Wikijunior:Bugs/Spider

Jumping Spiders actively hunt, and as their name suggests they jump on top of their prey when they attack.

Description

Spiders are members of a group of animals without backbones, called the Arthropods, meaning “joint legged.” They are not insects. They belong to the Class Arachnida, along with the tick, dust mite, and scorpion.

Spiders never have antennae or wings. Their eight legs have seven segments each. The tip of each leg usually has two or three claws and many bristles or hairs.

Spiders do not have an internal skeleton or backbone. They instead have an external skeleton, called an exoskeleton. It is tough and fairly rigid like a suit of armour. It is flexible for ease of movement. Muscles are attached to the inside. The exoskeleton supports and protects a spider’s soft delicate internal organs.

The head and thorax are combined to form the cephalothorax. Sometimes there are spikes, lumps, or odd shapes on it. There is a narrow pedicel (waist) connecting the cephalothorax to the abdomen. When viewed from above a tough covering or shield called a carapace can be seen.

At the front of the head are chelicerae (jaws). There are sharp teeth and fangs to pierce the exoskeleton of a spider’s prey. Each fang has a tiny hole connected to poison sacs. The fangs move on hinges and inject poison when the spider bites.

Scientists use the number and arrangement of a spider’s eyes to identify its family. There is normally a group of six to twelve simple eyes. One pair of eyes forms an image, while the surrounding eyes detect movement.

The silk with which a spider builds webs comes from a special organ in the abdomen called a spinneret. Most spiders have three or four pairs of spinnerets to deliver liquid silk to the outside.

People often confuse spiders with their eight-legged relations called harvestmen which look similar but have a different body arrangement. Unlike spiders, harvestmen have a body with head, thorax and abdomen fused into a single structure.

Spiders range in size from very small to relatively large. The small Branch Tipped Spider is only 1/16 of an inch long. The Orb-Weaver Spider is 3/8 of an inch long. The large Black and Yellow Argiope is 1-1/8 inches long.

Tarantulas are large hairy spiders. They can be 1-3/8 inches or more in length. Female Cobalt Blue Tarantulas can be 2-3/4 inches long. Brown Recluse Spiders have a leg span about the size of a quarter. Their body is about 3/8 of an inch in length and about 3/16 of an inch in width. Females can be 2 to 7 inches. They are commonly referred to as "fiddleback" or "violin" spiders because of a violin-shaped marking on the back. Often, this feature is very faint depending on the species and on how recently it has molted. Their color ranges from light tan to dark brown. They are covered with many fine hairs that give them a velvety appearance. They have long, thin, brown legs also covered with fine hairs. Brown Recluse Spiders have six eyes instead of the usual eight.

Habitat

Spiders are familiar predators found in nearly every habitat. Some species live in damp marshes and some live in dry deserts. Spiders that do not spin webs live in silk-lined burrows, while others have no permanent safe place. Some spiders live part of their lives on the surface of freshwater ponds and lakes. A few dive below the surface where they can survive for short periods of time. Spiders that live along the coastline are able to tolerate immersion in salt water when the tide is in. Only true water spiders, Argyroneta, are able to live completely under water.

Water Spiders build their homes underwater, spinning silken tents that they fill with bubbles of air. They are found throughout temperate Asia and Europe, including Great Britain and Ireland. They live in lakes, ponds, and ditches.

American House Spiders live in sheltered corners of houses, barns and other buildings. Their range is throughout the United States and Canada.

Brown Recluse Spiders are native to the United States. They are found mainly in the central Midwestern states southward to the Gulf of Mexico. Occasionally they are transported in materials and may be found outside this area. In favorable habitats, their populations are usually dense.

Brown Recluse Spiders seek and prefer seclusion. They usually occupy dark, undisturbed sites indoors or outdoors. Indoors, they may be found in attics and basements. They may seek shelter in shoes, folded linens, and behind furniture. Outdoors, brown recluse spiders may be found beneath logs and under loose stones in piles of rocks.

Southern Black Widows live among fallen branches and under many kinds of objects, including furniture and trash. Their range is from Massachusetts to Florida, west to California, Texas, Oklahoma, and Kansas. They are most common in the south.

Tarantulas are nocturnal and hide in dark cavities or burrows during the day. They come out to hunt by touch when it gets dark. They are sometimes called banana spiders, because tropical species arrive in cargoes of fruit. There are about 30 species in the United States.

