246 AGRONOMY 



insects are said to have an incomplete metamorphosis. The 

 great majority of insects, however, have a complete metamor- 

 phosis. When hatched they show no sign of the kind of adult 

 insects they are designed to be. They begin life as wormlike 

 creatures called caterpillars or worms, though it should be 

 understood that they are not closely related to the true worms, 

 such as the earthworm. The young worms, or more properly 

 the larvce, feed voraciously until they reach maturity, increas- 

 ing rapidly in size and casting their skins from time to time 

 as these become too small. When full-grown they stop feed- 

 ing and either spin a cocoon about themselves or creep away 

 into some safe shelter under a loose piece of bark, along old 

 fences, or even in the soil, where they remain motionless 

 for several days, weeks, or months, during which time they 

 undergo great changes in form and structure. This stage is 

 called the pupa stage and is the one in which large numbers 

 pass the winter. At length there emerges from the dull and 

 motionless pupa a winged insect, often brightly colored, 

 which flies away to mate and deposit eggs upon the proper 

 food plant and thus start the life cycle anew. 



Forms of insects that cause injury. Crops may be injured 

 by insects in either the larval or adult stage. An insect is 

 seldom equally harmful in both stages. Usually the greatest 

 damage is caused by the voracious larvse, the mature insects 

 often living on the nectar of flowers and frequently being 

 beneficial as agents for the transfer of the pollen. In some 

 cases the larvae are much less destructive than the mature 

 insects, possibly because they feed on plants that are not 

 valued by man, while others, like the potato bug and the 

 asparagus beetle, in both their larval and adult stages are 

 injurious to crops. Some of the more harmful insects are 

 mentioned in this book. Many others, less widely distributed, 

 though often as destructive in restricted localities, may be 

 found in any work on entomology. As with plant diseases, 



