hymenoptera_ (ὑμήν membrane, πτερόν a wing). Four wings of membranous
consistency; the front pair larger than the hind, which are always small
and do not fold up in repose. Mouth mandibulate, sometimes provided also
with a tubular proboscis. Metamorphosis very great.

5. _Coleoptera_ (κολεός sheath, πτερόν a wing). Four wings; the upper
pair shell-like in consistency, and forming cases which meet together
over the back in an accurate line of union, so as to entirely lose a
winglike appearance, and to conceal the delicate membranous hind pair.
Mouth mandibulate. Metamorphosis great.

6. _Lepidoptera_ (λεπίς scale, πτερόν a wing). Four large wings covered
with scales. Mouth suctorial. Metamorphosis great.

7. _Diptera_ (δίς double, πτερόν a wing). Two membranous wings. Mouth
suctorial, but varying greatly. Metamorphosis very great.

8. _Thysanoptera_ (θύσανος fringe, πτερόν a wing). Four very narrow
fringed wings. Mouth imperfectly suctorial. Metamorphosis slight.

9. _Hemiptera_ (ἡμι half, πτερόν a wing). Four wings; the front pair
either leather-like with more membranous apex, or entirely parchment-like
or membranous. Mouth perfectly suctorial. Metamorphosis usually slight.

We must again ask the reader to bear in mind that numerous exceptions exist to these characters in most of the great Orders; for instance, wingless forms are not by any means rare in several of the Orders.

Before remarking further on this system we will briefly sketch two other arrangements of the Orders of Insects, for which we are indebted to Packard and Brauer.

PACKARD'S CLASSIFICATION.

Packard has devoted much attention to the subject, and has published two or three successive schemes, of which the following is the most recent:[109] the definitions are those of the author himself, but the information in brackets is given to institute a concordance with the system we adopt:—

1. _Thysanura._ Wingless; often with a spring (equivalent to our
_Aptera_).

2. _Dermaptera._ Front wings minute, elytra-like (= _Forficulidae_, a
part of our _Orthoptera_).

3. _Orthoptera._ Wings net-veined; fore wings narrow, hind wings folded
(= our _Orthoptera_ after subtraction of _Dermaptera_).

{174}4. _Platyptera._ Four net-veined wings; mouth parts adapted for
biting (= _Termitidae_ and _Mallophaga_, parts of our _Neuroptera_).

5. _Odonata._ Wings net-veined, equal (= _Odonata_, a division of our
_Neuroptera_).

6. _Plectoptera._ Wings net-veined, unequal (= _Ephemeridae_, a part of
our _Neuroptera_).

7. _Thysanoptera._ Mouth beaklike but with palpi (= our _Thysanoptera_).

8. _Hemiptera._ Mouth parts forming a beak for sucking. No palpi (= our
_Hemiptera_).

The above eight Orders form the group AMETABOLA, while the following eight constitute the METABOLA:—

9. _Neuroptera._ Wings net-veined; metamorphosis complete (= a small part
of our _Neuroptera_).

10. _Mecaptera._ Wings long and narrow (for a small part of our
_Neuroptera_; the _Panorpatae_ of Brauer).

11. _Trichoptera._ Wings not net-veined (= our division of _Neuroptera_
with the same name).

12. _Coleoptera._ Fore wings sheathing the hinder ones (= our
_Coleoptera_).

13. _Siphonaptera._ Wingless, parasitic. Flea (= a division of
_Diptera_).

14. _Diptera._ One pair of wings (= our _Diptera_ after subtraction of
_Siphonaptera_).

15. _Lepidoptera._ Four wings (and body) scaled (= our _Lepidoptera_).

16. _Hymenoptera._ Four clear wings; hinder pair small; a tongue (= our
_Hymenoptera_).

Although this system of the Orders of Insects has some valuable features it is open to very serious objections, to which we can only briefly allude. The Order Hemiptera with its extensive divisions, Heteroptera, Homoptera, Coccidae, and Anoplura exhibiting great differences in structure and considerable divergence in metamorphosis, is treated as only equivalent to the little group Panorpatae (scorpion-flies); these latter being considered a distinct order, although they are not very different in structure or metamorphosis from the Orders he calls Neuroptera and Trichoptera. The arrangement appears to be specially designed with the view of making the Orders adopted in it fall into the two groups Ametabola and Metabola. The propriety of such a course is more than doubtful since very few of the Ametabola are really without metamorphosis, in the wide sense of that term, while the Metabola include Insects with various kinds of metamorphosis. Indeed if we substitute for the term Ametabola the more correct expression, "Insects with little metamorphosis," and for Metabola the definition, "Insects with more metamorphosis but of various kinds," we then recognise that the arrangement {175}is, like all others, a quite artificial one, while it is of little value, owing to the development of so few Insects being hitherto fully ascertained.

