Popular Science Monthly/Volume 70/January 1907/Fossil Insects and the Development of the Class Insecta
|FOSSIL INSECTS AND THE DEVELOPMENT OF THE CLASS INSECTA|
ADJUNCT CURATOR, ROYAL IMPERIAL NATURAL HISTORY MUSEUM, VIENNA, AUSTRIA
To the majority of mankind, who supposedly are inclined to look on the bright side of life, the sound of the word 'insects' ever recalls the picture of a wide-awake boy with a green net and possibly with a botanical box of the same hue, but more vivid in color, chasing along after the variegated butterflies and beetles. He seldom overtakes them, but positively assures us that he already has a 'nearly complete collection of insects of fifty or more species.' With this same word' insects 'many a pessimist, however, will bring to mind only the small troublesome pests of his home, perhaps even of his own worthy person, or certain minute organisms to which he indirectly ascribes the cause of the more and more frequently recurring adulteration of his wine. In each instance, the matter will be quickly despatched either with a good-natured smile or with a gentle imprecation, and only rarely does Homo sapiens attempt to make clear to himself what the word 'insects' really signifies.
That insects constitute a subdivision of the Arthropoda, to which group spiders, crabs and myriapods also belong, and that they are distinguished by the possession of only six legs and four or two wings, have with other details doubtless been acquired at school, where, too, knowledge was surely gained of many forms because of their usefulness (bees and silkworms) or because of their injurious character (moths, bark-scarabs and plant-lice).
Of the immense part that insects play in the household of nature and especially in science, however, of their truly wonderful diversity in bodily structure, of their organization, habits of living and development, as well as of the number of species, the greatest ignorance still prevails everywhere.
In proof of this not too flattering assertion, therefore, we will at once proceed to give a statistical summary, strictly in round numbers, of the insects now existing and scientifically recorded and named.
About 3,000 species of grasshoppers and crickets (Locustidæ and Gryllidæ) are known, whose music fills the woods and meadows of both hemispheres; and about 4,000 species of their nearest relatives, the locusts, or Acrididæ, to which group the notorious migratory locusts also belong. It is estimated that there are about 2,500 kinds of specters, or walking-sticks (Phasmidæ), which inhabit tropical regions chiefly and are noted for their close resemblance to twigs and leaves. Much smaller is the number of those creatures called earwigs, although they are neither worms nor crawl into the ears; scientifically they are termed Dermaptera, and comprise about 500 species. Less noteworthy are the 200 forms of small thrips, or Physopoda. The stately, but harmless, praying-crickets, or Mantidæ, are represented throughout the world by only 800 different species. On the other hand, about 1,200 kinds of cockroaches, or Blattidæ, are known, and this family unfortunately includes the small Croton-bug and its larger black cousins. In warm countries, with these troublesome creatures are associated about 400 different species of white ants. The very small insects called body-lice, book-lice or wood-lice, which belong to the Corrodentia or Copeognatha, are represented in almost equal numbers. Mallophaga (bird-lice, which should not be confounded with bird-ticks) already number 1,300 species, for nearly every kind of bird has its special parasite. On the other hand, luckily, only 50 species of true bloodsucking lice have become known, a relatively high percentage of which afflicts mankind. One hundred and sixty thousand species is certainly not too large a figure to include the hosts of beetles, or Coleoptera, which people every corner of the globe, and may be obtained in the region of perpetual ice as well as in salt marshes. We are acquainted with but 52,000 species of Hymenoptera, or membrane-winged insects, among which are the many 'wild' relatives of the honeybee, the colonies of ants, the true wasps, digger-wasps, ichneumon-flies, gall-flies, saw-flies, golden-wasps and wood-wasps. Of dragon-flies, or libellids (Odonata), there may be about 2,300 different kinds at present described, while 300 species of May-flies (Plecoptera) and stone-flies (Perlidæ) are recognized. True Neuroptera (netted-winged insects), which also include the ant-lions and lace-winged flies, number 1,400 species; Panorpidæ, or scorpion-flies, about 100, and caddice-flies, or Phryganeidæ, 1,200. After the beetles, the forms most abundant in species are the butterflies, or Lepidoptera; of these science has disclosed the existence of about 55,000 species up to the present time. Next come the much less noted two-winged insects, or Diptera, of which two main groups, Orthorrhapha (midges, gnats, horse-flies, etc.) and Cyclorrhapha (true flies), with 14,000 and 30,000 species, respectively, share in the sum total of insect forces. The number of fleas, or Suctoria, is small in comparison, only 100 species as yet being known, one of which lives on the blood of mankind. Further, if the 30,000 kinds of bugs, cicadas and plant-lice included in the Hemiptera, or half wings, are counted, the round sum of 360,000 species of insects now known is reached.
