Getting to Know Insects and Spiders
WHAT ARE INSECTS AND SPIDERS?
Insects, spiders, and their relatives are all arthropods (arthro-pawds), or animals with a hard outer skeleton and several pairs of jointed limbs. Arthropods form the largest group of animals on Earth. They are found almost everywhere, from the deepest ocean trenches to the tallest mountain peaks. There are nearly one million species of insects known, nearly half of all the different plants and animals combined. There may be as many as ten to thirty million insect species total. Even a "typical" backyard may be the home to several thousand species of insects and spiders. Estimates of species numbers vary because scientists still know so very little about them. Millions of new species await discovery, especially among insects and mites on land and crustaceans in the ocean.
It is estimated that for every human being alive today, there are as many as two hundred million individual insects. Just the total weight of all the ants in the world, all nine thousand different kinds, is twelve times greater than the weight of all the humans on the planet. Despite their amazing numbers and the fact that they are found virtually everywhere, insects and other arthropods are still very alien to us, as if they were beings from another planet. They move on six or more legs, stare with unblinking eyes, breathe without noses, and have hard skinless bodies made up of rings and plates, yet there is something strangely familiar about them, too. Arthropods have to do all the things people do to survive, such as find food, defend themselves from their enemies, and reproduce. They also rely on their finely tuned senses to see, touch, hear, smell, and taste the world around them.
Because of their numbers and the fact that they eat almost everything that is plant, fungus, or animal, arthropods have a huge impact on all the species sharing their habitats. They pollinate flowers, disperse seeds, recycle dead organisms, and bury animal waste. Plant-feeding species provide a natural pruning service that keeps plant growth and populations in check, while flesh-eaters control the populations of other animals. They, in return, are an important food source for fishes, reptiles, amphibians, birds, mammals, and other arthropods.
Many different kinds of scientists study the lives of insects, spiders, and their relatives. Entomologists (EHN-tih-MA-luh-jists) examine the lives of insects, while arachnologists (uh-rak-NA-luh-jists) look at spiders and their relatives. Myriapodologists (mi-RI-ah-po-DAL-luh-jists) focus their attentions on millipedes, centipedes, and their kin. Invertebrate zoologists (in-VER-teh-breht zu-AH-luh-jists)and some marine biologists study marine crustaceans, sea spiders, and horseshoe crabs. It is the work of all these scientists that has provided the information found on these pages.
OLDER THAN DINOSAURS
Arthropods were swimming in lakes, crawling on land, and flying through the air long before dinosaurs. In fact, millipedes are one of the oldest land animals on Earth and have been around for about four hundred million years. Insects are more than 380 million years old. Scientists know this by studying their fossils (FAH-suhls), or remains of animals that lived long ago, usually found set into rock or earth. Scientists who study fossils are called paleontologists (PAY-li-un-TA-luh-jists). Paleontologists study fossils to understand how life has developed and changed over time. The location and chemical makeup of fossils helps paleontologists to determine their age. By studying fossils scientists know that some groups of organisms, such as horseshoe crabs, millipedes, silverfish, and cockroaches, have changed very little over millions of years. The process of organisms changing over time is called evolution (EH-vuh-LU-shun). Organisms must adapt in form and behavior to survive in an environment that is also changing. Studying fossils not only reveals clues about the evolution of these ancient animals but also gives a glimpse of the environments in which they lived.
How to become a fossil
For arthropods, the process of fossilization (FAH-suhl-ih-ZAY-shun) begins when footprints, bodies, or body parts are quickly covered in mud or sand made up of fine particles. The finer the particles, the greater the detail preserved in the fossil. Under just the right circumstances, the mud and sand will be compressed, or squeezed, as more mud and sand settle on the remains, until it becomes rock. This process takes millions of years. Fossils are only impressions of the ancient animals. Their tissues are replaced, molecule by molecule, with surrounding minerals. In time, the remains of the arthropod are transformed physically and chemically and resemble the surrounding rock.