BRAUER'S CLASSIFICATION.

Professor Brauer has recently proposed[110] to adopt 17 Orders or chief groups of Insects, arranging them as follows:—

I. APTERYGOGENEA (with one order).
1. _Synaptera_ (= _Aptera_ of our system).
II. PTERYGOGENEA (= all the other Insects of our arrangement).
2. _Dermaptera_ (= _Orthoptera_, Fam. _Forficulidae_ in our
arrangement).
3. _Ephemeridae_ (= a division of _Neuroptera_ in our arrangement).
4. _Odonata_ (= a division of _Neuroptera_ in our arrangement).
5. _Plecoptera_ (= _Neuroptera_, Fam. _Perlidae_ in our
arrangement).
6. _Orthoptera_ (= our _Orthoptera_ - _Forficulidae_ and +
_Embiidae_).
7. _Corrodentia_ (= the families _Termitidae_, _Psocidae_, and
_Mallophaga_, of our _Neuroptera_).
8. _Thysanoptera_ (as with us).
9. _Rhynchota_ (= _Hemiptera_ with us).
10. _Neuroptera_ (= the families _Hemerobiidæ_ and _Sialidæ_ of our
_Neuroptera_).
11. _Panorpatae_ (= the family _Panorpidae_ of our _Neuroptera_).
12. _Trichoptera_ (= the division _Trichoptera_ of _Neuroptera_).
13. _Lepidoptera_ (= as with us).
14. _Diptera_ (= our _Diptera_ - _Aphaniptera_).
15. _Siphonaptera_ (= _Aphaniptera_, a division of _Diptera_ with
us).
16. _Coleoptera_ (= _Coleoptera_).
17. _Hymenoptera_ (as with us).

The chief characters on which Brauer bases his system are: (1) The existence or absence of wings. (2) The condition of the mouth, and whether it undergoes radical changes in the ontogeny, arriving thus at the categories Menognatha, Metagnatha, and Menorhyncha, as we have mentioned on p. 161. (3) The metamorphosis; the grouping adopted being Ametabola, Hemimetabola, Metabola. (4) The number of the Malpighian tubules; Oligonephria, Polynephria. (5) The nature of the wings, the relative proportions of the thoracic segments, and some other characters.

Brauer's treatise is accompanied by a valuable and in many respects very sagacious consideration of the generalised characters of the Insecta; as a classification based partly on generalisations and partly on structures, it is, so far as the present {176}condition of our knowledge goes, a good one. But it is of little value as a practical guide, and as a basis for theoretical speculation it cannot be treated as of importance, because the generalisations it makes use of are premature, owing to the small proportion of the forms that have been examined. And even now the groups adopted are known to be subject to many exceptions.

Thus it begins by a division of Insecta into winged and wingless; but the winged division is made to comprehend an enormous number of wingless Insects, whole subdivisions of Orders such as the Mallophaga being placed in the winged series, although all are without wings. This first division is indeed entirely theoretical; and if a classification on generalisations were adopted, it would be more natural to begin with the old division into Homomorpha and Heteromorpha, and treat the Order Aptera as the first division of the Homomorpha, while the Heteromorpha would commence with the Ephemeridae and Odonata, in which, though the individual in the early part of the ontogeny is very different from the perfect Insect, there is no marked division of the later larval and the pupal stages. Brauer's system is also defective inasmuch as it takes no account of the embryological or oogenetic processes, though these are of equal importance with the later phases of the Ontogeny. Even as regards the division into Orders, it is far from being free from reproach; for instance, the separation of the Dermaptera from the Orthoptera, while Rhynchota remains intact, although including a more extensive series of heterogeneous forms; the division of the Neuroptera into widely separated groups, each of which is treated as equivalent to the great Orders, such as Coleoptera (in which Strepsiptera are included), Hymenoptera, and Diptera, is not reasonable. The association of Mallophaga and Termitidae, while Dermaptera are separated from Orthoptera, is also undeniably arbitrary, and other similar disparities are to be seen on scrutinising the details of the system.

On comparing the three arrangements we have outlined, it will be seen that the chief discrepancies they present come under two heads: (1) The treatment of the Neuroptera, opinions differing as to whether these Insects shall be grouped as a single Order, or shall be divided into numerous Orders; and as to what, if this latter course be adopted, the divisions shall be. (2) The treatment of the parasitic groups Mallophaga, Aphaniptera, etc. {177}It must be admitted that whichever of the three systems we have sketched be adopted, the result is, as regards both these points, open to criticism. The Order Neuroptera, if we take it in the broad sense, differs from the other Orders in the greater variety of metamorphosis exhibited by its members; while if, on the contrary, it be dismembered, we get a number of groups of very unequal extent and not distinguished from one another by the same decisive and important characters as are the other Orders of which they are considered equivalent. The discrepancy exists in nature, and can scarcely be evaded by any system. A similar observation may be made as to the parasitic groups, viz. Mallophaga, Anoplura, Aphaniptera, and Strepsiptera. If these be treated as separate Orders the result is not satisfactory; while, if they be associated with the larger groups to which they are respectively nearest allied, it is almost equally unsatisfactory.