Estimates have been given showing that not more than one sixth of all forms actually existing have as yet been described and named, so that the number of species (not individuals) now living in the present period of the earth's history may be placed at about 2,000,000.
It is quite conceivable that man in his effort to understand nature everywhere surrounding him should not be satisfied merely to study all these existing insects and arrange them in a system of orders, families, genera, etc., but he would also wish to know how this greatest division of the animal kingdom, in specific numbers about doubly exceeding all other groups, has been developed, and how and when it has attained its present size.
If we would really learn the primitive history of the insect tribe, and not construct it in a speculative manner, we must descend into the depths of the earth in order to see whether or not a fortunate chance has possibly preserved some remains which might afford us an insight into the insect life of previous ages.
If, as mentioned above, the number of various species of insects now existing be taken in round numbers at 2,000,000, and for each species at least 1,000,000,000 individuals yearly, which, judging from the swarms of bees and gnats, colonies of ants and termites, parasites of plants (often millions living on a single tree), certainly seems legitimate, an annual total of 2,000,000,000,000,000 (two thousand trillions) individuals is obtained, while during the time that man has inhabited the earth some hundreds of trillions must have existed.
And of all these trillions of insect remains, which moderately computed (about 100 to a gram) represent 1,000 billion kilograms in weight, we have as yet found but a few hundred examples, and these have been accidentally enclosed in gum (copal), in peat-beds, or finally buried in hardened mud. They have thus become more or less well preserved, and again by chance have fallen into our hands. All these forms clearly demonstrate that the species of insects have not materially altered during man's sojourn on the earth.
It may now be concluded that these results must lead only to discouragement, for they show very plainly how small a percentage of the insect world escapes complete destruction, and how slight is the prospect of securing any of these remains.
Notwithstanding this, it has already come to pass that quite a number of fossil insects have been brought to light from analogous deposits of older periods, and the explanation may be partly found in the fact that even these older strata are to be estimated not only by thousands, but probably by millions of years, so that the sum total of vanished and preserved forms must evidently increase accordingly. Although in comparison with the hosts of living forms, researches hitherto made have resulted in the insignificantly small number of about 10,000 species of fossil insects, yet these few afford us a glimpse into the insect life of past ages. Such a collection of extinct species, moreover, much exceeds in numbers the recent forms in most university and private collections, which have become the basis of so many bold hypotheses. We can thus see or at least have some idea how in the course of millions of years the present mighty tree has grown up from so small and tender a plant.
Of the fossil insects thus far obtained, the larger part have come from that important period immediately preceding the age of man. This is designated the Tertiary period, or the age of mammals. Those insect remains preserved in fossil gum (Baltic amber), like artistic and permanent microscopic preparations, are indeed well known, and of these many thousand specimens have been accumulated in museums. On the other hand, less noted, but not less numerous, are the wonderful impressions found in many places in laminated shales, as in Œningen (Baden), Radoboj (Croatia), in Italy, on the Rhine, in Provence, in North America, etc. These are to be likened to nature's own printing and provide us with an atlas of the Tertiary fauna in which we find very many species that can scarcely be distinguished from those living to-day. With the exception of bird-lice, lice and fleas, all the principal existing groups of insect throngs are represented in Tertiary time, but the remarkable bizarre forms which especially delight our eyes to-day were much fewer in number then than now. Thus very few large butterflies and no striking types of beetles, such as we are accustomed to see in all shop-windows of the dealers, have been discovered.
