F. M. Carpenter.
A Review of our Present Knowledge of the Geological History of the Insects.
Psyche 37:15-34, 1930.

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19301 Geological History of Insects
A REVIEW OF OUR PRESENT KNOWLEDGE OF THE GEOLOGICAL HISTORY OF THE INSECTS.1
During the past ten years, subsequent to the publica- tion of Handlirsch's general account of fossil insects in Schroder's "Handbuch der Entomologie" in 1920, many important and unexpected specimens have been discovered. So profound an effect have these new fossils had upon in- sect paleontology that I venture to invite your attention this evening to a review of our present knowledge of the geological history of the insects, and to a discussion of the main problems which await solution.
First, let us consider what important discoveries the past decade has ~itnessed.~ The Carboniferous rocks, unfor- tunately, have not made a very large or an unusual con- tribution. The British coal measure insects, comprising sixty species, have been monographed by Bolton in a work which has added a great deal to our knowledge of certain extinct orders. Pruvost has described a splendid series of new fossils collected at the famous Commentry beds in the central plateau of France, and Bolton has published on a smaller assemblage of insects from the same locality and now contained in the British Museum. In this country, Cockerell has written a comprehensive account of the Car- boniferous insects of Maryland. But interesting as all these coal measure insects are, they seem to be quite typical of those which have previously been found in this horizon, 'Annual address of the retiring president of the Cambridge En- tomological Club, Jan. 14, 1930. Contribution from the Entomological *Laboratory of the Bussey Institution, Harvard University, No. 330. 'Although Handlirsch's account of fossil insects in the "Handbuch" was published in 1920, it did not include the results of several impor- tant works which appeared a few years earlier. For this reason, the latter (as Tillyard's "Mesozoic Insects of Queensland") are mentioned here.
Psit-he 37:lS.M (1930). hup //psyche einclub org/17/37-015 html



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16 Psyche co arch
including such groups as the Paleodictyoptera, the Blat- taria, the Megasecoptera, etc. In marked contrast to this, the Permian has unexpectedly furnished us with a remark- able series of specimens, representing groups which have not previously been reported from this horizon. In 1920
Handlirsch listed from the Permian 97 species of insects, aside from cockroaches; now there over 250, also exclusive .
of cockroaches, and many additional species are contained in unworked collections recently obtained from Permian deposits. For the most part, these new fossils have been secured at three widely separated localities, in Australia, North Russia, and Kansas. The Australian and Kansan specimens have almost exclusively been studied by Tillyard, while the Russian material has been investigated by Mar- tynov. Of course such fossils have thrown much light on the geological ranges and the phylogenetic origin of our existing groups of insects. We now know that many recent orders had a much longer geological history than had been supposed; for the first time the Mecoptera, Neuroptera, Coleoptera, Diptera, Odonata, Hornoptera, Psocoptera, and Orthoptera3 have been found in the Paleozoic. The Mesozoic has likewise contributed much to our col- lections during the past decade. The Triassic, which had previously been nearly a blank as far as insects were con- cerned, is now represented by about 120 species, described by Tillyard from Queensland. Martynov has published on another remarkable series from the Jurassic of Turkestan, and his studies are by no means complete. Mention should
also be made of Tillyard's monograph of the Liassic dragon- flies of England, and Ping's study of the Cretaceous insects of China. The latter work is particularly interesting since it has brought to light the only promising insect deposit of the Cretaceous which has yet been found. The Tertiary, of course, has played a large part in the recent advances of insect paleontology. The Baltic amber insects have received much attention at the hands of several specialists ; certain groups, as the Thysanura, Colembola,. and Paussidae, have been completely revised. Cockerel1 has continued his description of the Florissant and Green River



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19301 Geological History of Insects 17
insects, as well as those of the Tertiary of England and others contained in the Burmese amber. Very recently Martynov has described the insect fauna of a newly dis- covered Tertiary bed in Russia, and Pongraz has written several papers on the fossil insects of Hungary, redescrib- ing many obscure insects which Heer first studied seventy years ago. A few Tertiary insects have been collected at new localities in this hemisphere,-in Tennessee, Washing- ton, Nevada, and Argentina, all of which give much promise of providing us with a more complete series of fossils in the future.
If we take the conventional bird's-eye view of the geolog- ical history of the insects, we are at once struck by the antiquity, not only of their natural groups, such as the orders, families, and genera, but of their habits and ethol- ogy. As far back as the Oligocene, about 50 million years ago, the social Hymenoptera had already differentiated into several castes, and the ants, at least, had accomplished this by the Middle Eocene. The Baltic amber ants, as demon- strated by Wheeler, show definite polymorphism and even the higher stages of development such as ergatoid and pseu- dogynic females, and ergatomorphic males. Some of these -
Oligocene ants had also "learned to attend" plant-lice, just as many of the modern species do. Whether or not some of the amber insects belong' to species which are still existing on earth, is an open question. In the case of the ants, there are eight species which are morphologically identical with certain living ones. If these species actually are identical -and there is no evidence to the contrary-then they have existed without apparent structural change for some 50 million years4. Aside from the probability of the specific identity of some of our fossil and recent insects, it is cer- tain that most of the genera of the Tertiary are still slur- viving. Of course many of the genera which used to be more or less cosmopolitan, these many millions of years ago, are now restricted to much smaller areas,-as the dipterous genus, Glossina, or the formicid, Oecophylla. As we be- come more and more acquainted with the tropical insect 4I have used the time estimates advanced by Dr. A. Holmes, in his "Age of the Earth" (London, 1927).