We may mention that Packard and Brauer have in their treatises discussed the question of super-orders, and have gone so far as to propose names for them. These two authorities do not however agree in their conclusions; and as the names proposed are of little practical value, and are but rarely met with, we need not explain them or discuss the comparative merits of the two systems.

The divisions of inferior value to the Order are, after repeated scrutiny by many naturalists, becoming of a more satisfactory character, and notwithstanding various anomalies, may be, many of them, considered fairly natural.[111] Unfortunately entomologists have not been able to agree on a system of terminology, so that for these subdivisions terms such as sub-order, series, legion, section, tribe, etc., are used by different authorities in ways so various as to cause much confusion. In the following pages the terms sub-order and series will be used in a somewhat vague manner, the term sub-order being preferred where the group appears to be an important one and of a fairly natural character, while the word series will be adopted when the groups are connected in a conventional manner. The designation "family" will be used for groups of subordinate importance; and as regards this term we may remark that systematic entomologists are making genuine efforts to define the "families" in an accurate and comprehensible manner. The endeavour to make these systematic {178}families dependent throughout the Class Insecta on characters of similar morphological value has, however, scarcely been entered on, and it is perhaps not desirable, seeing how very small a portion of the Insects of the world have been critically examined, that much effort should be yet expended on an attempt of the kind. It must be admitted that the species of Insects should be obtained before they can be satisfactorily classified, and it is estimated[112] that at least nine-tenths of the Insects of the world are still unknown to entomologists.

GEOLOGICAL RECORD.—Although Insects have a very long pedigree, it is as yet a very imperfect one. The remains of creatures that can be referred to the Class Insecta have been found, it is said, in Silurian strata; only one or two of these very early forms are at present known, and the information about them is by no means satisfactory; if Insects at all—as to which some doubt exists—they apparently belong to very different forms, though, like all the earliest fossil Insects, they are winged. In the strata of the Carboniferous epoch numerous Insects have been detected, in both Europe and North America. These earlier Insects are by Scudder called Palaeodictyoptera, and separated from the Insects around us on the ground that he considers there existed amongst these palaeozoic Insects no ordinal distinctions such as obtain in the existing forms, but that the primeval creatures formed a single group of generalised Hexapods. Brauer does not accept this view, considering that the earlier Insects can be referred to families existing at the present time and forming parts of the Orthoptera, Neuroptera, and Hemiptera. The discrepancy between these two authorities depends to a great extent on the different classifications of existing Insects that they start from; Scudder treating the wings as of primary importance, while Brauer assigns to them only a subordinate value. From the point of view taken in the present work Scudder's view appears to be in the main correct, though his expression as to the primary fossil Insects forming a single homogeneous group is erroneous. The Neuroptera, still in existence, certainly form a heterogeneous group, and it is clear that the Palaeozoic fossils represent a more diverse assemblage than the present Neuroptera do.[113]

{179}In the more recent rocks Insect remains become comparatively numerous, and in Mesozoic strata forms that can satisfactorily be referred to existing Orders are found, the Palaeodictyoptera of Goldenberg and Scudder having mostly disappeared; the Blattidae or cockroaches do not apparently present any great discontinuity between their Palaeozoic and Mesozoic forms. The Tertiary rocks afford us fairly satisfactory evidence to the effect that Insects were then more numerous in species than they are at the present day. At Florissant in Colorado the bed of an ancient lake has been discovered, and vast quantities of Insect remains have been found in it, the geographical conditions indicating that the creatures were not brought from a distance, but were the natural fauna of the locality; and if so we can only conclude that Insects must have been then more abundant in species than they are now.

Scudder has informed us[114] that not only were Insects abundant in the Tertiaries, but that their remains indicate conditions of existence very similar to what we find around us. "Certain peculiarities of secondary sexual dimorphism accompanying special forms of communistic life, such as the neuters and workers in Hymenoptera and the soldiers among the Termitina, are also found, as would be expected, among the fossils, at least through the whole series of the Tertiaries. The same may be said of other sexual characteristics, such as the stridulating organs of the Orthoptera, and of peculiarities of oviposition, as seen in the huge egg-capsules of an extinct Sialid of the early Tertiaries. The viviparity of the ancient Aphides is suggested, according to Buckton, by the appearance of one of the specimens from the Oligocene of Florissant, while some of the more extraordinary forms of parasitism are indicated at a time equally remote by the occurrence in amber of the triungulin larva of _Meloe_, already alluded to, and of a characteristic strepsipterous Insect; not only, too, are the present tribes of gall-making Insects abundant in the Tertiaries, but their galls as well have been found."