Some of the most detailed remains of ancient arthropods are preserved in hardened tree sap called amber (AM-bur). Amber comes from the sticky sap, or resin (REH-zin), of trees. Trees produce resin to heal wounds and to defend against insect borers. Insects, spiders, and other organisms became trapped and completely encased in the sticky stuff. The resin quickly hardened and eventually fell to the ground, where it was buried by decomposing plants and soil. Storms washed the hardened resin into low-lying areas that were eventually covered by the ocean. These ancient sea bottoms eventually changed into layers of limestone and sandstone that are now filled with scattered bits and chunks of fossilized resin, or amber. Over millions of years, as new mountains and islands formed, the sea bottoms were lifted above sea level, exposing pieces of amber with the ancient remains of arthropods. The oldest known insect fossils in amber are about one hundred twenty million years old.
Only a tiny fraction of all the arthropods that ever lived during ancient times were preserved as fossils. The remains of species living on land were less likely to be preserved than those living in freshwater and marine habitats. Fossilization in stone or amber depends on animals dying in the right place, at the right time, and under the right circumstances. The odds of this all happening are extremely low. Even today arthropods are quickly eaten by other animals or decompose and break up within hours or days of their deaths.
Flights of fancy
Insects are one of only four groups of animals (with pterosaurs, birds, and bats) to have achieved true flight and were the first to take to the air. The power of flight gives many insects the opportunity to find food and mates over wide areas. Flying insects also have the ability to avoid being eaten by other animals and to colonize new and suitable habitats. Birds, bats, and pterosaurs all evolved wings from their forelimbs, or front legs, but insects did not have to give up a pair of legs to fly. So where did insect wings come from?
Based on the study of fossils, the first winged insects appeared between three hundred fifty and three hundred million years ago. Their wings may have evolved from flexible structures that were first used as gills for breathing underwater. Or they may have developed from stiff projections growing out of their midsection, or thorax, and eventually evolved into more flexible, winglike structures. But why did the first insect wings evolve? Did the oldest winged insects use them for gliding through their habitat, or did they use them as solar panels to collect heat to warm their bodies? The discovery of even older fossils of winged insects may help to unravel the mystery.
DRESSED FOR SUCCESS
One way to measure the success of any group of animals is to look at biodiversity (BI-o-dih-VUHR-seh-tee), or the variety of species in a particular place. Another way is to count the numbers of individuals of a particular species. By either measure, arthropods are the most successful group of animals on Earth. The physical features that make them so successful are the size and structure of their bodies. Most arthropods range in length from 0.04 to 0.4 inches (1 to 10 millimeters). This allows them to live in numerous small habitats where larger animals cannot hope to make a living. Furthermore, their bodies are wrapped in a hard, protective, external covering called the exoskeleton (EHK-so-SKEH-leh-tin). Small and armored, arthropods are perfectly suited for living and reproducing on land or in the water.
A suit of armor
The exoskeleton works both as skin and skeleton. It protects the animal from harm as it swims, crawls, burrows, or flies through the habitat, and it provides a means of support for the muscles and internal organs inside. The exoskeleton is made up of several layers that are composed mostly of chitin (KYE-tehn), a complex material that is made of fibers and combines with a protein to make the exoskeleton light, tough, and flexible, just like fiberglass. The surface of the exoskeleton is covered with small pits, spines, and hairlike structures called setae (SIH-tee). Some setae are sensitive to touch and sometimes help to protect the body from injury. In most insects and spiders, a waxy layer covering the exoskeleton helps to maintain the moisture levels inside the body. Millipedes, centipedes, and crustaceans do not have this protection.
The exoskeleton is divided into two or three body regions. Each region is made up of a series of armored plates that are sometimes closely joined together to increase strength or distinctly segmented to maintain flexibility. The appendages (mouthparts, antennae, legs) are jointed, or divided into segments to increase their flexibility. In fact, the name arthropod means "jointed foot." All plates and segments are joined together by a thin, flexible membrane of pure chitin.
The mouthparts of arthropods are made up of two to four pairs of appendages. These appendages come in a variety of forms and are used as lips, jaws, and fangs. Insect jaws may form piercing-sucking or lapping mouthparts for drinking plant or animal fluids.
The eyes, if present, are either compound or simple. Compound eyes are made up of several to tens of thousands of individual lenses and are used for seeing images. Some species, as well as all larval insects or young, have only simple eyes, which have just one lens for each eye. Simple eyes are used primarily to distinguish light and dark. One or two pairs of antennae (an-TEH-nee), or sense organs, are covered with setae that are especially sensitive to touch and often have special pits for detecting certain odors.
