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 any-
Centipedes are strictly carnivores and actively hunt for small animals, usually insects. Occasionally larger centipedes will catch and kill a small mammal, such as a young mouse. (Arthur V. Evans. Reproduced by permission.)
Many insects and arachnids scavenge dead animals. This female scorpionfly and a mite are picking over the remains of a cricket. (Arthur V. Evans. Reproduced by permission.
thing 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. Man-tids 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
Most species lay their eggs somewhere in their habitat. Some prepare special chambers for their eggs. (Arthur V. Evans. Reproduced by permission.)
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,
These Chinese mantid nymphs are hatching out of their egg case. (Arthur V. Evans. Reproduced by permission.)
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 shortlived 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.
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