{180}CHAPTER VII

THE ORDER APTERA–DEFINITION–CHIEF CHARACTERISTICS–THYSANURA–CAMPODEA– JAPYX–MACHILIS–LEPISMA–DIVERSITY OF INTERNAL STRUCTURE IN THYSANURA– ECTOTROPHI AND ENTOTROPHI–COLLEMBOLA–LIPURIDAE–PODURIDAE–SMYNTHURIDAE– THE SPRING–THE VENTRAL TUBE–ABDOMINAL APPENDAGES–PROSTEMMATIC ORGAN– TRACHEAL SYSTEM–ANURIDA MARITIMA–COLLEMBOLA ON SNOW–LIFE-HISTORIES OF COLLEMBOLA–FOSSIL APTERA–APTERYGOGENEA–ANTIQUITY AND DISTRIBUTION OF CAMPODEA.

ORDER I. APTERA.

_Small Insects with weak outer skin, destitute throughout life of wings
or their rudiments, but with three pairs of legs; antennae large or
moderate in size._

The above definition is the only one that can at present be framed to apply to all the Insects included in our Aptera. Unfortunately it is far from diagnostic, for it does not enable us to distinguish the Aptera from the larvae or young individuals of many Insects of other Orders. There are, however, certain characters existing in many species of Aptera that enable their possessors to be recognised with ease, though, as they are quite wanting in other members, they cannot correctly be included in a definition applying to the whole of the Order.

We are thus brought in view of two of the most important generalisations connected with the Aptera, viz. that these Insects in their external form remain throughout their life in a condition resembling the larval state of other Insects, and that they nevertheless exhibit extreme variety in structural characters.

The more important of the special characters alluded to above {181}as being possessed by some but not by all members of the Order are (1) a remarkable leaping apparatus, consisting of two elongate processes at the under side of the termination of the body; (2) a peculiar ventral tube, usually seen in the condition of a papilla with invaginated summit, and placed on the first abdominal segment (see Fig. 100, p. 194); (3) the scales covering the body; (4) the existence of abdominal appendages in the form of long cerci or processes at the termination of the body, or of short processes on the sides of the under surface of the abdominal segments.

Throughout the Order the general shape approximates to that of a larva; this is shown by the diagrammatic section of the body of _Machilis_ (Fig. 90). There is a succession of rings differing little from one another, except so far as the head is concerned; even the division of thorax from abdomen is but little evident, and although in some of the forms the three thoracic segments may differ considerably among themselves, yet they never assume the consolidated form that they do to a greater or less extent in the imago stage of the other Orders. Fig. 90 shows the larva-like structure of the body, and also exhibits the inequalities in size between some of the dorsal and the corresponding ventral plates. This phenomenon is here displayed only to a small extent, so that the true relations of the dorsal and ventral plates can be readily detected; but in the higher Insects want of correspondence of this kind may be much more extensive.

The respiratory system is in many of these Insects very inferior in development, and may even be, so far as tracheae and spiracles are concerned, entirely absent, but in other members of the Aptera it is well developed. In the other internal organs there is also great variety, as there is in the external structure.

A brief explanation as to the term Aptera, which we have adopted as the name of this Order, is necessary. This name was used by Linnaeus for our Insects, but as he associated with them various other heterogeneous forms which were afterwards separated, his "Aptera" became completely broken up and ceased {182}to be recognised as an Order of Insects. The term was, however, revived by Haeckel and Balfour several years since, and applied quite properly to the Insects we have in view. Subsequently Packard and Brauer, recognising the claims of these Insects to an isolated position, proposed for them the names Synaptera and Apterygogenea, and Packard has also used the term Cinura. There is, however, clearly an advantage in retaining the termination "ptera" for each of the Orders of Insects; and as the fact that "Aptera" of Linnaeus included many Insects is not a sufficient reason for refusing to apply the term to a portion of the forms he used it for, we may, it is clear, make use of the Linnaean name with propriety, it being explicitly stated that the Order does not include by any means all the apterous forms of Insects.

The Order includes two sub-orders, viz. (1) _Thysanura_, in which the hind body (abdomen) is composed of ten segments, and there is no ventral tube on its first segment; and (2) _Collembola_, in which the hind body consists of not more than six segments, the first of which is furnished beneath with a peculiar tube or papilla.

THYSANURA.

Our knowledge of this important sub-order has been recently much increased by the works of Grassi[115] and Oudemans.[116] Very little is known, however, of the extra-European forms, there being great difficulties in the way of collecting and preserving specimens of these Insects in such a way as to render them available for study and accurate comparison. Grassi and Rovelli[117] recognise four families among the few European species of Thysanura, viz. Campodeidae, Japygidae, Machilidae, Lepismidae. Campodeidae is perhaps limited to a single species, only one having been satisfactorily established, though several descriptions have been made of what are supposed to be other species.