Adults have three or more pairs of jointed legs and, in many insects, one or two pairs of wings. The legs come in a variety of shapes and are used for running, jumping, climbing, digging, swimming, and grasping prey. The abdomen contains the internal and reproductive organs, as well as special appendages used for defense (as in scorpions), steering (in horseshoe crabs), or spinning silk (as in spiders). The abdomen is distinctly segmented in most arthropods but not in ticks, mites, and nearly all spiders.
A PEEK INSIDE
Inside arthropod bodies are incredibly powerful muscles that make mouthparts chew, antennae wiggle, legs dig, and wings fly. The nervous system helps to coordinate these and other movements. The brain is located inside the head. Trailing behind the brain is a nerve chord that runs along the entire length of the arthropod's underside. Along the nerve chord are bundles of nerves called ganglia (GANG-lee-uh) that help control the various parts of the body. A pair of ganglia controls each pair of appendages. All abdominal segments have a pair of ganglia except in millipedes, where each segment has two pairs. Most body segments of millipedes are actually two segments joined as one. This is also why millipedes have two pairs of legs on most of their body segments.
The blood of fishes, amphibians, reptiles, birds, and mammals circulates inside arteries and veins. This is called a closed circulatory system. But arthropods have an open circulatory system. They have a tube that runs along their backs. A series of pumps, or hearts, inside the abdomen, or body trunk, pumps the blood forward in the tube. Eventually, it spills out behind the head into various body cavities. The blood usually does not carry oxygen, but it does carry nutrients and chemicals called hormones (HOR-moans) that help the body to function. All the tissues and organs are bathed in blood. The blood eventually moves back to the abdomen where it enters the tube through tiny holes located between the hearts.
Arthropods do not have lungs. The respiratory, or breathing, system of most species living on land is made up of a series of holes and tubes. Oxygen enters the body through a series of holes along the sides of the body called spiracles (SPIH-reh-kuls). Each spiracle is attached to a network of tubes, or trachea (TRAY-key-uh). The trachea carry oxygen throughout the body. Carbon dioxide, a waste product of living tissues, is expelled out of the body through the same system. Some spiders have a tracheal system, but most use book lungs. Book lungs are made up of folded tissue inside the abdomen that resembles the pages of a book. Aquatic insects either trap a bubble of air over their spiracles or use gills or gill-like structures. A very thin layer of exoskeleton covers the gills, allowing dissolved oxygen in the water to pass through and enter the tracheal system. Some species have no respiratory system at all. Instead, oxygen in the water simply seeps in all over their bodies.
Animals are classified into various groups on the basis of having similar features. Sharing these similar features suggests that they share a common ancestor or history. The more features they share, the closer the relationship. Arthropods are grouped in the Phylum Arthropoda (AR-thruh-PO-duh) because they all have the following special features: exoskeletons, segmented bodies, pairs of jointed appendages, open circulatory systems, and a ventral nerve cord that runs down the underside of the animal. Arthropods are further divided into three smaller groups, or subphyla (sub-FAI-leh).
The subphylum Cheliceriformes includes sea spiders, horseshoe crabs, scorpions, spiders, ticks, mites, and their relatives. Their bodies are divided into two major regions, the forebody, sometimes called the cephalothorax (SEH-fe-lo-THO-raeks), and abdomen. The forebody has six pairs of appendages, including the pinchers or grasping arms, claw-like pedipalps (PEH-dih-paelps), and eight walking legs. They never have antennae. The reproductive organs are located at the front or rear of the abdomen. The abdomen sometimes has a tail-like structure that is used as a rudder (horseshoe crabs), a defensive weapon (scorpions), as a sensory organ (whip scorpions), or a silk-producing organ (spiders and mites). There are about sixty-one thousand species, most of which live on land.
The Uniramia includes arthropods with only one pair of antennae and legs that are not branched at their bases. Insects and their relatives have bodies that are divided into three major regions: the head, thorax, and abdomen. The head has five pairs of appendages, including the mouthparts and one pair of antennae. Adults have three pairs of legs and sometimes one or two pairs of wings. Their reproductive organs are located toward the rear of the abdomen.
