14 MISC. PUBLICATION 2738, U. 8S. DEPT. OF AGRICULTURE 
called caterpillars; those of flies with two clear wings are called mag- 
gots; and those of wasphke flies with four clear wings are called 
grubs, slugs, or false caterpillars. The larvae feed and grow, the final 
size which they attain being influenced to some extent by the abun- 
dance of food and moisture. As they increase in size they molt or 
shed their skins several times. The larvae transform to the pupal or 
resting stage, and the pupae in turn change to the fourth stage, the 
adult insects. Growth takes place only in the larval stage. Although 
some adult insects do some feeding, none of them increase in size. 
Their chief function in life is to mate and produce eggs, and thus 
initiate another life cycle. 
Scale insects, aphids, bugs, and termites undergo what is called an 
“incomplete metamorphosis”; that is, they have only three forms—the 
egg, the nymph, and the adult. Growth takes place during the 
nymphal stage, in which the insect has very much the form and ap- 
pearance of the adult but lacks fully developed wings. 
Certain insects, such as the termites, aphids, and ants, have several 
specialized adult forms, Thus, in addition to the usual stages, there 
may be workers, soldiers, and secondary sexual forms. Certain scale 
insects and aphids give birth to living young without producing eggs. 
Others are able to reproduce by means of eggs laid by virgin females, 
which develop without being fertilized. In some cases, as among the 
gall midges, larvae are able to give birth to similar larvae without 
passing through other stages. These are all exceptions to the general 
rule. 
Insect damage to trees may be caused in any one of several ways. 
Adults of some species cause injuries by feeding on the leaves, twigs, 
or tender cambium, or by slitting bark or leaves in order to deposit 
egos. Adult bark beetles do considerable damage in constructing egg 
tunnels under the bark. Most commonly, however, the damage is done 
by the larvae or nymphs in their feeding on various parts of the tree. 
No damage is ever done by the insects while in the egg or pupal stages. 
The principal methods of feeding by which insects injure trees are 
chewing, sucking, and gall forming. The great majority of forest 
insects belong to the chewing group, and in the larval or the adult 
stage, or both, these chew and ingest plant material. This group in- 
cludes the leaf eaters, the cambium miners, and the wood borers. 
Aphids, scale insects, and bugs feed by sucking plant juices by means 
of slender mouth parts which they insert into the tender portions of 
the tree. A group of specialized insects irritate portions of the tree 
and thus cause it to form a swelling or gall which encloses them. The 
method of feeding has an important bearing on the methods of 
control. 
The important forest insects might be classified, for the purpose of 
discussion, according to their natural relationships, according to the 
species of trees attacked, according to the parts of the tree affected, 
or according to the stage of the life of a forest tree upon which they 
inflict their greatest injury. For the purposes of this publication, it 
seemed that the last-mentioned arrangement would be the most helpful 
for the forest field man. In this publication, therefore, the western 
forest trees will be followed through their life cycle, from seed to 
final finished product, and at each step the insects that are of greatest 
importance in injuring them will be discussed. 
