EFFECT OF PHYSICAL FACTORS IN ECOLOGY OF INSECTS IN LOGS 27 
ples taken from the logs. The method was too slow and cumbersome 
to give satisfactory results. 
Influence of the Character of Food 
Food is one of the important factors limiting insect development 
in logs. Some forms are very exacting in their food requirements 
and their development is only possible within narrow limits. Others 
are not so exacting and are therefore found under a wider range of 
conditions. Insects working in freshly cut logs may be divided into 
several ecological units on the basis of food. 
1. Insects requiring fresh cambium. These forms are living in a 
highly nutritious medium and are therefore not greatly dependent 
upon the aid of microérganisms. Examples of this group are 
many Ipidae. 
. Insects requiring fresh cambium for early development and which 
are later able to complete their growth and development in the 
outer sap wood. Many Cerambycidae and Buprestidae come under 
this head. 
3. Insects requiring fresh cambium during their early stages, but 
later capable of entering both sap wood and heart wood to com- 
plete their development. Examples are Monochammus and Chal- 
cophora. 
4. Insects which live in the wood during the entire development period, 
such as the Siricidae. 
. Insects which do not depend directly upon the wood for food, but 
live upon organisms growing in the wood. This group may be 
exemplified by Gnathotricus. 
The maximum length of the insect’s developmental period is limited 
by the time that its food remains in a usable condition. Insects of the 
. first group must complete their development while the cambium is still 
fresh and green. Thus their developmental period must be short. The 
second and third groups require green cambium for only a compara- 
tively small portion of the entire developmental period, therefore their 
cycle may be almost indefinitely lengthened. 
i) 
ol 
Influnce of Light and Heat 
Insects react in much the same way to both light and heat. Since 
as a rule these factors are closely associated with one another and, 
under outdoor conditions, usually vary synchronously, it is often im- 
possible to determine whether the effects are produced by the one or by 
the other. For this reason it seems logical that they should be considered 
together. 
