February, 1931 



EVOLUTION 



Page five 



In the Permian we find a few other Hving orders, — the 

 dragon-flies {Odonata) , May-flies, scorpion-flies, Neuroptera, 



1. A member of the extinct order Prodonata from the Permian of 

 Kansas. Existed only during Carboniferous and Permian. Some were 

 more than 2 V-2 feet across the wings. 



Similarly, the wings of these Carboniferous insects were un- 

 specialized; the hind pair were of the same size and shape as 

 the front pair, and the veins were alike. As we follow the 

 insects through the geological ages we can trace the evolution 

 of the peculiar features of the modern species. In some groups, 

 such as the grasshoppers and allies, the hind wing has in- 

 creased in size at the expense of the fore wing; in other groups, 

 such as the may-flies, the hind wings have become smaller and 

 smaller, while the front pair have enlarged. In the Permian 

 may-flies, for example, the wings were just alike, in the Meso- 

 zoic species the hind wings were a little smaller than the fore, 

 and in all existing species the hind wings are so small as to be 

 almost useless for flight, — in some they have been entirely lost. 



Of course, as the size and shape of the fore and hind wings 

 became different in these groups of insects, their venation was 



2. Primitive Dragon-fly from the Permian of Kansas; (specimen in 

 Harvard Museum of Comparative Zoology). Note that none of the 

 specialized features of later dragon flies are present. 



also modified; so that in living insects the veins of the fore 

 wing follow a different arrangement from those of the hind 

 wing. The evolution of these changes can be traced in the 

 fossils. Again, in the Carboniferous insects the veins were 

 distributed equally over the wings, but gradually assumed 

 other positions as the wings themselves changed in shape or 

 mode of operation. The predatory insects, for example, in 

 which the ability to fly rapidly was essential, developed pecu- 

 liarities in the veins which strengthened the wing membrane 

 for powerful strokes. 



Recent investigations on fossil insects have filled in many of 

 the gaps in the geological history of these creatures, so that 

 we are able to obtain a bird's-eye view of their evolution from 

 a consideration of the geological ranges of the major groups, 

 or orders. Most of the insects of the Carboniferous and Per- 

 mian were of orders dying out before the middle Mesozoic. 



The cockroaches, or at least insects which were like them, 

 but a little more primitive, have existed since the Upper Car- 

 ^ boniferous; but they have undergone considerable modification 

 in structure and the number of species has greatly increased. 

 In the Carboniferous the cockroaches comprised nearly SOfo 

 of all the species, but they steadily died off during the geo- 

 logical ages until now they make up less than a half of one 

 percent of the existing insects. 



3. Primitive May-fly from Permian of Kansas. The wings of 

 these insects were ahke, but in existing species the hind wing is less 

 than one-third as large as the front one. 



true bugs (Homoptera) , and the bark-lice (Psocoptera) ; but 

 they were all much simpler than the later types and belong to 

 extinct families. In the Mesozoic the wasp group (Hymenop- 

 tera) makes its first appearance, as well as the caddis-flies, 

 thrips, and the heteropterous bugs, and the ear-wigs. In the 

 Tertiary we find the white ants (Isoptera) and butterflies 

 (Lepidoptera) for the first time. 



The significant fact concerning this sequence of insect 

 groups in geological time is that the groups appear in the same 

 chronological order as we should expect from a study of the 

 structures of their living representatives. The primitive orders 

 appear first, followed in turn by more highly specialized mem- 

 bers until the present complex, intricate insects developed. 



4. Dragon-fly from the Mesozoic of Bavaria. Note how much 

 more complex rhe veins are than in the Permian specimen. (Fig. 2.) 



BROMS ON THE AIR 



A LLAN Broms, Science Editor of EVOLUTION, speaks 

 over the radio regularly three times a week as follows: 



Saturdays, 11:30 A.M. Station WOR 



Saturdays, 5:00 P.M. Station WEVD 



Sundays, 2:00 P.M. Station WPAP 



Some of his subjects this month are: "How Big Is The 

 Universe?" "Blood Will Tell," "What Is The Sun Made Of?" 

 "History In A Tree Trunk," etc. Listen in with your friends 

 and write your itnpressions to these radio stations. If Natural 

 Science radio talks are appreciated, there will be more of them. 