Diet

All spiders feed almost entirely upon prey that they catch for themselves. A few species eat food which has been taken by other spiders. They usually eat other arthropods, mainly insects, woodlice and centipedes. One family feeds exclusively on other spiders. Very large species, such as the “bird-eating” spiders, are able to consume small animals.

Orb Weaver Spiders coat some of their silk with droplets of sticky liquid to catch prey. Other species create a bewildering maze of dry silk threads where insects get lost.

Water Spiders anchor their nests to water plants. During the day, they sit in the nest with their front legs in the water, waiting. When a water insect passes by, it is quickly pounced on, killed, and drawn into the nest. When they venture out of their nest to hunt, Water Spiders carry an air bubble on the abdomen to breathe from. Their genus name is Argyros, from the Greek for “silver” because the air bubble shines with a silvery hue through the water.

Some spider species stalk and ambush their victims on the ground. Most spin webs to catch prey in the air. Intricate spider webs can be seen between plants, branches, or buildings. There are many different web shapes, weaving patterns, and web locations. Each spider species spins only one kind of web. Scientists often identify a species by its distinctive web.

American House Spiders spin irregular webs made of sticky strands. Their webs catch dust as well as prey and are known as “cob webs”.

Southern Black Widows spin irregular meshed webs with a funnel-shape to hide in.

Most species of Tarantulas line the top of their burrows with silk, but do not spin webs to catch prey. They prey on insects, lizards, and other small animals.

Brown Recluse Spiders are largely scavengers, preferring dead insects and meat.

Defense

Most species have poor vision and feel movement through body hairs or slits in the exoskeleton. Brown Recluse Spiders are not aggressive, and normally bite only when crushed, touched or disturbed. There are reports of people having been bitten in bed after rolling over onto one of these spiders. Others have been bitten after accidentally touching one while cleaning storage area.

Southern Black Widows are the most feared of all spiders. Female Black Widow venom is very poisonous to people. Despite their reputation, they often escape rather than stab, unless they are guarding an egg. Males do not bite.

Some spiders defend themselves in strange ways indeed. ... Orb spiders make body doubles of themselves out of dead bugs and silk to defend themselves from predators while in their webs. The green lynx spider can spit venom up to a foot away, so you don't have to even come in contact with it to become a victim.

Reproduction Spiders undergo simple metamorphosis with only three main growth stages, egg, spiderling, adult. The pupal stage is omitted and the young spiderlings often look like small versions of adults. Because the exoskeleton cannot stretch, spiders can grow only by shedding or molting the exoskeleton several times.

Most spiders lay eggs in silken sacs. Egg sacs may be placed in the web or attached to twigs or leaves. Some species carry the sac around until the eggs hatch. Young spiderlings are often cannibalistic, eating their own kind.

Female American House Spiders spin pear-shaped brownish silken cocoons around the egg mass and hang it in a web.

Female tarantulas continue to molt after reaching maturity and may live to twenty years.

Special behavior

Some spiders have up to six types of silk glands to make silk for different uses. They use silk to catch and bind prey, to build webs and cocoons, and to spin draglines. Many spiders pull a dragline when walking. It serves as a life line for jumping clear of danger. Dragline silk is twice as elastic as nylon.

All young spiders release long silken strands, which they use like a parachute to ride the wind from one place to another. This process is called parachuting.

Spiders save their prey for a later meal by wrapping it in hundreds of thin threads called a swathing band.

Female Southern Black Widows often eat the smaller males after mating with their leader, earning the name “widow.”

When young Water Spiders hatch, they leave the nest covered with bubbles trapped by hydrophobic (water repelling) hair so they can breathe under water until they find a nesting place of their own.

Human effects

Few spiders bite people. The venom of most is harmless. Some South American species of Tarantulas have deadly venom, but the bite of North American Tarantulas is no more dangerous to people than a wasp or bee sting.

Spiders are part of folklore and legend in many parts of the world. They are considered beneficial to humans because they help keep insect populations in check. They help to balance nature in general.

References

Shea, G., & Bickel D. (Eds.). (2004). Encyclopedia of Discovery, Reptiles and Insects. San Francisco, CA: Fog City Press.

Milne, L., & Milne, M. (2000). National Audubon Society Field Guide to North American Insects and Spiders. New York, NY: Alfred A Knopf.

Preston-Mafham, K. (1984). Spiders of the world. New York, NY: Facts on File Publications.

Ohio State University Fact Sheet http://ohioline.osu.edu/hyg-fact/2000/2061.html

Understanding science and nature. Insects and spiders. (1993). Alexandria, VA: Time-Life.