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Psyche
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faunas, we even find genera which were first known to us only as fossils and supposed to have been extinct. Such a genus is Archipsoczis, described by Hagen for an amber Psocid, but subsequently found to be existing in the tropics of both the New and the Old World. Another instance is the ant Gesomyrmex, which was originally described by Mayr from the amber, and which was found many years later in the region of Borneo.
Now if we go back another hundred million years on the geological calendar, to the Middle Mesozoic, we are unable to recognize definitely any existing genera, but we do find many families quite familiar to us at the present time. Of course, as one would naturally expect, there is a marked difference in the development of the several orders. All the Mesozoic Trichoptera, for example, belong to extinct fami- lies, but many of the Orthoptera and Diptera can be in- cluded within modern families. When we reach the Permian, another 50 million years away, we observe that our recent families no longer make their appearance, but we are still able to recognize several existing orders, as the Mecootera, Neuroptera, Odonata, Diptera, etc., including types with complete metamorphosis. However, receding another 50 million years~rnaking a total of about 250 million-we come to the Upper Carboniferous, in which our earliest winged insects have been found. Here we find an assem- blage quite unlike that of any other period, including such primitive forms as the Paleodictyoptera, and a few more highly specialized groups as the Protodonah and Megase- coptexa. Only one recent order, BIattaria, has been rec- og-nized without question in the Upper Carboniferous3. Tillyard has described from even older rocks, the Devonian of Scotland, the remains of some arthropods which he con- siders to be true Collembola, but I do not believe that ento- mologists in general have accepted his conclusions. The absence of winged insects from strata below the Upper Carboniferous is particularly disconcerting, for, primitive as these coal measure insects may be, they are nevertheless Tillyard maintains that the obscure Met~opator qmsilns HandL, from the Pottsviile series of the Upper Carboniferous, ie a true Mecop teran, but this Is very dubious.
Bee G. C. Crampton's discussion, ,
Psyche, 37, 1330; p. 93.




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19301 Geological History of Insects 19
perfectly respectable insects and so far along the line of the Insecta that they show no definite relationship with other Arthropods.
With this general survey of insect paleontology in mind, I suggest that we now examine more carefully the geological ranges of the larger and more prominent insect orders. Of the extinct ones certainly the most interesting, phylo- genetically, is the Paleodictyoptera. These generalized cre- atures, which are usually regarded as the ancestors of all the other winged groups, were developed into many diver- sified families before the end of the Carboniferous ; but for some reason their glory was brought to an abrupt end, for only one species is known to have persisted into the Per- mian. Another interesting Carboniferous order was the Megasecoptera, the members of which were unique among the other known species of the period in that they possessed petiolate wings, not very much unlike those of the damsel- flies. These insects appear to have completely died out be- fore the Permian, but some recent groups are supposed to be their direct descendants,-as the Odonata and Mecop- tera. The order Protodonata, another assemblage which has never been found living, is especially famous because of the large size attained by some of its members, Meganeura of the Commentry of France having a wing-expanse of about 29 inches. This order, in contrast to the foregoing, persisted through the Permian, but apparently became ex- tinct during the Triassic. All the rest of the Carboniferous insects, excluding a few very small orders with obscure affinities, seem to fall into what we may call the Blattoid, or cockroach, complex. Handlirsch and others have attempted to divide them into separate orders, such as the Protorthop- tera and Protoblattaria, but these groups overlap in many respects. This complex, in my opinion, represents the con- verging branches which later lead to several distinct orders of insects; it represents, in other words, the trunk of the conventional phylogenetic tree, where the several branches had joined-or were in the act of joining-into one. I sus- pect that as more and more Upper Carboniferous insects are found this complex will become even more jumbled ; and I also believe that when Lower Carboniferous insects are discovered, as they eventually must, we shall find the Paleo-