This Insect (_Campodea staphylinus_) is, so far as external form goes, well known, from its having been figured in many works on natural history on account of its having been supposed to be {183}the nearest living representative of a primitive or ancestral Insect. The creature itself is but little known even to entomologists, although it is one of the commonest of Insects over a large part of Europe. It is numerous in the gardens and fields about London and Cambridge, and abounds in damp decaying wood in the New Forest; if there be only one species, it must possess an extraordinary capacity for adapting itself to extremes of climate, as we have found it at midsummer near the shores of the Mediterranean in company with the subtropical white ants, and within a day or two of the same time noticed it to be abundant on the actual summit of Mount Canigou, one of the higher Pyrenees, where the conditions were almost arctic, and it was nearly the only Insect to be found. The species is said to exist also in North America and in East India. It is a fragile, soft Insect of white colour, bending itself freely to either side like a Myriapod; the legs are rather long, the antennae are long and delicate, and the two processes, or cerci, at the other extremity of the body are remarkably similar to antennae. It has no eyes and shuns the light, disappearing very quickly in the earth after it has been exposed. If placed in a glass tube it usually dies speedily, and is so extremely delicate that it is difficult to pick it up even with a camel's hair brush without breaking it; so that we may fear it to be almost hopeless to get enough specimens from different parts of the world to learn what differences may exist amongst the individuals of this so-called primitive Insect. Meinert, a very able entomologist, considers that there is really more than one species of _Campodea_.

Campodeidae as a family may be briefly defined as Thysanura with the trophi buried in the head and with the body terminated by antenna-like processes. We shall consider some of the anatomical peculiarities of this interesting Insect after we have {184}briefly reviewed some of the external characters of the other Thysanura.

The second family (Japygidae) consists of one genus _Japyx_, of which there are, no doubt, several different species in various parts of the world, such having already been detected in tropical Africa, in Malasia, and in Mexico, as well as in Madeira and Europe. The commoner species of the latter continent, _Japyx solifugus_, lives in moss or in shady places on the edges of woods. It possesses a great resemblance to a newly-hatched earwig, and the writer has found it in France under a stone in company with a number of the tiny creatures it was so much like. This species has been found as far north as Paris, but has not been met with in Britain. The family Japygidae is, like the Campodeidae, entotrophous, and is distinguished by the body being terminated behind by a pair of forceps instead of antennary organs.

The other two families of Thysanura, Machilidae and Lepismidae, are ectotrophous—that is, the parts of the mouth are not buried in the head, but are arranged in the fashion usual in mandibulate Insects.

Only one genus of Machilidae is known, but it is no doubt very numerous in species, and probably is distributed over most of the globe. _Machilis maritima_ is common in some places on the coast of England. Another species (_M. polypoda_) occurs amongst dead leaves in the New Forest, and we have also observed a species of the genus under the loose stones that frequently form the tops of the "dykes" or piled walls in Scotland. In more southern Europe species of _Machilis_ are commonly met with on the perpendicular faces of very large stones or rocks, over which they glide with wonderful facility. The scales on the bodies of these rock-inhabiting species form pretty patterns, but are detached with such facility that it is almost impossible to obtain specimens in satisfactory condition for examination.

In Machilidae the dorsal plates of the hind body are reflexed to the under surface so as to form an imbrication covering the sides of the ventral plates, and the eyes are largely developed; by which characters the family is distinguished from the Lepismidae. The pair of large compound eyes (Fig. 92, _O_) is a remarkable feature, being indeed unique in the Aptera. The structures (_o_, _o′_) that Oudemans considers to be simple eyes have, in external appearance, a resemblance to the fenestrae of the {185}Blattidae; Grassi states, however, that not only are they eyes, but that they are of almost unique structure, being, in fact, intermediate between simple and compound eyes.

The mode of development of the compound eyes of _Machilis_ is of considerable interest, but unfortunately very little is known about it, even the period at which the eyes appear being uncertain. Judging from analogy with the Orthoptera, we should suppose them to be present when the Insect leaves the egg, and Oudemans apparently considers this to be the case, but Bolivar states[118] that in the early stages of _Machilis_ the eyes are only simple eyes; these being replaced by compound eyes in the later life. The writer has observed very young individuals of _Machilis polypoda_, and found the eyes to be evidently compound.

