12 MISC. PUBLICATION 273, U. 8S. DEPT. OF AGRICULTURE 
birds. Such injury is usually inconsequential in its effect upon a 
forest as a whole, and trees show remarkable powers of recovery from 
limited mechanical injury unless insects or fungi enter to complicate 
the situation. 
In most cases of damage by insects the source of injury is readily 
apparent from the very start. but even in such cases it 1s well to make 
certain whether other conditions are partly responsible before taking 
steps to control the insect pests. If insects are not the primary cause 
of injury, little benefit can be expected from the effort to control 
them. 
A forest. officer should become familiar with the appearance and 
characteristics of those insects capable of killing or injuring trees and 
destroying wood products on the area under his care. The insects 
he really needs to know are comparatively few, but ability to recog- 
nize the injurious forms comes only after considerable study, not only 
of the insect stages but of their typical work, whether it be markings 
on the bark and wood, tunneling of needles, or deforming of termi- 
nals. In the following discussions special emphasis is placed upon 
the habits and typical work of the most injurious forms; for it is 
through these that the forester first becomes acquainted with the de- 
structive species, and only after considerable experience does he learn 
to recognize insect adults and larvae dissociated from their work and 
from typical host trees. 
Adult insects can be distinguished from other small invertebrate 
animals by the fact that they have jointed bodies of three parts (head, 
thorax, and abdomen), breathe through tracheae, and have one pair 
of antennae and three pairs of legs. 
The larval form is the one most frequently encountered by the for- 
ester; but unfortunately it is difficult to distinguish insects when in 
this form by any simple characters. Usually, however, for the for- 
ester’s purpose it is sufficient to be able to recognize the larvae as 
those of insects of a certain group. ‘The forester easily acquires the 
ability to recognize some of the more common forms through becom- 
ing familar with their work. 
The insects most important from a forestry standpoint are in- 
cluded in seven main groups or orders under the large class Hexa- 
poda or Insecta. These common groups (fig. 4) include the beetles 
(Coleoptera), butterflies and moths (Lepidoptera), wasps (Hymenop- 
tera), flies (Diptera), scales and aphids (Homoptera), bugs (Hemip- 
tera), and termites (Isoptera). There are a number of other orders 
of insects, but these are less frequently encountered. Some small ani- 
mals closely related to insects, and frequently confused with them, 
are of importance in forestry. The mites, belonging to the class 
Arachnida, are sometimes injurious to trees. The spiders, belonging 
to the same class, are predacious and usually beneficial. The mil- 
lipedes and centipedes, belonging to the classes Chilopoda and Dipiop- 
oda, are occasionally of importance in the forest. 
Most insects pass through either three or four stages of develop- 
ment. The beetles, wasps, flies, butterflies, and moths pass through 
four such stages, and so are said to undergo “complete metamorphosis.” 
The adult female lays eggs, from which the second stage, the larvae, 
develop. The larvae usually are soft bodied and wormlike. The 
larvae of beetles are called grubs; those of moths and butterflies are 
