Section 1

The Insect World

• The insect body is divided into three tagmata.The head has mandibles and one pair of unbranched antennae; the thorax has three pairs of jointed legs and, in many species, one or two pairs of wings; and the abdomen has 9 to 11 segments but neither wings nor legs in adults.

• Insects live in almost every terrestrial and freshwater environment. Factors responsible for their success include their ability to fly, exoskeleton, jointed appendages, small size, and short life span.

• Insects negatively affect humans by competing for food, transmitting diseases, and destroying buildings and other manufactured products. However, insects are also beneficial. They serve as food for other animals, pollinate flowers, make valuable products such as honey, and recycle nutrients.

• The mouthparts of insects are often specialized for tearing and cutting solid food or for sucking or soaking up liquid food.

• Insects have an open circulatory system that transports nutrients through the body. Gas exchange occurs by means of air-filled tracheae that reach deep into the body. Malpighian tubules remove cellular wastes from the hemolymph while conserving water.

• Insect sensory structures include simple and compound eyes, sound-sensing tympana in some species, and sensory hairs on the antennae and other body parts.

• Most insects go through metamorphosis. In incomplete metamorphosis, a nymph hatches from an egg and resembles the adult but has undeveloped reproductive organs and no wings. The nymph molts several times to become an adult.

• In complete metamorphosis, a wormlike larva hatches from an egg and molts several times before becoming a pupa. The pupa molts to produce the adult, which resembles neither the larva nor the pupa.

• Insects can defend themselves by stinging, using camouflage, or releasing noxious chemicals. Insects that are dangerous or taste bad often have warning coloration that makes them recognizable to predators. The warning coloration of a dangerous species may be mimicked by harmless species.

Vocabulary entomology (p. 742) labrum (p. 746) labium (p. 746)

tympanum (p. 747) ovipositor (p. 748) metamorphosis (p. 748)

incomplete metamorphosis

(p. 748) nymph (p. 748)

complete metamorphosis

(p. 748) pupa (p. 749)

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