SEGMENTED SCHIZOCOEIS 



243 



eyes are present in the group. The compound eyes 

 are of interest in that each eye receives a portion of 

 the image and any slight movement will cause the 

 insect to be instantly aware of it. 



The adaptations of insects are numerous and a few 

 will be mentioned. Mouth parts are varied, often in 

 relation to specific food items. Feeding habits are 

 extremely varied, but some type of holozoic nutri- 

 tion is the rule. The ovipositor, an egg-laying ap- 

 paratus, is modified into a sting in bees, wasps, and 

 some ants. This structure is 4 inches long in some 

 wasps and can penetrate wood. Needless to say, the 

 sting is limited to the females. Much behavior is 

 instinct, a complex sequence of invariable automatic 

 responses to a stimulus. The social insects, ants, 

 bees, wasps, and termites, have complex societies 

 with workers, kings, queens, and soldiers. Defense 

 mechanisms are varied. Most insects depend upon 

 speed to escape harm; others "play dead." The bite 

 of many insects, sting of wasps and bees, and sting 

 "hairs" of some caterpillars are effective deterrents to 

 predators. Chemical secretions help still others; 

 some bugs and other insects produce foul odors, 

 bombardier beetles produce a "smoke screen," and 

 ants secrete an irritating liquid. Many insects 

 blend with their background or resemble plants or 

 nonliving things in their environment. Some resemble 

 ants, wasps, or spiders and supposedly gain some 

 protection from such mimicry. 



Insects in general are considered harmful, because 

 of wood borers, termites, fleas, mosquitoes, weevils, 

 scale insects, and the like, but their overall benefit 

 greatly surpasses their harm. They provide food for 

 many animals. They pollinate many plants that 

 could not exist without them. Bees produce honey 

 and wax; scale insects, shellac; and gall insects, tan- 

 nic acid. The edible crimson pigment, cochineal, is 

 produced by a Mexican scale insect. The silkworm is 

 the source of raw silk. Burrowing insects aerate the 

 soil. Most scavengers are beneficial. Some pre- 

 daceous insects feed almost entirely upon harmful 

 insects. Finally, certain insects are used for scientific 

 research that is directly or indirectly beneficial to 

 man. For example, much knowledge of heredity 

 came from studying the fruit fly, Drosophila. 



The insects have amazing reproductive powers, 

 laying from a few dozen to many thousand eggs. 

 Most insects lay eggs, but in some the eggs are re- 

 tained and hatch in the female's body. Most eggs 

 must be fertilized, but in some insects this may not be 



necessary, males are unknown. In aphids and gall 

 wasps fertilized and nonfertilized eggs alternate sea- 

 sonally. In a few insects larvae produce other larvae. 

 In the honey bee sex is determined by fertilization, 

 fertilized eggs producing females. In some insects a 

 single egg develops into many individuals; perhaps 

 more than a thousand. Eggs are laid singly or clus- 

 tered and in a site that is usually protective or offers 

 food to the young. Hatching takes from a few hours 

 to many months. Escape of young from the egg is 

 accomplished either by chewing mouth parts; by a 

 temporary, sharp head process; or by internal pres- 

 sure due to movement or air inflation of the insect. 



Growth is accompanied by many molts, each in- 

 volving replacement of the external skeleton and lin- 

 ings of the tracheae, foregut, and hindgut. Replacing 

 structures are formed prior to each molt and are 

 larger than molted parts because the insect inflates 

 before the new structures harden. Most insects have 

 four to eight molts, but some have over twenty. 

 Bristletails molt after becoming adults, but most 

 adult insects neither molt nor increase in size. The 

 developmental form between molts is called an instar. 

 The first instar is between hatching and the first molt, 

 and so on. In most insects the different instars are 

 distinct. In some, the young and adults are alike. 

 This latter condition is called direct development 

 (Figure 14.18). Other insects show some to many 

 changes, that is, metamorphosis. 







Figure 14.18 Direct development in on insect, a silverf)sh (Thysanura). 

 From left to right, egg, immature, and adult. 



Metamorphosis is of two main types, simple or 

 gradual which normally has no resting or pupa stage, 

 and complete or complex which does have a pupa 

 stage. In forms having simple metamorphosis the 

 hatchlings resemble the adults and are called nymphs 

 (Figure 14.19). The principal changes from nymph 

 to adult are in size, body proportions, formation of 



