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Spiders
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Spiders are not insects. They are close relatives of ticks, mites and scorpions, which all belong in the group called arachnids. Unlike insects, which have three main body sections and six legs, spiders have two body sections and eight legs. The eyes, mouthparts and legs are found on the front section of the body, known as the cephalothorax. The second section, the abdomen, bears the parts of the respiratory system (spiracles and/or book lungs depending on the type of spider), the digestive and reproductive systems, and the external organs used for spinning silk or webbing. Most spiders are identified by size, color, markings on the body and the number (usually six or eight) and arrangement of eyes.
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Spiders are soft-bodied arachnids with two body parts: the fused head and thorax, called the cephalothorax, and the abdomen... known as the opisthosoma. Spiders have four pairs of walking legs and a fifth pair of appendages, located just behind their mouthparts, known as pedipalps. Pedipalps are not used for locomotion but often assist in touching and maneuvering prey. Each male pedipalp is equipped with a special structure that is used to transfer sperm to the female. This apparatus makes the male palp enlarged; it is often described as resembling a boxing glove. The mouthparts, called chelicerae, each end with a fang.
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Spiders usually have six fingerlike silk glands called spinnerets located beneath their abdomen. The silk comes from inside the spider's body as a liquid, thicker than water. When a spider wants to make a web, it squeezes the silk out of the two small holes at the back of its body called spinnerets. The moment it hits the air, the silk dries into a line that looks like a long strand of hair. Many spiders use their sticky silk webs to catch food, which consists of tiny animals. Some spiders use silk as draglines, which are long lines of silk the spider hangs onto as the wind blows it through the air.
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Spiders have provided benefits to humans and are an important part of the ecosystem. Some species are beneficial predators of pests in crops. But, they ... prey on insects that are beneficial, such as the honey bee. Spider silk has been used for the cross hairs in gun sights and other instruments. In the early 1700's, spider silk was used by a man in France to make fine gloves and stockings. The cannibalistic habits of spiders prevents them from being raised together to make silk production practical.
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Spiders have developed several different respiratory anatomies, based either on book lungs, a tracheal system, or both. Mygalomorph and Mesothelae spiders have two pairs of book lungs filled with haemolymph, where openings on the ventral surface of the abdomen allow air to enter and diffuse oxygen. This is ... the case for some basal araneomorph spiders like the family Hypochilidae, but the remaining members of this group have just the anterior pair of book lungs intact while the posterior pair of breathing organs are partly or fully modified into tracheae, through which oxygen is diffused into the haemolymph or directly to the tissue and organs. This system has most likely evolved in small ancestors to help resist desiccation. The trachea were originally connected to the surroundings through a pair of spiracles, but in the majority of spiders this pair of spiracles has fused into a single one in the middle, and migrated posterior close to the spinnerets.
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Spiders produce silk, secreted as a liquid through the spinnerets, which hardens on contact with air. Different types and textures of silk may be used to construct snares or webs, egg sacs, draglines and ballooning threads. Some spiders use web snares to trap prey, and all species construct a silk sac to deposit eggs. Many spiders attach draglines of silk to the substrate at intervals wherever they go, appearing to have a silk thread to hang onto when knocked from their perch. Some spiderlings sail through the air (ballooning) on wind currents. Young spiders climb to a high point and release silk strands until the drag from the wind is sufficient to support their weight.
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Spiders