Centipedes, millipedes, and their relatives have bodies that are divided into two major regions. The head is followed by a long trunk-like body. The head has four pairs of appendages, including the mouthparts and one pair of antennae. Adults have one or two pairs of legs on most body segments. Depending on the species the adults have eleven to 382 pairs of legs. Their reproductive organs are located at the end of the body or just behind the head. There are about 818,000 species of insects, millipedes, centipedes, and their relatives that live on land or in freshwater habitats.
The Crustacea include crabs, lobsters, crayfish, shrimp, barnacles, beach hoppers, pillbugs, and their relatives. Their bodies are divided into two major regions: the head, which is usually covered by a broad shield, or carapace (KARE-a-pays), and the body trunk. The head has five pairs of appendages, including the mouthparts and two pairs of antennae. The appendages of crustaceans are usually branched at their bases. The abdomen may also have paired appendages underneath. The reproductive organs are usually found on the midsection or near the front of the abdomen. There are about sixty-seven thousand species, most of which live in the ocean.
Classifications help scientists sort and identify species, as well as organize and locate information about them. But the system of classification is not carved in stone. As understanding of these animals continues to improve, classifications will also change. Groups will be combined, divided, added, or discarded. This constant state of change is sometimes frustrating, but the goal is to have a classification that reflects the true relationships of all organisms based on their evolutionary history.
Arthropods grow by breaking out of their rigid exoskeletons. Most species molt, or shed their exoskeletons, only as larvae (LAR-vee). Larvae are immature animals that are not able to reproduce. However, crustaceans, arachnids, some insects, and other arthropods will continue to grow and molt throughout their adult lives. Each stage between molts is called an instar (IHN-star). The number of instars varies among species, ranging from three to more than twenty times in insects. The number of larval molts remains the same for each species. With each molt a soft pale body escapes from its old exoskeleton through a special escape hatch. After a few hours, days, or weeks the new exoskeleton darkens and hardens. This process of change and growth is called metamorphosis (MEH-teh-MORE-feh-sihs).
There are four basic types of metamorphosis. Some millipedes and centipedes, as well relatives of insects known as proturans, develop by anamorphosis (ANN-eh-MORE-feh-sihs). Their larvae hatch from eggs with fewer body segments than they will have as adults. Additional segments and legs are added as they molt. When wingless diplurans, springtails, silverfish, and bristletails molt, the only noticeable change is that they are larger. They molt many times as larvae and will continue to molt after they reach adulthood. Grasshoppers, true bugs, dragonflies, and some other winged insects develop by gradual metamorphosis. The larvae strongly resemble the adults when they hatch, but they lack developed wings and reproductive organs. These insects stop molting once they reach the adult stage. Beetles, butterflies, moths, flies, fleas, ants, bees, wasps, and others develop by complete metamorphosis. They have four very distinct stages: egg, larva, pupa, and adult. They do not continue to grow or molt once they reach adulthood.
Spiders and other arachnids lay their eggs in a protective sac. The eggs hatch into helpless prelarvae that are unable to move. Their legs are not fully developed, and their bodies show traces of segmentation not visible in the adults. The prelarvae molt into larvae, which still show traces of segmentation on the abdomen but have legs that are more fully developed. The larvae molt into nymphs (nihmfs), or spiderlings. The very active spiderlings leave the egg sac and resemble small versions of the adults. Many arachnids continue to molt after they reach adulthood.
Arthropods engage in all kinds of behaviors that help them to survive and reproduce. They not only have to find food, but they also need to avoid their enemies, find and select mates, and secure a future for their young.
Ecologists (ih-KA-luh-jists), scientists who study where and how organisms live, sometimes divide arthropods into different groups based on what they eat. Herbivores (URH-bih-vorz) eat plants, and carnivores (KAR-nih-vorz) eat animal flesh, while omnivores (AM-nih-vorz) eat both plants and animals. Another way to look at the feeding ecology of arthropods is by viewing them as generalists or specialists. For example, generalist herbivores eat all kinds of plants, but specialists feed on only one kind of plant or a small group of closely related species. Parasitoids (PAE-re-SIH-toyds) live and feed inside the bodies of certain kinds of animals (hosts) and eventually kill them. Parasites are also specialists, attacking only certain animal hosts but seldom killing them.