The remaining family of Thysanura, the Lepismidae, is in certain respects the most highly developed of the Order. The covering of scales found on the body is very remarkable in some of the species, especially in the genus _Lepisma_ (Fig. 93, _L. cincta_); the thoracic segments are different from one another {186}and from those of the abdomen, and the tracheal system is more highly developed than it is in the Machilidae. Several genera are known, but only two members of the family have yet been detected in Britain. One of them (_Lepisma saccharina_), occurs only in houses, and is sometimes called the silver fish; it is, when full grown, less than half an inch long, and is covered with scales that give it a feebly metallic lustre. Like the other Thysanura, its movements are very perfect. It is said that it is occasionally injurious by nibbling paper, but the writer's observations lead him to doubt this; its usual food is doubtless farinaceous or saccharine matter. _Thermobia furnorum_, our other British Lepismid, has only recently been discovered; it is found in bakehouses at Cambridge and elsewhere. The bakers call these Insects fire-brats, apparently considering them to be fond of heat.

Much valuable information as to the anatomy of Thysanura has been obtained by Grassi and Oudemans, and is of great interest. Taking four genera, viz. _Campodea_, _Japyx_, _Machilis_, and _Lepisma_, to represent the four families constituting the sub-order, we will briefly enumerate some of the more remarkable of the characters of their internal anatomy. _Campodea_ has a very inferior development of the tracheal system; there are three pairs of spiracles, which are situate on the thoracic region; the tracheae connected with each spiracle remain distinct, not uniting with those coming from another spiracle; there are thus six separate small tracheal systems, three on each side of the body. _Japyx solifugus_ has eleven pairs of spiracles, of which four are thoracic; the tracheae are united into one system on each side by means of lateral tubes; thus there are two extensive tracheal systems situate one on each side of the body, there being a single transverse tube, placed near the posterior extremity, uniting the two lateral systems. In _Machilis_ there are nine pairs of stigmata, two of them thoracic, seven abdominal; the tracheae from each spiracle remain unconnected, so that there are eighteen separate tracheal systems, some of which are considerably more developed than others. The Lepismidae have ten pairs of stigmata, and the tracheae connected with them are completely united into one system by longitudinal and transverse tubes. Besides these differences there are others, of considerable importance, in the position of the stigmata.

{187}All the Thysanura possess salivary glands. In _Campodea_ there are about sixteen extremely short Malpighian tubules, or perhaps glands representing these organs; _Japyx_ is destitute of these structures; _Machilis maritima_ has twenty elongate tubules; in _Lepisma_ also they are long, and apparently vary in number from four to eight in different species. The proportions of the three divisions of the alimentary canal differ extremely; there is a very large proventriculus in _Lepisma_, but not in the other families; coecal diverticula are present on the anterior part of the true stomach in _Machilis_ and in _Lepisma_, but are wanting in _Campodea_ and in _Japyx_.

The dorsal vessel seems not to present any great differences in the sub-order. Grassi says there are no alary muscles present, but Oudemans describes them as existing in _Machilis_, but as being excessively delicate.

The ventral chain of nerve-ganglia consists in _Campodea_ of one cephalic ganglion, one sub-oesophageal (which clearly belongs to the ventral series of ganglia), three thoracic, and seven abdominal. In the other families there are eight instead of seven abdominal ganglia.

The structure of the internal sexual organs is very remarkable in the Thysanura. In _Campodea_ there is one extremely large, simple tube on each side of the body. In _Japyx_ there are seven small tubes on each side, placed one in each of the successive abdominal segments, and opening into a common duct. In _Machilis_ there are also seven tubes opening into a common duct, but the arrangement is no longer a distinctly segmental one. In _Lepisma_ there are five egg-tubes on each side, the arrangement being segmental in the young state but not in the adult condition. In _Campodea_ nutrient cells alternate with the eggs in the tubes, but this is not the case in the other families. Fig. 94 shows the ovaries in three of the Thysanura; in the drawing representing this part in _Machilis_ (C), one of the two ovaries is cut away for the sake of clearness.

The male organs in _Campodea_ are very similar in size and arrangement to the ovaries, there being a single large tube or sac and a short vas deferens on each side of the body. In _Japyx_ there is a sac on each side, but it is rendered double by a coecum at its base, and there are long and tortuous vasa deferentia. In _Lepisma_ there are three pairs of coeca on each {188}side, segmentally placed and opening into a common duct. In _Machilis_ there are three retort-shaped sacs on each side opening near one another into a common duct, the vasa deferentia are elongate, and are very curiously formed, being each double for a considerable length, and the separated portions connected at intervals by five transverse commissural ducts.

One of the characteristic features of Insect structure is the restriction of articulated legs to the thoracic region. In the Thysanura there exist appendages occupying a position on the hind body somewhat similar to that of the legs on the thorax. These appendages are quite small bodies, and are placed at the hind margins of the ventral plates of the abdomen, one near each side; they are connected by a simple joint to the sternite and are provided with muscles. They are found in _Campodea_ on segments 2 to 7; in _Lepisma_ on 8 and 9, in the allied _Nicoletia_ on 2 to 9; in _Japyx_ on 1 to 7, being, however, more rudimentary than they are in _Campodea_. In _Machilis_ they attain perhaps their greatest development and exist on segments 2 to 9; moreover, in this genus such appendages occur also on the coxae of the second and third pairs of thoracic legs. Oudemans thinks they help to support the abdomen, and that they also assist in leaping; Grassi considers that they are supporting agents to some extent, but that they are essentially tactile organs. He calls them false legs "Pseudozampe."

