THE PHYLA ARTHROPODA AND ONYCHOPHORA 



Importance of Insects to Man. Although a lew insects, such as the 

 silkworm and the honeybee, are specihcally useful, a vast array of insect 

 species are detrimental to man. Again, some insects have important roles in 

 the economy of nature, in the pollination of flowers, as food for many animals, 

 and as destroyers of other insects harmful to livestock and crops. More often, 

 however, the insect is destructive of the plant and animal life most necessary 

 to man. Estimates made for the United States in recent years place the total 

 insect damage to crops, domestic animals, and stored products at millions of 

 dollars annually. In the past man had to reckon more with his fellow mam- 

 mals; in the present and for the future the insects, above all other forms of 

 life, strive with man for the control of his environment. Finally, many insects 

 are dangerous to man as pests of his body and his habitations and, most im- 

 portant of all, as hosts and carriers of the microorganisms of various diseases 

 of humans. 



With increasing knowledge of habits, life cycles, and ecological relationships 

 of economically important insects, it has become possible to institute various 

 measures for their control or eradication. Natural factors of insect control, 

 such as bacterial and parasitic diseases aflfecting insects themselves, have 

 been advantageous. Mechanical methods of picking the pests from plants and 

 destroying them, as well as cutting and burning infested plants, are of 

 considerable value. 



The most widespread technique of insect control is by means of chemicals. 

 These are sometimes used to repel insects or to attract them to poisoned food 

 and into traps; but most commonly they are employed as insecticides. Arsenic, 

 sulfur, and fluorine compounds are among the oldest and most common insect 

 poisons, along with a variety of soaps, oils, nicotine compounds, and an in- 

 creasing number of very effective synthetic organic products. Among these 

 last is DDT (dichloro diphenyl trichloroethane) made famous by its extensive 

 use in World War II. Many ingenious devices have been developed for the 

 application of these insecticides to growing plants. One of the most effective 

 is the use of airplanes and helicopters to spray or dust crops and woodlands 

 on a large scale, or to reach areas not easily accessible by other means. 



Insecticides exert their effects on insects in a variety of ways, notablv as 

 "stomach poisons" when ingested with food, as in the case of arsenic com- 

 pounds, or as contact agents by clogging the spiracles or tracheal tubes, as 

 oil sprays do. The action of DDT and some of the newer organic insecticides 

 is somewhat different: DDT has been shown to operate as a nerve poison, 

 blocking the action of a specific enzyme which normally inhibits constant ex- 

 citation along nerve pathways. Under the inffuence of DDT, a susceptible 

 organism is thrown into violent and continuous muscular spasms which end 

 with the death of the animal. Not all insect species of economic importance 

 are susceptible to DDT, however; and of those which are, some populations 

 rapidly develop strains that are markedly resistant to concentrations of the 

 poison ordinarily fatal to the species. In the same manner, populations of 

 scale insects have developed in California which are resistant to the fatal 



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