60 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
Although complete control of forest insects seldom results from the 
activities of parasites alone, the host population is often greatly re- | 
duced, and it is believed that natural enemies contribute ‘materially 
in alleviating conditions during an outbreak of the host and especially 
in lengthening the period between outbreaks. | 
Gener ally speaking, the type of management that encourages the | 
development of a forest composed of a mixture of tree species is also 
advantageous to the parasitic insect fauna existing there. Both forest 
management and the utilization of natural enemies offer permanent 
types of control. 
THE ZOOLOGICAL POSITION OF INSECTS AND 
SOME OF THEIR COMMON RELATIVES 
It is of interest to most technical workers inquiring into the name 
and habits of an animal or plant to have it oriented in the generally 
recognized classifications. Its position in relation to other known 
forms helps materially in knowing it. 
Insects and some of their relatives, such as the spiders, mites, 
scorpions, and pseudoscorpions, some of which are briefly treated here 
because of their common association with the forest or forest products 
and forest wildlife, all belong to the phylum of invertebrates known as 
Arthropoda, which have bodies composed of several segments, some 
ot which carry jointed appendages. Here also belong ‘the crawfish, 
millipedes, crabs, and many other abundant forms of life that do not 
come into the classification of forest fauna. 
The relationships of the pertinent forms are shown in the following 
outline: 
Arthropoda 
Classes : 
Grustacea 2b) in Se see ee Crawfish, shrimp, wood lice* or gribble. 
Diplopoda=== 2 Seesi acess Millipedes. 
Chilopodavs=2 Skies sees Centipedes. 
Aur CHING Ay see re eee Spiders, ete. 
Orders: 
Scorpionidas. oe. — Scorpions. 
Pseudoscorpionida__ Pseudoscorpions. 
AT AIC) Cate ae re Spiders. 
Phalancidas: =25 225 Daddy-long-legs, or harvestmen. 
NCAT INA ee Mites, redbugs, red spiders, ticks. 
ELENA OG ae ee eT SCCES g 
Mollusea 
Class: 
Pelecypoda___- Bee ee Shipworms: 
*Wood lice and shipworms are treated together in the discussion under the 
heading of marine borers. 
* Subdivided at length in later discussion. 
CRAWFISH AND SHRIMPS 
Crass CRUSTACEA 
The members of the class Crustacea, represented by crawfish, crabs, 
and shrimps, and also sowbugs and wood lice, are almost entirely 
aquatic and breathe by means of gills. Asa group, they are of utmost 
importance as a source of fish food in our streams. These, together 
with aquatic insects, form the bulk of the food of many species of fish. 