Still more remarkable and obscure in function are the vesicles found near the appendages; we figure a pair after Oudemans, showing them in the exserted state. In the retracted state the outer portion of the vesicles is withdrawn into the basal part _P_ (Fig. 95), so that the vesicles are then only just visible, being {189}concealed by the ventral plate. The abdominal appendage is not retractile. In _Machilis_ there are twenty-two of these vesicles, arranged either two or four on one ventral plate of the hind body. They are also present in _Campodea_, where there are six pairs. They are usually said to be absent in _Japyx_ and in _Lepisma_, but Haase shows[119] that _Japyx_ possess a pair placed behind the second ventral plate of the abdomen. The vesicles appear to be exserted by the entrance of blood into them, and to be retracted by muscular agency. Much difference of opinion prevails as to their function; it appears probable that they may be respiratory, as suggested by Oudemans.

The scales found on the bodies of the Ectotrophous Thysanura may be looked on as modified hairs, and are essentially similar to those of the Lepidoptera, and they drop off as readily as do those of the Lepidoptera.

Stummer-Traunfels, who has recently published[120] the results of his researches on the mouth-organs of Thysanura and Collembola, confirms the division of the Thysanura into Entotrophi and Ectotrophi, and considers that the Collembola agree with the former group. The German author therefore proposes to divide our Aptera, not into Thysanura and Collembola, but into Ectognathi and Entognathi, the former group consisting of Machilidae and Lepismidae, the latter of Campodeidae, Japygidae and the various families of Collembola. We think it far more natural, however, to retain the older division into Thysanura and Collembola.

COLLEMBOLA.

The sub-order Collembola, which we have defined on p. 182, consists of small Insects, many of which possess the capacity of leaping, or springing suddenly, and when disturbed or alarmed naturally make use of this means of escaping. Their leaps, however, appear to be made quite at random, and very frequently do {190}not have the result of taking the creature into concealment, and in such circumstances they may be rapidly and frequently repeated until the Insect feels itself, as we may suppose, in a position of safety. Three families may be very readily distinguished, viz. (1) Lipuridae, in which no leaping apparatus is present; (2) Poduridae, a leaping apparatus exists near the extremity of the abdomen; the body is subcylindric and evidently segmented; (3) Smynthuridae, a leaping apparatus exists: the body is sub-globular with comparatively large head and abdomen, the intervening thoracic region being small; the segmentation of the body is obscure.

The study of the Collembola is much less advanced than that of the Thysanura, comparatively little having been added to our knowledge of the group since Lubbock's monograph of the British forms was published by the Ray Society in 1873. Why the Collembola should be neglected when the Thysanura attract so much attention is as inexplicable as many other fashions are.

The family Lipuridae consists of a few very small and obscure Insects of soft consistence. They move slowly, and, owing to the absence of any leaping power, attract attention less readily than the other Collembola do. Two genera are generally recognised, and they should probably form separate families; indeed, in Lubbock's arrangement they do so. In one of the genera (_Anoura_) the mouth is very imperfect, no mandibles or maxillae having been detected, while in the other genus (_Lipura_) these organs exist.

In the members of the family Poduridae, including the Degeeriidae of Lubbock, a saltatory apparatus is present in the form of appendages attached to the fifth abdominal segment (Degeeriides), or to the fourth (Podurides). These appendages are during life flexed beneath the body, but in dead specimens usually project backwards, having the appearance of a bifid tail. Poduridae are of elongate form, somewhat like small caterpillars, and are frequently prettily marked with variegate colours. Fig. 97 represents an arctic form closely allied to our native genus _Isotoma_.

{191}The peculiar shape of the members of the Smynthuridae is sufficient for their identification. They possess a very convex abdomen, and very near to it a large head, the intervening chink being occupied by the small thorax. The segmentation of the body is not easily distinguished. Nicolet states that the thorax consists of three segments and the abdomen of the same number, and that when the Insect emerges from the egg these divisions can be perceived. In after life the posterior part of the thorax becomes amalgamated with the abdomen, so that it is difficult to trace the divisions, but there appears to be no information as to the manner in which this change occurs. Some of these minute Insects frequent trees and bushes, and their leaping powers are very perfect, so that it is difficult to capture them. The family includes both the Smynthuridae and the Papiriidae of Lubbock.

