ARTHROPODA. 287 



they become pests and threaten the existence of the plants and 

 animals on which they prey, and likewise that they become 

 important elements in the food supply of animals which prey 

 on them. It is only by their great reproductive power that 

 they can hold their own against their many enemies, the birds 

 and other insectivorous animals, and the accidents of climate, 

 etc. 



From a human point of view they may be the greatest pests 

 or the greatest helpers. In the voracious larval stage they 

 devour waste material as scavengers, strip vegetation, spread 

 disease, produce silk, and furnish food to the higher animals. 

 In the adult stage they may destroy crops; cross-fertilize 

 flowers in their search for nectar, which they may store for 

 themselves and their young to be intercepted in the case of 

 the bee by man ; may spread contagious diseases ; may devour 

 stored grain or by their mere presence become a nuisance to 

 man and the domestic animals. In both stages they may be 

 parasites on man and other animals. Few of the arthropods 

 are directly useful as food to man, though lobsters, cray-fish, 

 shrimps, etc., are important items in our food supply. Many 

 special devices of structure and of instinct have arisen making 

 their continued existence in the presence of their enemies 

 possible. Indeed there is no group of animals in which so 

 many and such interesting adaptations to the special conditions 

 of life are found as among the arthropods. All are provided 

 with some degree of external protective covering. Many are 

 so colored and shaped as to be inconspicuous in their natural 

 environment. Some are endowed with offensive odor and 

 taste, some with stinging organs. Others which are them- 

 selves perfectly harmless are so much like forms which are 

 repulsive or dangerous, as to be preserved thereby from their 

 enemies (see Chapter VIII). 



Many insects as ants, bees, and wasps are strikingly social 

 in their habits, and show a high degree of differentiation 

 among themselves. Among the bees a special class of females ' 

 the queens lay the fertilized eggs, the other females the 



