Economic Importance of Animals 549 



Injurious or Detrimental Insects in General. — The insects of this 

 group might be considered from the following viewpoints: (1) those 

 which annoy and attack man and other animals, (2) those which attack 

 and injure plants and crops, and (3) those which destroy and diminish 

 the values of man's commodities. 



Insects may attack man in such ways as the following: They may 

 live in or on the body as internal or external parasites. They may serve 

 as secondary hosts for certain disease-producing organisms which with- 

 out the insect could not exist for any period of time. Some species may 

 inject poisons into the body by means of stingers, nettling hairs, or 

 mouth parts. Others may influence the tastes and odors of foods be- 

 cause of repulsive odors and secretions which they produce. 



Insects may injure plants and crops in a great variety of ways. The 

 examples given will at least give some idea of the methods in which 

 this can be accompHshed. They may attack the underground stems 

 and roots; they may suck the vital sap; they may chew and destroy the 

 flowers, bark, stems, and foliage; they may bore in stems, leaves, and 

 fruits; they may construct damaging nests and shelters in various plants; 

 they may deposit eggs in or on some part of the plant which will later 

 develop into destructive forms; they may transport other injurious 

 insects to new plants and establish them there at the expense of the 

 latter; they may inject disease-producing organisms, such as bacteria, 

 Protozoa, and fungi, into plant tissues; they may destroy parts of plants, 

 particularly the leaves, which will prevent or hinder the normal process 

 of photosynthesis. If this is done, normal growth and other plant activi- 

 ties may be highly impaired. 



Insects may destroy and diminish the value of man's commodities, 

 such as foods, clothing, books, furniture, papers, drugs, bridges, houses, 

 lumber, collections of plants, and animals in museums. The above may 

 be accomplished in many ways, as can be shown by the following 

 examples: Insects may increase the expense and labor for sorting, pack- 

 ing, transporting, and preserving foods. Certain kinds, such as termites 

 (Fig. 283), may destroy wooden houses, bridges, and similar articles. 

 Clothes moths (Fig. 301) may destroy large quantities of clothing and 

 upholstered furniture. Carpet beetles may destroy rugs,, carpets, and 

 similar objects. Papers may be destroyed by such insects as the silver- 

 fishes (Fig. 273). Foods may be contaminated by insect secretions, 

 excretions, eggs, etc., even though the food may not be eaten by the 

 insects themselves. Certain species of powder-post beetles (order Co- 

 leoptera), known as lead cable borers, eat holes through leaden cover- 

 ings of aerial telephone cables, causing short circuits (Fig. 272). 