Suitable foods are found by sight, smell, touch, and taste. Herbivores chew or suck fluids from all parts of plants, including roots, trunks, stems, buds, leaves, flowers, fruits, and seeds. Some species even bleed leaves of their sticky or toxic resins before eating them. Many collect a variety of plant materials and store them as food, while others simply use them as mulch for growing their own food. Some predators (PREH-duh-ters) actively hunt their prey, while others sit and wait to ambush them. Spiders build webs that are specifically designed to trap insects and other arthropods. Omnivores are opportunists and eat anything they find, even scavenging dead plants and animals. Some even feed on the waste products of other animals.
Insects, spiders, and other arthropods rely on many different strategies to defend themselves against predators. For example, large horned beetles avoid being eaten simply by being large and horned. Some orb-weaving spiders have hard spiny bodies that would make them an unwelcome mouthful even to the hungriest of predators. Millipedes coil up their bodies to protect their delicate heads, legs, and undersides, exposing only a series of hard plates along their backs. Others whip or kick spiny antennae and legs at their attackers. Tarantulas and other spiders rear up, flash their fangs, and adopt threatening poses. If this fails to work, many tarantulas will brush a cloud of painfully itchy hairs off their bodies into the faces of predators. While many arthropods bite, run, jump, burrow, swim, or fly to escape, others simply remain still or fall to the ground to get out of sight. Some rely on the protection of other well-defended species, such as ants.
Others startle would-be predators by suddenly flashing bright colors or eye spots. Mantids strike out with their spiny front legs to display their bright colors. The hind wings of some grasshoppers and stick insects are also brightly patterned, but they usually remain hidden under the forewings. Moths suddenly spread their plainly patterned forewings to reveal hind wings marked with large "eyes" or bright contrasting bands of color. Centipedes and caterpillars have "false heads" that either direct attacks away from sensitive parts of their bodies or simply confuse predators hoping to make a sneak attack.
Many insects and spiders use camouflage to stay out of sight, blending in with backgrounds of living or dead leaves. Stick insects, grasshoppers, katydids, and mantids may go a step further by actually having bodies shaped like sticks, stones, leaves, or flowers. Arthropods that conceal themselves by looking like another object, living or dead, are called cryptic (KRIP-tik). They even act like the objects they mimic by remaining very still, although stick and leaf mimics sometimes gently sway back and forth, as if they were in a gentle breeze. Some spiders and caterpillars avoid detection by looking like something unappetizing, such as bird droppings.
Biting, stinging, bad tasting, and foul smelling arthropods are often brightly marked or distinctively colored as a warning to potential predators. The colors, patterns, and body shapes of harmful species, especially ants, bees, and wasps, frequently serve as models for other species that do not bite or sting. Species that resemble each other in color or behavior are called mimics.
The mating game
In some species, males are rare or unknown. The females lay unfertilized eggs that usually develop into more females. This process is called parthenogenesis (PAR-thuh-no-JEH-nuh-sihs). But most arthropods reproduce by mating. Males usually mate as many times as possible, but females mate only once, just a few times, or many times, depending on the species. In some species males and females gather at a food resource, such as a patch of flowers, sapping limbs, decomposing bodies, or piles of dung. Some males claim these resources as territories and engage in battles with other males to win the favor of a nearby female.
Many males and females find one another by releasing pheromones (FEH-re-moans), chemicals that are especially attractive to members of the opposite sex of the same species. Others use flashing lights or sounds to attract one another. Once they get together, many species engage in courtship behaviors that help them to establish each other's suitability for mating. Courtship may involve biting, grappling, touching, leg waving, wing flapping, flashing mouthparts, and vibrating bodies.
In species that live on land, the male usually grasps the female with his legs or jaws and deposits sperm or a sperm packet directly into her body. These packets not only contain sperm but also provide nutrition for the female so she can produce bigger and better eggs. The act of mating may be brief or last several hours. To prevent the female from mating with other males, some males will remain with their mate until she lays her eggs. In some species, such as honeybees and many spiders, males leave part or all of their reproductive organs in the female's body to block mating attempts by other males.
