On the other hand, many of the insects are harmful or dangerous enemies 

 of man. Some are carriers or spreaders of diseases, such as malaria and typhoid, 

 and some destroy crops. It has been estimated that the American farmer each 

 year loses one-tenth of his crops to insect pests, and in occasional outbreaks, 

 such as those of grasshoppers or chinchbugs, entire crops may be destroyed over 

 large areas. The damage done to property by such insects as clothes-moths, 

 carpet'beetles and termites is too well known to warrant discussion. 



Because of the immense numbers of insects, their universal distribution, 

 and the economic importance of many forms the study of insects, or Entomob 

 ogy, has developed into practically a science of its own. For this reason we do 

 not attempt any but the most general discussion or keying of them here. Any 

 of the standard entomologies can be used for a more comprehensive knowledge 

 of this group. These books usually refer to more technical works for specialized 

 study. 



The development of insects is an interesting subject for study. A few, 

 like the silver-fish and springtails, undergo no marked changes as they grow. 

 Most insects, however, have a complicated metamorphosis, the young of some 

 not even remotely resembling the adults. Forms like the butterfly and the 

 house'fly, which in their early stages are quite unlike the adults, are called 

 larvae. Other forms, like the grasshopper and the dragon-fly, in which the 

 young as they grow become more and more like the adults, are called nymphs 

 or naiads. If each molt leaves the insect more like the adult than its preceding 

 stage, the immature insect is called a nyynph. In some forms, such as stone-flies, 

 may-flies, dragon-flies and damsel-flies, the immature insect has developed 

 special structures adapting it for an aquatic life, which are lost when the insect 

 becomes mature. In this case the immature insect is called a naiad. An insect 

 has a non-living outer coat or exoskeleton, which it must shed periodically in 

 order to grow. The period between each shedding is called an instar. The 

 insects that undergo complete metamorphosis, such as butterflies and moths, 

 acquire at about the fifth instar a form quite different from the earlier cater- 

 pillar form. In this new condition the insect, now called the pupa, takes no 

 food and shows little outward sign of life. The outer skin is called the chry^ 

 salis. In some cases, especially in some of the moths, the caterpillar, as it 

 prepares for the pupal stage, spins from its salivary glands a covering of silk 

 threads called a cocoon and passes the pupal stage within this. Commercial 

 silk is obtained from such a cocoon, spun by the silk-worm, a native of south- 

 eastern Asia. Although apparently lifeless, the insect is now undergoing a 

 great transformation. The caterpillar is being rebuilt into a totally different 

 form. The pupal instar may last only a few weeks or it may extend over a 

 winter. When it ends, the pupal skin splits and the insect emerges in the 

 imaoQ or adult form. 



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