Even though the character of the Tertiary fauna in general did not vary essentially from that now in existence, still the distribution of forms over the earth must have been far different. For instance, in Germany we find elements that now are met with only in tropical lands, from which follows many a conclusion as to the variations of climate and of the plant world. Moreover, the numerical distribution of species in kindred groups was likewise not the same as that at present in force, since among the Tertiary Hymenoptera a much smaller percentage of bees is found, among the Diptera there are more gnats than flies, among the Orthoptera far more grasshoppers than locusts, and only very few walking-sticks, etc.
Further, when it is stated that in the Tertiary period no single type of insect has been hitherto identified which does not still exist, and that therefore the numerous amber preparations and the impressions so beautifully preserved are as yet capable of giving no direct answer to our question, we must then turn to the next older period,
Tabular Summary of the Development of Insects in the Various Geological Periods
The signs denote that, compared with the same group as now existing, a group falling in a given period was relatively more abundant, smaller, or equally developed, respectively, in the next younger period.
the Mesozoic, or the age of reptiles. Of its three chief divisions, Cretaceous, Jurassic and Triassic, the first mentioned and youngest has thus far yielded only a small number of fossil insects. During the Cretaceous, the flowering plants came into existence, and on this account it may be concluded that a multitude of new conditions were furnished for many kinds of insect forms. The bees and various other honey-eaters could thus have originated. The fact that insects immediately adapted themselves to these new plants is to be seen in the few specimens thus far obtained; that is, in the galls and eaten places on the leaves of the oak, willow and Eucalyptus, etc. Other than these, unfortunately, but little evidence of insects has been found in the Cretaceous.
On the contrary, the remains of this group preserved in Jurassic deposits are very large in number. These have been discovered in England, Spain and Russia, but nowhere in such quantities and remarkable preservation as in the Jura of Franconia in northern Bavaria, where in previous epochs a shallow sea between coral reefs became filled up with the finest calcareous silt. Many of the insects which peopled the neighboring land found their graves in this mud. By a fortunate chance, after perhaps millions of years, these forms have now come to us, for this same hardened mud is to-day used by us as lithographic stone or paving-stone.
Now what does this rich collection of Jurassic insects teach us? It shows that in that period probably an entire series of groups of living forms either then had no existence or were just in the process of evolution. As yet are found no locusts, no earwigs, termites, thrips and wood-lice. Of the Diptera, the only representatives are those which are in the minority to-day; of the Hymenoptera, the wood-wasp, saw-fly and ichneumon-fly alone appear to have been present, while bees, ants, etc., are wanting. Some primitive forms of butterflies have been discovered, but these were at first erroneously regarded as cicadas. Grasshoppers were abundantly developed and some of them, judging from the structure of their legs, may have run about on the water or wet mud quite as water-striders, a genus of aquatic insects, do at the present time. Through their changed habits of living, these water locusts thus appear to have modified the legs no longer needed for jumping, and in this way the specters, or walking-sticks, may have finally originated. Dragon-flies, May-flies, Neuroptera and Hemiptera were represented in great variety, and of the last group there were aquatic species as well as those terrestrial; also small cicadas. Beetles, too, were not wanting, although no particularly striking forms are to be distinguished.
The fact that Jurassic insects were so extremely abundant clearly indicates a warm climate, and the school children of Bavaria would have to provide themselves with much larger nets should the thousands of past generations of insects celebrate a joyous resurrection, for the size of these Jurassic representatives was from four to five times that of many forms now existing in the Danube region.
