MAN IN A WORLD OF INSECTS — DE LONG 431 



are carried on. Many forage above the surface, carrying organic 

 materials below, where new tunnels or burrows are continuously made. 

 The soil is thus aerated and continuously enriched by their excretions 

 and the decomposition of their dead bodies. In this mamier soil 

 insects improve the physical properties of the soil and add to its 

 organic content. We should hasten to add that soil insects vary- 

 greatly in their feeding habits, and some that are root-feeding forms 

 in the larval stage, such as white grubs and wireworms, are quite 

 injurious and are of much more hami than benefit to man. There is 

 no question, however, that many soil-inhabiting forms are beneficial 

 and are of value to man. 



Insects destroy noxious plants. — A survey of insect feeding habits 

 has established the fact that a large proportion of insects feed on 

 plants but only a small number of these are considered pests. Many 

 of the others may be beneficial by destroying cacti, noxious weeds, or 

 undesirable deciduous plants. It often happens that when a plant 

 is introduced into a new geographic area it thrives to such an extent 

 that it becomes a pest. In some cases plant-feeding insects have been 

 introduced to bring this plant under control. The pricklypear cacti 

 ( Opuntia spp.) were at one time introduced into Australia, and by 1925 

 they had spread over some 25 million acres to form a dense, im- 

 penetrable growth. In 1925 a moth, C actohlastis cactorum (Berg.), 

 the larvae of which burrow in the cactus plants, was introduced into 

 Australia from Argentina. As a result of the continuous feeding of 

 these moth larvae the dense cactus growth is now reduced to about 1 

 percent of the area it occupied in 1925. 



Weed-feeding insects are not always beneficial. In some cases the 

 weeds may serve only as an early seasonal food plant for the produc- 

 tion of large populations which will later in the season attack and in- 

 jure cultivated plants or crops. This type of problem is seen in the 

 case of sugar-beet leafhopper. On the other hand, the insect may 

 change its food preference from a wild to a cultivated host. The 

 Colorado potato beetle, for example, originally fed on wild species of 

 Solanum and later changed to potato. 



The esthetic value of insects. — The brilliant colors and color patterns 

 of insects have been utilized by artists, jewelers, and designers. Some 

 of the butterflies, moths, and beetles have provided basic patterns in 

 many types of art. Because of their larger size their patterns are more 

 often observed, but some of the smaller insects are just as brilliantly 

 colored. For instance a tropical leafhopper, Agrosoma puJchella 

 (Guerin), has a brilliant black, white, and red pattern of bars or 

 stripes which is frequently used in Mexican and Central American 

 art. The ecology and abundance of this insect account for its use in 

 color designs there. It occurs in the lower tropical areas on shrubbery 

 and is commonly found along streams where the natives launder their 



