Spiders are a large (more than 38,000 described species worldwide), distinct, and widespread group. The earliest evidence of spiders comes from a 380 million year old (Devonian) fossil. Spiders occur in many types of habitats and are often very abundant. Typical temperate habitats may support up to 800 individual spiders per square meter. Point estimates of spider diversity suggest that more than 600 species may be found in a single hectare of tropical forest.
Unique derived characters that define spiders include cheliceral venom glands (rarely lost, e.g., the family Uloboridae), abdominal spinnerets, and the modification of the male pedipalps into sperm transfer organs.
Although spider bites are widely feared, few species are dangerous to humans. Brown recluse spiders (and their relatives, genus Loxosceles) and black widow spiders (genus Latrodectus) are the most medically important spiders in the United States.
In the United States, brown recluse spiders are found mostly in the Midwest and Southwest. Brown recluse venom causes a small, dry, irregular necrotic lesion that heals very slowly. Contrary to popular belief, brown recluse bites cannot be conclusively diagnosed from the wound. Serious medical conditions including Lyme disease, chemical burn, and Anthrax infection have been misdiagnosed as brown recluse bites, delaying proper treatment. It is not uncommon for such misdiagnoses to occur outside the range of the brown recluse spider.
Widow spiders are widespread in the United States. Black widow venom is a neurotoxin. Typical symptoms of envenomation include swelling of the lymphatic nodes, profuse sweating, rigidity of the abdominal muscles, facial contortions, and hypertension. Antivenom is available to counteract the effects of Latrodectus envenomation.
The ability to produce silk has evolved independently in several arthropod lineages. However, spiders are the only group to use silk throughout their lives. Silk is spun through spinnerets located on the posterior part of the abdomen. In addition to its conspicuous use as a snare to trap prey, silk is used to line burrows, construct retreats and molting chambers, make sperm webs, protect developing eggs, and as a dragline.
Individual species are able to produce up to seven distinct types of silk, each with a specialized function. Complex snares including orb webs incorporate several distinct types of silk. Some spiders periodically eat their web and are capable of rapidly recycling most of the protein into fresh silk. Some spiders, especially small species and immature individuals, use silk in a form of airborne travel called ballooning. To balloon, the spider climbs to a high point and releases silk into the air. When the drag on the silk exceeds the spider's mass, the spider releases itself into the air.
Silk is a protein fiber produced in glands that terminate in spigots on the abdominal spinnerets. In the gland, silk is a water-soluble liquid protein soup. As the silk is spun, it passes through an acid bath. The acid hardens the silk by causing the molecules to reorient. Complimentary regions of the silk molecule align and bond together in multi-layered stacks, forming protein crystals. These crystals are interspersed in a matrix of loosely arranged amino acids. The protein crystals give the silk its strength while the loose matrix provides elasticity.
The physical properties of silk are remarkable. Tensile strength is the greatest stress a material will tolerate before failure. Silk is stronger than most natural materials and is about half as strong as steel. However, silk is extremely extensible; it can tolerate substantial distortion (i.e., strain) before failure. The product of stress and strain is expressed as toughness and is the total amount of energy a material will absorb before failure. Silk has extremely high toughness; steel tolerates very little distortion in shape, and so is not very tough.
Basal araneomorph spiders produce adhesive silk from a plate-like cribellum just anterior to the spinnerets. The cribellum is covered in tiny spigots. Cribellate silk is composed of hundreds of very fine dry silk fibers around a few thicker core fibers. The physical basis of stickiness in cribellate silk is not well understood, but the adhesive force is proportional to the surface area contact between the silk and the object being held. Cribellate silk is combed out from the cribellum using the calamistrum, a group of specialized, curved setae on the metatarsus of the fourth leg.
Theridiid spiders usually have a comb of specialized setae on the ventral part of the fourth tarsus. They use this comb to through large drops of viscous sticky silk in the course of subduing prey. This is known as a sticky-silk wrap attack. Theridiids do not bite their prey until they are completely immobilized by silk. Tarsal combs can also be found in other spider families, including deinopids, nesticids, and synotaxids. The function of the nesticid and synotaxid comb is similar to that of the theridiid comb; deinopids also wrap prey in silk. However, they use a different kind of silk and the role of the deinopid comb in prey-wrapping behavior is unknown.
Adult male spiders have their pedipalps modified into sperm transfer organs. The form of the male palpal organ is extremely variable and is critical in spider taxonomy. Minimally, the palpal organ consists of a bulb containing a sperm duct and a terminal embolus. The bulb is attached to the palpal tarsus, which is called the cymbium. The cymbium may be ventrally excavated, partially enclosing the palpal bulb. The testes are located in the abdomen. There is no connection between the testes and the pedipalps. Instead, sperm is extruded from a furrow in the ventral part of the abdomen. A sperm web is typically spun to receive the sperm. To transfer the sperm, the spider inserts his embolus into the sperm droplet and draws the fluid into the palpal organ, probably by capillary action. During copulation, the male transfers his sperm to the female by inserting the embolus into the female genitalia.
Adult female spiders usually have a sclerotized plate called an epigynum located on the ventral part of the abdomen. The epigynum typically consists of a pair of copulatory ducts that conduct the embolus (terminal portion of the male palpal organ) to the spermathecae, where the sperm are stored. Separate fertilization ducts conduct the sperm to the uterus, where the eggs are fertilized. Fertilization is the prevue of the female, who releases the stored sperm as the eggs pass from here ovaries to the silken egg case she has constructed. This system of female genitalia is known as entelegyne. Some spiders including the most primitive groups do not have separate copulatory and fertilization ducts. Instead, the sperm enters and leaves the spermathecae through the same system of ducts. This system of female genitalia is known as haplogyne.
In some species of the spider subfamily Erigonine (family Linyphiidae), males have their head regions modified with various assortments of lobes, pores, pits, sulci, and modified setae. In the few species with modified heads for which mating behavior has been observed, the female interacts with these modified structures, often inserting her fangs into the sheath-like sulci of the male. During copulation, the male may secrete liquid from concentrations of pores associated with the sulci. It has been suggested that these secretions have nutritional value and are offered to the female as an incentive to mate. Note that the mating biology of the species illustrated here has never been observed. Similar head modifications can also be found in some other spiders, most notably members of the theridiid subfamily Argyrodinae.
All images were taken by Jeremy Miller using the Amray 1810 at the Smithsonian's National Museum of Natural History Scanning Electron Microscope Facility with assistance from Scott Whittaker. Most images were taken as part of: "Assembling the Tree of Life: Phylogeny of Spiders," a National Science Foundation grant (DEB 0228699) to W. Wheeler, L. Prendini, J. Coddington, G. Hormiga and P. Sierwald. Additional images were taken as part of Miller's dissertation research, supported by an NSF-PEET grant to G. Hormiga and J. Coddington (DEB 9712353). Some information on spider biology was modified from Miller, J. A. & D. Ubick. 2004. Spiders. Pages 105-128 in: Borror and DeLong's Introduction to the Study of Insects. Seventh Edition. C.A. Triplehorn & N.F. Johnson.
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