But these fertile years were apparently preceded by others more barren. At least this impression is gained when we contemplate the swarms of insects that lie buried in a stage still lower—the Lias, or black Jura. The discovery of some rich localities in Switzerland, in Mecklenburg and in England, for instance, have yielded almost absolutely dwarf species. On the average, these forms were even smaller than those inhabiting the same regions to-day; truly starved species. In fact, at that time there were as yet no butterflies, few Hymenoptera, and no other striking insects. The beetles and gnats found were small and insignificant. On the other hand, caddice-flies and scorpion-flies were abundantly.represented, the latter of which now play only a limited part. There were also dragon-flies of moderate dimensions, bugs and small cicadas similar to our frog-hoppers; grasshoppers and locusts, and the ever-present cockroach as well.
From the long Triassic period that stored up a large part of the material from which the imposing dolomite towers were subsequently formed, we as yet unfortunately know only some insignificant beetles and Neuroptera. Hence, we can turn at once to that very ancient period called the Paleozoic. On important but purely material grounds, this epoch stands very close to mankind in general, since it includes the most valuable coal deposits, the mining of which has materially aided our present studies. In and near the coal in many places in Europe and North America has been found a great number of impressions of insects whose investigation furnishes us with an entirely new world of forms.
Although in the upper beds of this period no more beetles and Neuroptera are found, yet caddice-flies and scorpion-flies, gnats and locusts, too, are wanting. So much the more do the cockroaches increase! May-flies and stone-flies were already represented, and Hemiptera as well, but of a form that it is not known whether they should be pronounced cicadas or bugs.
In addition we also find insects that it may not be possible to arrange in the established classification of living forms, although affinities with the latter are undoubtedly to be recognized. The deeper we descend into the coal period, these forms more and more increase in number, while modern types gradually become less and less frequent. It may therefore be concluded that in the Carboniferous forms the direct ancestors of many of the insect groups previously mentioned are to be sought, and hence corresponding names have been chosen for them: as Protodonata, the ancestors of the Odonata, or libellids; Protorthoptera, ancestors of the Orthoptera, or locusts, etc. Nearly all these insects attained a considerable size; indeed, there were many the span of whose wings measured much more than half a meter—they were literally giants!
These forms, too, decrease in number, and at last there appears to us a quite distinct fauna of primitive creatures, whose structure was of the simplest order, and who were apparently without adaptation to the definite modes of life which we are accustomed to see in nearly all existing insects. These primitive forms we call 'Palæodictyoptera,' and among them it is possible to distinguish a series of different genera and species, all, however, having common characters and standing in about the same degree of relationship to existing groups.
These palæodictyopteres, therefore, constitute the first shoot of the giant tree which we have to-day in the insect world.
As has been frequently indicated, we also see that the race of insects has by no means remained unaltered since primitive times, but that it has been subjected to precisely the same changes as have other groups of animals. And the conclusions to be drawn from these mutations are manifold. In the first place, they permit us to erect a natural system in accordance with actual descent; they permit us to weigh the characters accurately and to distinguish between those which are old and inherited and those that are recent and acquired. Moreover, they afford us many and far-reaching conclusions regarding the climate and the nature of the soil in those times and regions, as well as the distribution of land and water, etc. Finally, by this means we are also enabled to penetrate a very little into the future. And this further shows us that eventually neither the boy with the green net nor the imprecating pessimist will be so very far wrong, for the immediate future probably belongs to the brilliantly colored insects, on the one hand, and, on the other, to the troublesome and offensive vermin, the parasites of man, animals and plants. These two extremes appear to us to-day in their greatest development.
- Translated from the German by Lucy Peck Bush, Peabody Museum, Yale University. (Mitt. d. Sect. f. Naturk. d. Osterreich. Tour.-Klub, April, 1905, pp. 25-30.)