EVOLUTION OF INSECTS CARPENTER 349 



The Cretaceous insect fauna is virtually unknown, since very few 

 specimens have been found. The gap is an unfortunate one, for a 

 rapid development of the flowering plants and of the vertebrates 

 took place during this long period. It is not surprising that insects 

 of the early Tertiary period consist almost exclusively of families 

 now living and to a large extent of living genera. The Lepidoptera 

 and Isoptera first appear in early Tertiary rocks, but the nature of the 

 earliest representatives shows that these groups arose in the Mesozoic. 

 The insects of the Tertiary are better known than those of any equiv- 

 alent interval of geologic time, largely because of the Baltic amber, 

 which was formed from the resin of pine trees about 50 million years 

 ago and which has preserved types of insects that would almost cer- 

 tainly not occur in rock formations. For example, two specimens 

 of fleas, presumably from a rodent inhabiting the amber forest, have 

 been found in the amber. The amber inclusions have also enabled 

 more exact comparisons with living insects than ordinary preservation 

 would permit. There are several instances of genera being recognized 

 and established for amber species and subsequently being found in 

 existence. More remarkable still is the occurrence in the amber of 

 certain species of insects, mostly ants, which are apparently identical 

 vrith some species now living. The Baltic amber has also furnished 

 proof of the existence of social habits among the insects of that time, 

 for the ants that occur there include, in addition to males and females, 

 major and minor workers. The extent to which the complex habits 

 of living ants had already been acquired in the early Tertiary is shown 

 by the presence of plant lice attended by ants in search for honey dew, 

 and by the presence of mites attached to the ants in the same mamier 

 as is characteristic today. It is worth noting, however, that by no 

 means all of the families of insects had acquired such evolutionary 

 stability by the early Tertiary period. The bees preserved in the 

 amber, for example, belong to extinct genera. 



A study of Tertiary insects also contributes to our understanding 

 of the geographical distribution of living families and genera, many 

 of which occupied very different regions from those now inhabited 

 (see pi. 2, fig. 2; pi. 3, fig. 1). An example of this is shown in plate 3, 

 figure 2, which depicts a peculiar scorpionfly from mid-Tertiary 

 shales in Colorado; it belongs to a group now restricted to parts of 

 Asia. Hundreds of examples of such changes could be given [12]. 

 The best known of these is the occurrence in the Colorado Tertiary 

 of tsetse flies (Glossinidae), now confined to Africa. Incidently, the 

 suggestion has been made by several mammalogists that trypan- 

 osomiasis, a protozoan disease now transmitted by the tsetse flies in 

 Africa, might have been a factor in the extermination of some of the 

 Tertiary mammals in North America. 