Other groups of arthropods do not mate directly. For example, male spiders must first transfer their sperm to special containers on their pedipalps before they are ready to mate. They use the pedipalps to put the sperm directly into the female's reproductive organs. Male horseshoe crabs climb on the back of the females and release their sperm onto her eggs as she lays them in the sand. Silverfish males deposit a drop of sperm on the ground and then guide the female over it so she can pick it up with her reproductive organs. Male millipedes, centipedes, scorpions, and other arachnids put their sperm packets on the ground. Then they engage in a variety of courtship behaviors to guide the females over the packets. In some arthropods the females must find these packets without the help of males.
Parental care is rare among arthropods. In most species it consists only of a female laying her eggs in places where they will not be eaten or destroyed, preferably near food that is suitable for the young. However, in a few species, the female keeps the eggs inside her body until they hatch or are "born." The eggs are nourished by their own yolks. This type of development is called ovovivipary (O-vo-vai-VIH-pe-ree). Vivipary (vai-VIH-pe-ree) occurs in some flies and parasitic true bugs. The females produce only one or a few eggs at a time and keep them inside their bodies. The eggs are nourished by the mother's body, and the larvae are born alive.
Most species lay their eggs somewhere in their habitat, either singly or in batches. Some species have special egg-laying tubes called ovipositors (O-vih-PA-zih-terz) that place their eggs out of harm's way deep in the soil or wood or inside plant or animal tissues. Others have special glands that allow them to glue their eggs to surfaces or surround them in protective cases. Some species prepare special chambers for their eggs, provide them with all the food the larvae will need to develop, and then leave. Females of a few species guard the eggs until they hatch. Some will even remain with the young for a short period, but the greatest level of parental care is seen in the social insects.
True social behavior is defined by overlapping generations of the same species living and working together to raise their young. They also cooperate in gathering food and defending, repairing, and expanding the nest. Insects are the only truly social arthropods. Social insects include all termites and ants but only some bees and wasps. They live in colonies with up to one million individuals. The tasks within each colony are divided among distinctly different forms or castes. The castes include workers, soldiers, and reproductives (queens and males).
Workers form the majority of the colony. They care for the young and the queen and perform all other tasks in the nest. They divide the labor among themselves on the basis of age or size. Some ants and termites also have a soldier caste. Soldiers are usually larger than the workers and sometimes equipped with powerful jaws to drive away intruders. Both workers and soldiers are sterile and cannot reproduce. The workers and soldiers of ants, bees, and wasps are always sterile females, but in termites they are male or female.
The reproductive caste consists of queens and males. Each colony has at least one queen, and she is usually the mother of the entire colony. She may live many years, laying millions of eggs in her lifetime. Males are short-lived and usually die after mating with the queen. However, termite kings usually stay with the queen long after they mate.
Colony members communicate with pheromones to identify nest mates, recruit other members of the colony to find food or defend the nest, and to coordinate other activities. For example, honeybee queens use a pheromone called queen substance to hold the colony together. Workers pick up the pheromone as they lick and groom the queen. As they feed one another, they pass it along to other workers in the colony. The queen substance "tells" the workers to feed and care for the queen and her eggs and lets all the members of the colony know that she is alive and well. Every worker in the colony will know if their queen has died within a day, even though only a few workers will have actually had contact with the body.
Other arthropods occasionally gather in groups to feed, mate, or temporarily guard their young, but they are not truly social. There are about 40 species of spiders that live in groups in large webs and feed together. A huntsman spider, Delena cancerides, lives under bark in groups of up to three hundred individuals. These groups consist mainly of young spiders with just a few adults. They work together to attack and kill insect prey, as well as defend their shelter against spiders from other colonies.
DANCES WITH PLANTS
Arthropods, especially insects, have had a long and close relationship with flowering plants that dates back between 135 and sixty-five million years ago. From the plant's point of view this relationship is both negative and positive. A negative example is their relationship with herbivorous insects. As herbivores, insects have strongly influenced the evolution of flowering plants. Over millions of years, plants have evolved several ways of defending themselves against insects. Many have developed bad-tasting chemicals, or toxins (TAK-sihns), that discourage herbivorous insects from eating their stems and leaves. Those plants that survived insect attacks were able to pass along their characteristics to the next generation through their seeds. At the same time, the defensive strategies of plants have influenced the evolution of insects. They have evolved systems within their bodies that breakdown these toxins into harmless chemicals so they can continue to eat the plant. Those insects that were able to get enough food were able to pass their characteristics on to the next generation through their eggs. Over time, plants and insects continued to change, or evolve, into new species as they attempt to overcome the attacks and defenses of the other. The mutual influence that plants and insects have over each other's evolution continues today and is called coevolution (ko-EH-vuh-LU-shun).
