THE GROUPS OF ANIMALS 273 



must be several times as great. The insects furnish a greater share of 

 these than all other arthropods together. 



Arthropods are found in practically all situations that support life — in 

 fresh and salt water, in mud, burrowing in soil, on the surface of the earth 

 where they feed on animal or plant food, flying in the air, boring in trees 

 or herbaceous plants, and parasitic in or on animals. 



While most arthropods go through a fairly direct development, such 

 that they are readily recognized at all stages even by the uninitiated, 

 some of them, including many insects, have a striking metamorphosis 

 involving larva, pupa, and adult. In the larva there are groups of cells 

 forming the rudiments of the adult organs. These persist through the 

 pupa, but the rest of the larval organization disintegrates into a milky 

 mass which is doubtless partly used as nutrition for the growing adult 

 structures. 



Fig. 237. — A slider. Fig. 238. — A crab. Fig. 239.— A crayfish. 



(Courtesy of Carolijia (From. Van Cleave.) 

 Biological Supply Co.) 



Among the interesting features of arthropods is the social organization 

 among some of the insects. The bees, ants, wasps, and termites have 

 structurally different types of individuals which are also distinguished as 

 social castes, sometimes in a very complicated system. 



Worthy of note with respect to reproduction in the phylum is the 

 rather frequent occurrence of parthenogenesis. In some of the smaller 

 Crustacea there is diploid parthenogenesis, in which the eggs do not 

 experience a reduction division. Such parthenogenesis may be repeated 

 for many generations but is usually interspersed with bisexual reproduc- 

 tion at intervals. In bees and many other insects there is haploid parthe- 

 nogenesis, meaning that the egg which develops without fertilization has 

 undergone chromosome reduction. The haploid individuals thus pro- 

 duced are regularly males. 



Many members of this phylum are of economic importance to man. 

 Lobsters, crabs (Fig. 238), in some regions crayfishes (Fig. 239), and 

 shrimps are used as food, and bees collect honey in domestication. Small 

 aquatic forms are common food for game fishes. Insects often pollinate 

 flowers and are important to certain seed crops and fruits (figs). The 

 silkworm moth is a valuable adjunct to the textile industry. Many 

 species are injurious. They may destroy fruit or grain crops or shade 



