Section I

objectives

• Describe the distinguishing characteristics of arthropods.

• Explain the process of molting in an arthropod.

• List the five major subphyla of the phylum Arthropoda.

vocabulary arthropod appendage chitin compound eye molting trilobite tagma mandible chelicera figure 36-1

The arthropod exoskeleton consists of three layers that cover the epidermis. Wax in the outer layer is secreted by wax glands. Sensory hairs projecting from the exoskeleton allow arthropods to respond to vibrations and chemicals in their environment.

Sensory hair

Outer layer-

Middle-layer

Inner layer

Epidermis-

Wax gland

Sensory hair

Outer layer-

Middle-layer

Inner layer

Epidermis-

figure 36-2

This green cicada, Tibicen superbus, is in the process of molting. The outer layer of its old exoskeleton appears brown.

figure 36-2

This green cicada, Tibicen superbus, is in the process of molting. The outer layer of its old exoskeleton appears brown.

Molting

A rigid exoskeleton limits the size to which an arthropod can grow. So, each arthropod periodically sheds its exoskeleton and makes a new one in the process of molting. An anthropod goes through many cycles of molting during its life. Figure 36-2 shows an insect in the process of molting.

A cycle of molting begins as the tissues of an arthropod gradually swell. When the pressure inside the exoskeleton is very strong, a hormone that triggers molting is produced. In response to this hormone, the cells of the epidermis secrete enzymes that digest the inner layer of the exoskeleton. At the same time, the epidermis begins to make a new exoskeleton by using the digested material. Eventually, the outer layer of the old exoskeleton loosens, breaks apart, and is shed. The new exoskeleton, which is flexible at first, stretches to fit the enlarged animal.

The new exoskeleton takes a few hours to a few days to become as hard as the one that it replaced. During this time, the animal is vulnerable to predators and, if the arthropod is terrestrial, to desiccation. For these reasons, arthropods usually stay in hiding from the time that they begin to molt until their new exoskeleton has hardened. The "soft-shelled crabs" sold in some restaurants are crabs that have been caught immediately after molting.

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