16 MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



(Hemiptera), and termites (Isoptera). There are a number of 

 other orders of insects, but these are less frequently encountered. 

 Some small animals closely related to insects, and frequently con- 

 fused with them, are of importance in forestry. The mites, belong- 

 ing to the class Arachnida, are sometimes injurious to trees. The 

 spiders, belonging to the same class, are predaceous and usually 

 beneficial. The millipedes and centipedes, belonging to the classes 

 Chilopoda and Diplopoda, 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 metamor- 

 phosis. The adult female lays eggs, from which the second stage, 

 the larvae, develop. The larvae usually are soft bodied and worm- 

 like. The larvae of beetles are called grubs; those of moths and 

 butterflies are called caterpillars; those of flies with two clear 

 wings are called maggots; and those of wasplike 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 abundance of food and moisture. As they in- 

 crease 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 appearance of the adult, but lacks fully developed wings. 



Certain insects, such as the termites, aphids, and ants, have sev- 

 eral specialized adult forms. Thus, in addition to the usual stages, 

 there may be workers, soldiers, and secondary sexual forms. Cer- 

 tain 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 eggs. Adult bark beetles do considerable damage 

 in constructing egg tunnels under the bark. Most commonly, how- 

 ever, 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 



