1930 ] 
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 cm., 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 
