Spider Biology

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Just the facts

  • There are approximately 40,000 known species of spiders worldwide out of nearly 1.7 million described species of plants, animals, fungi, bacteria, and protists.
  • The order Araneae (the spiders) ranks seventh in total species diversity among all other groups of organisms, and undescribed species of spiders are being found every year.
  • Spiders differ from insects in having eight legs instead of six, two body parts rather than three, no antennae, mouthparts called chelicerae rather than the mandibles of insects, usually eight eyes rather than the variable number found in insects, specialized leg-like appendages called pedipalps (or palps), and structures called spinnerets through which silk is produced (Figs. 1 and 2).
  • Spiders are important predators of insects and other arthropods in every ecosystem in the world.
  • An early arachnologist by the name of William S. Bristowe estimated that one acre of land could be home to 2.25 million spiders.
  • The weight of insects eaten each year by spiders exceeds the total weight of the human population. Without spiders, humans would be overwhelmed by insects at every turn.

Capturing Prey

  • Not all spiders use webs to capture prey.
  • Some spiders, known as wandering spiders, use keen vision to hunt. These visually hunting spiders stalk their prey like wolves, jump on their prey like cats, or ambush their prey while sitting motionless and camouflaged against the background.
  • The majority of web-building spiders have poor eyesight and rely on the vibrations of struggling insects transmitted through the silk of the web to locate their food.
  • Spider webs come in a variety of shapes and sizes (see Fig. 3). Each different web type is often so characteristic of a certain type of spider that an arachnologist can identify the family of spider to which a web builder belongs without even seeing the spider itself.

The Uses of Silk

  • Spider silk emerges from the body of the spider in liquid form through tiny spiggots on the spinnerets (Figs. 1 and 2). The liquid silk turns solid when it is pulled, either as the spider uses a back leg to pull the silk out of its body or when the spider attaches the droplet to an object and pulls its body away.
  • Spider silk is used not only for web construction but also for wrapping active prey.
  • All spiders, whether web builders or wandering spiders, produce a dragline of silk as they move around in the environment. This dragline is the spider's insurance against injury. If the spider is crawling along a leaf and a sudden gust of wind knocks it off, the dragline will keep the spider from falling too far.
  • Most female spiders also use silk to protect their eggs inside a soft silken eggsac. Burrowing spiders line their underground homes with silk thus keeping them free of dirt and debris.
  • When you see a spider web, it is an almost sure bet that the resident is either a juvenile spider or an adult female.
  • Spiders, like all other arthropods, must molt their outer shell or skin several times in order to grow into an adult. As soon as male web-building spiders molt into adulthood, they lose the silk glands that allow them to produce the capture threads of a web although they retain the ability to produce non-sticky silk.
  • Eating is not foremost in the minds of these males! Adult males can be found wandering around looking for a female with whom to mate. Often male spiders are found lurking at the edges of a female's web.

How Spiders Mate

Male or female?
You can tell adult male spiders from females by looking at their pedipalps, or the leg-like first appendages (See Fig. 1). Males use their pedipalps as intromittent organs, or structures used to inject sperm into the female. The testes of male spiders are located in the abdomen. A male spider constructs a tiny platform of silk, called a sperm web, on which he deposits a droplet of sperm. He then sucks this sperm into specialized ducts found in his boxing glove-shaped pedipalps. When it comes time to mate, the male inserts his pedipalps into the female's genital opening (called her epigynum). Adult females have a dark, hardened (or scleritized) epigynal opening.

The mating ritual
Prior to mating, a male must communicate to a female that he is a mate and not a meal. It is a myth that female spiders eat the male after mating. Although this does sometimes occur, it is the exception not the rule. A male web-building spider sings a silk song to the female of his dreams by plucking her web in a pattern typical of his species. These courtship vibrations are very different than the vibrations produced by an insect caught in the web. A wandering male spider literally dances for his lady love. These males wave their front legs and tap their pedipalps on the ground in a specific pattern. If the female is receptive, she will respond to the courting male and will allow him to approach and mate with her.

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