From a plant's point of view, pollination is an example of a positive interaction with insects. Plants produce flowers with nectar and more pollen than they need to reproduce to attract insects. Flowers are like brightly colored, sweet-smelling road signs that encourage insect pollinators to stop and visit. These pollinators either accidentally or purposefully collect pollen from the flower. Many flowers depend on bees, flies, beetles, and thrips to carry their pollen from one flower to another so their seeds will develop. Some species of orchids rely on specific insects to pollinate them. In fact, the flowers of some species mimic female wasps. Male wasps pollinate the flowers as they attempt to "mate" with the flower. This very special type of interaction between a plant and an insect is another example of coevolution.
BENEFICIAL INSECTS AND SPIDERS
People depend on insects to pollinate their crops, ensuring that they have plenty of food, fiber, and other useful products. Honeybees are not only valued for their pollination services but also for their honey and wax. Silk from the cocoons of the silk moth, Bombyx mori, has been harvested for centuries. Each caterpillar must eat 125 pounds (56.7 kilograms) of mulberry leaves before it can spin a cocoon. About seventeen hundred cocoons are required to make one dress. China and Japan are the world's largest producers of silk.
Beekeeping and silkworm culture were the first forms of insect farming, but today many other insects are raised and sold for use as research animals, fish bait, and pet food. Others are used to combat weeds and insect pests. Butterflies and other tropical insects are raised and shipped alive to insect zoos and butterfly houses around the world. Money earned by butterfly farmers in Central and South America, Malaysia, and Papua New Guinea is used to support families, to maintain the farm, and to preserve or improve butterfly habitats.
Grasshoppers, termites, beetles, caterpillars, and crustaceans are important sources of food for humans and are even considered delicacies in many parts of the world. They are an excellent source of fat and protein. Western European culture has largely ignored insects as food but considers lobster, crab, and shrimp as delicacies.
Every species of arthropod is a flying, walking, or swimming pharmacy filled with potentially useful medicines and other chemical compounds. For example, venoms from insect stings are used to treat patients with rheumatism (RU-me-TIH-zem), a disease that affects the joints. The venom helps to increase blood flow to the diseased joints and stops the pain.
Arthropods also provide an early warning system for detecting changes in the environment caused by habitat destruction, pollution, and other environmental disturbances. Aquatic insects and arthropods living in the ocean are especially sensitive to even the smallest changes in water temperature and chemistry. The presence or absence of a particular species may demonstrate that a particular habitat is polluted as a result of illegal dumping, pesticides from nearby agricultural fields, or chemical waste from mining operations.
INSECTS AND SPIDERS AS SYMBOLS
People around the world have used insects as symbols to explain how the world began. There is a tribe in South America that believes a beetle created the world and, from the grains of sand left over, made men and women. In the American Southwest the Hopis believed that the world began through the activities of Spider Woman, the Earth Goddess. The sacred scarab beetle appeared in wall paintings and carvings and played an important role in the religious lives of the early Egyptians. Early Christians used insects as symbols of evil and wickedness, but eastern cultures, especially in China and Japan, often used them to signify good luck. The ancient Greeks used insects and spiders in their plays and fables about good and evil.
INSECTS AND SPIDERS IN ARTS AND CRAFTS AND IN LITERATURE
Images of insects have appeared on 6,000-year-old cave paintings, as well as on the walls of ancient Egyptian tombs. The Egyptians wore sacred scarab carvings and other insect ornaments. The ancient Greeks used insect images on jewelry and coins. Insects were also used to adorn decorative boxes, bowls, and writing sets in the Middle Ages. The hard and tough bodies of beetles have long been used to make pendants and earrings. Today, boxed collections of large tropical species are sold to tourists as decorative pieces.
Ants, bees, fleas, flies, grasshoppers, spiders, and scorpions are frequently mentioned in the Bible. Images of arthropods were once used to decorate the brightly painted borders of other important religious books and papers. They have influenced language with such words as lousy, nitpicker, grubby, and beetle-browed. Beetles have also inspired the names of a world-famous rock group and a well-known German automobile.
