30 



Artnropoas — Insects, Spiders, 

 ana Allies 



Insects are small arthropods encased in lightweight, waterproof and flexible 

 exoskeletons. Basic features of their success are — their ability to live on land, 

 their economy of space and food, and their production of many offspring. 

 Their exoskeleton protects them from the evaporation that would otherwise 

 be inevitable with small size and life in dry air. Sense organs and sensory cells 

 in abundance can be stimulated through the exoskeleton which thus becomes 

 a means of contact and adjustment to the surroundings. 



Insects have always lived with human beings; fleas have shared their blood; 

 cockroaches, their food; and silkworms provided them with draperies. Insects 

 have pressed upon humanity, hundreds of thousands of species to one of man. 

 They have crowded over the earth for ages, far longer than man has existed. 

 Many of them live together socially, ants, bees and others following inborn 

 patterns that bear undeniable resemblances to those of human society. Not 

 only are insects and man associated with one another, but among all animals 

 they are the two paragons of social life. 



Characteristics. Insects can fly. In this they are unique among invertebrates 

 as birds are unique among the vertebrates. There are relatively few adult 

 insects that cannot fly — primitive species and confirmed parasites such as lice 

 and fleas. Immature insects do not fly except the mayflies and these do so only 

 when they are in a subadult stage. 



All insects are clothed in an integument, the living epidermis or "skin" and 

 the nonliving exoskeleton or cuticle which it secretes (Figs. 30.1, 30.3). 

 The exoskeleton of insects differs from that of crustaceans in the absence of 

 lime and importance of chitin. The terms exoskeleton and cuticle are both 

 used for the secreted layer but the latter suggests its chemical content and 

 applies especially to insects. The best-known component of cuticle is chitin, a 



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