Section 3

objectives

• List the characteristics of arachnids, as represented by a spider.

• Explain the adaptations that spiders have for a predatory life on land.

• Identify the unique characteristics of scorpions, mites, and ticks.

• Compare the characteristics of millipedes and centipedes.

vocabulary arachnid pedipalp spinneret book lung trachea spiracle

Malpighian tubule

Walking leg

Simple eyes Pedipalp

Poison gland

Brain Stomach Gut

Ovary figure 36-9

Digestive gland

Malpighian tubule

Anus

Brain Stomach Gut

Ovary

Digestive gland

Simple eyes Pedipalp

Poison gland

Malpighian tubule

Anus

Spinnerets

Silk glands

Spiracle

Spinnerets

Silk glands

The major internal organs of a female spider are shown in this cutaway side view. The inset shows a closer view of a book lung, one of the spider's adaptations to life on land.

Spiracle

Word Roots and Origins spiracle from the Latin spiraculum, meaning "air hole"

figure 36-10

The black widow, Latrodectus mactans (a), and the brown recluse, Loxosceles reclusa (b), are the only two spiders in the United States whose venom is dangerous to humans. Note the red hourglass-shaped spot on the abdomen of the female black widow and the dark violin-shaped marking on the cephalothorax of the brown recluse.

figure 36-10

The black widow, Latrodectus mactans (a), and the brown recluse, Loxosceles reclusa (b), are the only two spiders in the United States whose venom is dangerous to humans. Note the red hourglass-shaped spot on the abdomen of the female black widow and the dark violin-shaped marking on the cephalothorax of the brown recluse.

The chelicerae of spiders are modified as fangs and are used to inject venom into prey. The venom is produced by poison glands in the cephalothorax and flows through ducts in the chelicerae to the tips of the fangs. Most spiders have eight simple eyes at the anterior end of the cephalothorax. Each simple eye has a single lens.

On the tip of the abdomen of many spiders are three pairs of organs called spinnerets, which are visible in Figure 36-9. Each spinneret is composed of hundreds of microscopic tubes that connect to silk glands in the abdomen. A protein-containing fluid produced in the silk glands hardens into threads as it is secreted from the spinnerets. Spiders use their silk threads to spin webs, build nests, and protect eggs. In some species, spider silk also aids in dispersing spiders from one habitat to another. Young spiders move to new habitats when the wind pulls them through the air by their threads.

The nervous, digestive, and circulatory systems of spiders are similar to those of crustaceans. Because spiders are terrestrial, however, their respiratory system is quite different. In some spiders, respiration occurs in book lungs, paired sacs in the abdomen with many parallel folds that resemble the pages of a book. The folds in a book lung provide a large surface area for gas exchange. Other spiders have a system of tubes called tracheae (TRAY-kee-ee) that carry air directly to the tissues from openings in the exoskeleton known as spiracles. Some spiders have both book lungs and tracheae.

The excretory system of spiders is also modified for life on land. The main excretory organs, called Malpighian (mal-PIG-ee-uhn) tubules, are hollow projections of the digestive tract that collect body fluids and wastes and carry them to the intestine. After most of the water is reabsorbed, the wastes leave the body in a nearly solid form with the feces. Thus, the Malpighian tubules help spiders conserve water in terrestrial environments.

Life of a Spider

Spiders feed mainly on insects, although some can catch fish, frogs, and even birds. Different species of spiders are adapted to capture their prey in different ways. Some chase after prey, some hide beneath trapdoors waiting for prey to approach, and some snare prey in webs spun from silk. When an insect becomes trapped in the sticky web, the spider emerges from its hiding place near the edge of the web and paralyzes the insect with its venom. Many spiders also immobilize their prey by wrapping them in silk. They can then consume the body fluids of the prey at a later time.

Spider venom is usually harmless to humans, and most spiders bite only when threatened. There are, however, two kinds of spiders in the United States whose bites can be dangerous to humans. They are the black widow and the brown recluse, shown in Figure 36-10. The female black widow has a bright red or orange mark shaped like an hourglass on the ventral surface of its abdomen. The venom attacks the nervous system.

The brown recluse has a violin-shaped mark on the dorsal surface of its cephalothorax. Therefore, it is sometimes called the "violin spider." The venom of the brown recluse kills and digests the tissues surrounding the bite.

A male spider is usually smaller than a female of the same species. When the male is mature, he transfers sperm to special sacs in the tips of his pedipalps. The sperm are then placed in the seminal receptacle of the female during mating. As soon as mating has occurred, the male darts away. If he is not quick enough, he may be eaten by the female. Eggs are fertilized as they pass out of the female into a silken case that she has spun. The female may carry the egg case with her or attach it to a web or plant. The young spiders hatch in about two weeks and undergo their first molt before leaving the case.

Scorpions

Scorpions, such as the one shown in Figure 36-11, differ from spiders in two ways. Scorpions have large, pincerlike pedipalps, which they hold in a forward position. They also have a segmented abdomen with a large stinger on the last segment, which can be curled over the body. Scorpions usually hide during the day and hunt at night, mostly for insects and spiders. They seize prey with their pedipalps and inject venom into the prey with their stinger. Only a few species of scorpions have venom that can be fatal to humans. Most scorpions live in tropical or semitropical areas, but some are found in dry temperate or desert regions.

Mites and Ticks

Mites and ticks are the most abundant and most specialized arachnids. About 30,000 species have been identified, but the actual number of species may be much larger than that. Unlike spiders and scorpions, mites and ticks have a completely fused cephalothorax and abdomen, with no sign of separation between them.

Most mites are less than a millimeter in length, and some are small enough to live on particles of dust. They can be found in freshwater, marine, and terrestrial habitats. Many mites are entirely free living, while others are parasites during at least part of their life cycle. Spider mites parasitize fruit trees and many other agricultural crops by sucking the fluid from their leaves. The larvae of harvest mites— also known as chiggers—attach themselves to the skin of vertebrates, including humans. They break the skin with their chelicerae and feed on blood, causing swelling and itching. Other species of mites live on the bodies of chickens, dogs, cattle, humans, and other animals, where they feed on sloughing skin, hair, and feathers.

Ticks, such as the one shown in Figure 36-12, are parasites that feed on their hosts by piercing the flesh and sucking on the blood. Many species of ticks carry bacteria and other microorganisms in their guts that may cause diseases in the host. While the tick feeds, the microorganisms are transmitted to the new host. Rocky Mountain spotted fever and Lyme disease are transmitted in this way. Ticks range in length from a few millimeters to 3 cm (about 1 in.).

figure 36-11

Scorpions, such as Paruroctonus mesaensis, have pincerlike pedipalps and a venomous stinger at the end of their segmented abdomen.

figure 36-11

Scorpions, such as Paruroctonus mesaensis, have pincerlike pedipalps and a venomous stinger at the end of their segmented abdomen.

figure 36-12

The lone star tick, Amblyomma americanum, is an arachnid that parasitizes humans and other mammals.

figure 36-12

The lone star tick, Amblyomma americanum, is an arachnid that parasitizes humans and other mammals.

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