86 
Psyche 
[April 
tances that do not occur with sufficient regularity to support 
species unable to breed elsewhere. An example of this condition 
is seen in the species Drosophila listed above which breed under 
bark only where the latter is wet and fermenting due to the 
presence of yeasts. 
Three of the more extensive orders of holometabolous in- 
sects, the Coleoptera, Hymenoptera and Lepidoptera are very 
generally associated with trees although by no means exclusively 
so. This is particularly true of the Coleoptera which is of the 
three without question the oldest from a geological standpoint. 
As was hinted at the outset it seems quite apparent that the 
Hymenopterous fauna gives evidence of long association with 
trees and as will be shown in a moment the same appears to be 
true of the other orders. This implies a rather lasting and un- 
changing environment. 
Contributory to a continuity of fauna is the uniformity of 
conditions within the woody tissue. The material composition 
of the wood and bark varies only to a very limited extent and 
these offer the same food materials, supplied with water in far 
more constant proportions than food materials that are tem- 
porarily or permanently affected by drought or superabundant 
moisture. Aside from climatic changes and seasonal fluctuations 
the temperature variations of such an environment are of quite 
different order from those to be encountered in the course of a 
free and active existence in the open. In all, the conditions of 
life in wood are conducive to conservatism as they offer few in- 
centives for progressive adaptations. There is no question that 
these facts have had a profound influence upon the rapidity of 
evolution of the insects concerned. Consequently, we find at the 
present day many primitive types persisting as relicts of an 
earlier xylophagous fauna still associated with woody tissue and 
still retaining a primitive series of parasitic enemies, at least 
among the Hymenoptera. With this fauna are other insects 
that have adopted this habitat more recently and some of these 
very patently represent acquisitions derived from other habitats. 
These points may be readily clarified by the citation of 
examples taken from several orders of insects. Indeed, a very 
representative series might be drawn from the present small col- 
