390 APPLIED BIOLOGY 



327. Useful Insects. Honey-bees and silkworms are the 

 only truly domesticated insects. (SeeShaler's " Domesticated 

 Animals.") A few others are directly useful for their prod- 

 ucts, " Spanish flies " (used in medicine), cochineal bug 

 (cultivated on cactus for the dyes cochineal and carmine 

 which their dried bodies yield), and the lac-insect (which 

 produces the valuable shellac used in varnishes). Some 

 natives of Africa, Australia, and Mexico eat certain insects. 



As agents in cross-pollination of flowers insects as a group 

 are worth vastly more than they destroy; but it happens 

 that many of the very destructive insects do not visit and 

 pollinate flowers. Our studies of plants have made it clear 

 that many of our most useful plants depend upon pollination 

 by insects, e.g., clover, alfalfa, fruit trees, most vegetables. 

 The Smyrna fig is now successfully cultivated in California 

 because an insect imported from Algeria pollinates the flowers. 



Some insects are valuable as destroyers of injurious insects. 

 Numerous insects have their insect enemies, some predatory 

 and some parasitic. As an example of predatory insects may 

 be mentioned the history of the fluted scale-bug which once 

 threatened to destroy the orange groves of California. The 

 scale originally came from Australia, and there the ento- 

 mologists of the United States Department of Agriculture 

 found a natural enemy in a species of lady-bird beetles. 

 Some of these beetles were imported to California and in a few 

 years practically exterminated this species of scale-bug. 

 Specimens of the beetles sent later to other countries have been 

 as successful in ridding orange and lemon trees of the de- 

 structive scale-bug. This is one example of the usefulness of 

 a predatory insect. Many with similar habits are constantly 

 keeping harmful insects in check. 



Parasitic insects are important .checks on injurious insects. 

 A large number of species of insects belonging to the Diptera 

 and Hymenoptera are parasitic during their larval stage. 

 The caterpillars of many moths and butterflies are frequently 