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COLEOPTERA
DIPTERA
HYMENOPTERA
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TRICHOPTERA
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THYSANOPTRA
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HETEROPTERA
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LEPIDOPTERA
I I I I
MESOZ'OIC




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19301 Geological History of Insects 21
dictyoptera, Megasecoptera, and the other extinct orders of the Upper Carboniferous, joining with this complex. If we examine a diagram showing the geological ranges of the larger existing orders of insects, we are at once im- pressed by the fact that ten of them have been found in the Permian; of these, seven have been recognized in the Lower Permian, and one, the Blattids, in the Upper Carboniferous. These ten Permian orders are a rather diversified lot, in- cluding the Mecoptera, Neuroptera, Odonata, Homoptera, Psocoptera, Coleoptera, Diptera, Plectoptera, Blattaria, and Orthoptera. Since their occurrence in the Permian marks the earliest record of all these insects, except the cock- roaches, let us see if these ancient representatives are as primitive as we might expect.
The presence of Mecoptera in the Lower Permian is par- ticularly striking, inasmuch as our recent members of this order develop with complete metamorphosis. Of course since no larval or pupal forms of the Permian Mecoptera have been found, we have no direct evidence that they, too, were holometabolous; but, as Tillyard has pointed out, they are so close to our existing types in other respects that no one would deny that they also possessed holometabolism. There are many features of these ancient scorpion-flies that are most unusual. Those from the Lower Permian of Kan- sas, with which we are best acquainted, are very minute, having an expanse of about 10 mm.,-less than that of any existing species. Some of the Permian forms had short beaks, like those of the Australian Choristidae; the anten- nae were shortened and possessed fewer segments than those of any known Mecopteran except the highly special- ized Bittacidae; the males of some genera had a genital structure essentially like that of the Bittacidae, also. Fi- nally, some of the Lower Permian species had a wing vena- tion more highly specialized than that of any recent types. The Neuroptera have not yet been taken in the Lower Permian, but those of the Upper Permian were actually more specialized in their venation than their existing re- -
latives. Both the Planipennia and Megaloptera were al- ready differentiated and as highly developed along their own lines as the Mecoptera were along theirs. Although Tillyard maintains that the absence of Neuroptera in the Lower Permian collections is sufficient to show that the



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22 Psyche i March
order was a later development than the Mecoptera, I be- lieve that the Upper Permian specimens demonstrate that the Newoptera are at least as old a group as the Mecop- tera. The finding' of true Odonata in the Lower Permian was one of the surprising discoveries of the decade; pre- viously no Odonata had been known from the Paleozoic, but subsequent to the finding of the Lower Permian fossils, an Upper Permian species has also been located. All these described Permian forma possessed petiolate wings, more narrow and elongate than any recent types. As in the case of the Mecoptera, the Permian Odonata were very small, some having a wing-expanse of about 4.0 cm. The absence
of Odonata with broad wings basally from the Permian has lead Tillyard to the conclusion that the Anisoptera and the Anisozygoptera were derived from the petiolate Zygoptera during the Triassic. For my own part, I do not accept this inference, but consider that both the anisopterous and zygopterous lines were already developed during the close of the Upper Carboniferous. The occurrence of true Homop- tera in the Lower Permian is of much significance, espe- cially since we are forced to admit that they appear to be the most highly specialized of all the insects of this horizon. Numerous Hornoptera, even more highly developed, have been taken in the Upper Permian of Australia and Russia. The finding of Psocids in the Lower Permian was perhaps the most surprising discovery made in the field of foasil insects for many years. At the time when these fossils were collected, the earliest record of the order was in the Baltic amber, of Tertiary age, some 200 million years later, but subsequent to the finding of the Permian specimens, Marty- nov has taken others in the Jurassic of Turkestan. The Permian Psocids were very similar in venation to the Ho- moptera, and were nearly as highly specialized. The Plec- toptera or Ephemerida have long been recognized as very primitive insects, so their presence in Lower Permian beds is not at all surprising. The Permian species, although more primitive than any recent forms, were nevertheless well developed along certain lines and show that the order originated far in the past. The other Permian orders, the Diptera, Orthoptera, and Coleoptera, are too sparsely rep- resented in our collections just now to enable us to form any definite idea as to how far they had developed along their