The two most characteristic organs of the Collembola are the spring and the ventral tube. The first of these is an elongate structure attached to the underside of the abdomen near its extremity, either on the penultimate or ante-penultimate segment. It consists of a basal part, and of two appendages attached thereto. It is carried under the Insect bent forwards, and is retained in this position by means of a catch which projects from the under surface of the third segment of the body, descending between the two branches of the spring, and passing under the extremity of its basal segment. It is considered that the spring is elastic, is flexed under the body by muscular action, and, being retained in this position {192}of restraint by the catch, when the latter is removed the spring extends by reason of its elasticity, and the leap is thus executed. Whether this is really the exact method of leaping is, however, doubtful, for Lubbock says that the catch "only exists in certain genera"; while in its structure it does not appear to be well calculated to retain in position an organ that by virtue of its elasticity is constantly exerting a considerable force.

The ventral tube is an anomalous and enigmatic structure. In the lower forms, such as _Lipura_ or _Anurida_, it consists merely of a papilla (Fig. 100, A, _a_) more or less divided by fissure into two parts. In the Smynthuridae it is more highly developed, and protects two long delicate tubes that are capable of being protruded, as shown in the outline profile of _Smynthurus fuscus_ (Fig. 99), which is taken from specimens preserved in balsam by Mr. J. J. Lister. The nature and use of this ventral tube have given rise to much discussion. Lubbock considered, and others have agreed with him, that it serves to attach the Insect to bodies to which it may be desirable the Insect should, when in the perpendicular position, adhere. Reuter[121] assigns a quite different function to this singular structure. He states that the hairs of the body are hygroscopic, and that the peculiar claws of the Insect having collected the moisture from the hairs, the ventral tube becomes the means of introducing the liquid into the body. These Insects possess, however, a mouth, and there seems to be no reason why a complex apparatus should be required in addition to it for so simple a purpose as the introduction of moisture to the interior of the body. Haase finds[122] that Collembola can crawl on glass without the aid of the ventral tube; he considers its function to be physiological, and that it may probably be respiratory as it has been suggested is the case with the vesicles of Thysanura. The function of the ventral tube is certainly not yet satisfactorily elucidated. The vesicles contained in it are said to be extruded by blood-pressure, and withdrawn by muscular action in a manner similar to that which we have described as occurring {193}in the case of the exsertile vesicles of the Thysanura. The processes in _Smynthurus_ bear glandular structures at their extremities. It has been suggested that the ventral tube of Collembola is the homologue of a pair of ventral appendages. The term Collophore has been applied to it somewhat prematurely, seeing the doubt that still exists as to its function.

Some of the Collembola possess a very curious structure called the prostemmatic or ante-ocular organ; its nature and function have been very inadequately investigated. The ocular organs of the Collembola consist, when they are present, of isolated ocelli placed at the sides of the head like the corresponding organs of caterpillars; the prostemmate is placed slightly in front of the group of ocelli, and has a concentric arrangement of its parts, reminding one somewhat of the compound eyes of the higher Insects. This structure is represented in Fig. 100, B, C; it is said by Sir John Lubbock to be present in some of the Lipuridae that have no ocelli, and he therefore prefers to speak of it as the "post-antennal" organ.

A very characteristic feature in the Collembola is the slight development of the tracheal system. Although writers are far from being in accord as to details, it seems that stigmata and tracheae are usually absent. In _Smynthurus_ there are, however, according to Lubbock,—whose statement is confirmed by Meinert and Tullberg,—a pair of stigmata situate on the head below the antennae, and from these there extends a tracheal system throughout the body. Such a position for stigmata is almost, if not quite unique in Insects; Grassi, however, seems to have found something of the kind existing in the embryo of the bee.

At present only a small number of species of the Order Aptera are known; Lubbock recognised about sixty British species, and Finot sixty-five as found in France. The North American forms have not received so much attention as the European, and the Aptera of other countries, though they are probably everywhere fairly numerous, are scarcely known at all. A few have been described from the Indo-Malayan region and some from Chili, and the writer has seen species from the West Indian and Sandwich Islands. All the exotic forms as yet detected are very similar to those of Europe.

The Thysanura are probably not very numerous in species, and appear to be in general intolerant of cold. With the Collembola {194}the reverse is the case. They are excessively numerous in individuals; they are found nearly everywhere on the surface of the ground in climatic conditions like those of our country, while no less than sixteen species have been found in Nova Zembla and one each in Kerguelen and South Georgia. One species, if not more, of _Podura_, lives on the surface of stagnant waters, on which the minute creatures may frequently be seen leaping about in great numbers after being disturbed.

In 1874 the plain of Gennevilliers in France was copiously irrigated; in the following year the soil was still very damp, and there existed numerous pools of stagnant water, on the surface of which _Podura aquatica_ was developed in such prodigious quantity as to excite the astonishment of the inhabitants of the region.