INSECTS AND SPIDERS AS PESTS
Insects are humanity's greatest competitors and cause huge economic losses when they feed on timber, stored foods, pastures, and crops. Termites and other insects infest and weaken wood used to build homes, businesses, floors, cabinets, and furniture. The larvae of clothes moths and carpet beetles destroy woolen clothing, rugs, and hides. Mites, moths, beetles, and other insects invade homes and infest stored foods and destroy books and other paper products. Crops lost to insect damage cause enormous economic hardship and may lead to starvation and death among hundreds or thousands of people. One-third to one-half of all food grown worldwide is lost to damage caused by insects and mites, not only by devouring the foliage but also by infecting plants with diseases.
Arthropods not only eat people's belongings, they also attack human bodies. The bites of blood-feeding mosquitoes, flies, fleas, lice, and ticks are not only irritating, they are also responsible for spreading diseases that can infect and kill people, pets, and farm animals. Over the centuries more people have died from diseases carried by arthropods than any other reason. Even today, more people die from malaria and yellow fever, diseases transmitted by mosquitoes, than from HIV/AIDS, cancer, accidents, and wars. Spiders, millipedes, centipedes, and other arthropods are not often pests but are considered nuisances when they enter homes. The venomous bites of some spiders and centipedes may be painful but are seldom life-threatening for healthy adults.
These and other pests are effectively controlled by integrated pest management, or IPM. IPM includes plowing fields to kill pests in the ground, rotating crops so that they will not have anything to eat, or planting other crops nearby that will give their enemies a place to live and prosper. Whenever possible, natural enemies are used to combat pests instead of pesticides. The use of predators, parasitoids, and diseases is called biological control. Spiders might be considered biological controls in some fields, but most species tend to eat anything they can catch, not just the pest. IPM depends on accurate identification of the pest and a thorough knowledge of its life history so that control efforts can be directed at the pest's most vulnerable life stages. However, if not used wisely, any pest control method may harm other species or their habitats.
Habitat destruction is the number one threat to all insects, spiders, and their relatives. Pollution, pesticides, land development, logging, fires, cattle grazing, and violent storms are just some of the events that damage or destroy their habitats. Introduced, or exotic, plants and animals can also have devastating effects. They compete with native arthropods for food and space. Native arthropods are usually capable of dealing with organisms that they have evolved with over millions of years, but they are often defenseless against exotic predators and diseases.
Loss of habitat and competition with exotic species affect the availability of food, mates, and egg-laying and nesting sites. The reduction or loss of any one of these resources can make a species vulnerable to extinction (ehk-STINGK-shun). Extinct species have completely died out and will never again appear on Earth. Arthropods that are widely distributed or feed on a variety of plants or animals are less likely to become extinct, but those living in small fragile habitats with specialized feeding habits are more likely to become extinct when their habitats are disturbed or destroyed. The fossil record shows that extinction is a natural process. Yet today, the loss of thousands of species of plants and animals each year, mostly arthropods, is not the result of natural events but is a direct result of human activities.
Scientists, politicians, and concerned citizens around the world have joined together to establish laws that protect arthropods and their habitats. The United States Fish and Wildlife Service helps to protect species threatened with extinction. They list seventy-seven species of arthropods as Threatened or Endangered, including forty-four insects (mostly butterflies), twelve arachnids, and twenty-one crustaceans. Some countries set aside land as preserves specifically to protect arthropods and their habitats.
The World Conservation Union (IUCN) publishes a list of species threatened by extinction. It places species in the categories Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Near Threatened, Data Deficient, or Least Concern. In 2003 the list included 1,252 species of insects, spiders, and other arthropods. The sad fact is that scientists will probably never know just how many arthropod species are threatened with extinction and need protection. For example, tropical rainforest and coral reef habitats are disappearing so quickly that scientists have little or no time to collect and study their arthropod species before they are lost forever. Humanity's health and well-being depend on preserving all life, not just species that are big, pretty, furry, or feathered. Maybe you can be one of the scientists of the future that helps to save an insect or spider from becoming extinct.
FOR MORE INFORMATION
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