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19301 Geological History of Insects 23
particular lines of evolution. But I believe that it is already evident from our discussion of the preceding groups that the Permian orders were far more highly specialized than they should be in the strata where they first make their appearance. Some of them, in fact, such as the Mecoptera, Neuroptera, Odonata, Homoptera, and Psocoptera, were so highly developed that they must have extended as distinct orders well back into the Carboniferous. Now if this is the case, one might wonder why these insects have not been found in the Upper Carboniferous. The explanation, I be- lieve, lies in the coarse nature of the Carboniferous strata in which the insects are preserved. The Lower Permian representatives of the Mecoptera, Odonata, Homoptera, and Psocoptera are very small, those of the first two orders be- ing much smaller than the average existing species of these groups; and all the Neuroptera of the Russian Permian and most of those of the Australian Permian (which is almost Triassic) are also small, their averaging wing-expanse being about two centimeters. The majority of the Carbon- iferous insect beds are composed of coarse material, and even the finest of them would hardly be capable of pre- serving such minute insects as those which we have just considered from the Permian. The average wing-expanse of the Carboniferous insects was approximately 10 em., not including the cockroaches. The wings of the latter were much smaller, but they also possessed the coriaceous texture of the recent species, and were consequently able to be pre- served regardless of their smaller dimensions. This selective nature of the Carboniferous strata has given rise to the notion that all the Carboniferous insects were "giants" ; but I do not believe this to be the fact, and predict that when some enterprising geologist discovers for us a Carboni- ferous insect bed with as perfect a preservation as the Wellington shales of Kansas, we shall find some very small insects belonging to the several recent orders which are so highly developed in the Permian.
Let us now consider the orders of insects which make their first appearance in the Mesozoic. There are five of these : the Trichoptera, Heteroptera, Dermaptera, Hymenop- tera, and Thysanoptera. Only one, the Heteroptera, has been found in the Triassic; the others are not known earlier than the Jurassic. These oldest Heteroptera were so well



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24 Psyche [March
developed along the lines of the recent species that it seems almost certain that the order must have existed in the Per- mian. The Trichoptera okthe Jurassic, on the other hand, are essentially more primitive than the extant species, many of them possessing a venation so similar to that of their contemporary Mecoptera that it is frequently difficult to distinguish the members of these two orders. Consequently, it is very doubtful if the caddis-flies will ever be found lower than the Triassic. The Hymenoptera are first represented in the Jurassic, by such forms as Siricoids, Oryssoids, and Ichneumonoids. This diversity of the Jurassic Hymenop- tera makes it rather obvious that the order had been some time in existence before that period, probably at least as far back at the lower Triassic. The only known Mesozoic Der- maptera and Thysanoptera have been taken in the Turk- estan beds, but they are both represented by characteristic types, not very much unlike certain recent species. We are now left with the two orders whose first record is in the Tertiary rocks,-the Lepidoptera and Isoptera. Only very few Lepidoptera have been found as fossils in this horizon, but these are absolutely modern in every respect, and there can be no doubt that the group arose some time in the Mesozoic.. The Isoptera are quite common in the Ter- tiary beds and are differentiated into many recent families, most of which, however, are now limited to much smaller geographical areas. There is every indication that the ter- mites will some day turn up in Jurassic and perhaps even Triassic strata.
From the foregoing discussion of the geological ranges of the larger insect orders, it is obvious that by far most of the orders have had a longer history than one would as- sume from the mere geological occurrence of the oldest species. In every case where we have enough fossils to util- ize, we see that the earliest representatives of each order are relatively highly specialized along the lines of the re- cent types; and this is particularly true of the Permian rec- ord. In order to have our diagram represent the probable true range of these orders, we must make several changes : The Mecoptera, Neuroptera, Odonata, Homoptera, Psocop- tera, Plectoptera, and of course the Blattids, would extend back to the Upper Carboniferous; the Hemiptera would re- cede to the Permian, and the Trichoptera, Dermaptera, and



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19301 Geological History of Insects 25
Thysanoptera to the Triassic, while the Lepidoptera and Isoptera would be shown as far back as the Jurassic only. Whether these estimations are substantiated or not will of course depend entirely upon the future discoveries in insect paleontology.
There is one other aspect of insect paleontology which I wish to discuss: the relative development of each order in the several geological periods. Those of you who have ne17er considered this aspect of entomology will probably be more or less surprised at some of the facts. The average entomologist9 if there be such a freak? is so accustomed to thinking of the insect orders in their present relative stand- ings that he never stops to reflect that there raust have been some period in the earth's history when the more predomi- nant of the recent orders were actually struggling for a footing; when some of our smaller groups7 now nearly ex- tinct' were the predominant ones. Or perhaps the entomol- ogist is, let us say? a hyrnenopterist? and so fond of his pets that he cannot imagine any period in the earth's history when they were not disconcertingly abundant. But a mo-