orb spidweb (46264 bytes)

All About Spiders

By Dr. Linda S. Rayor, Assistant Professor of Entomology, Cornell University

Spiders are beneficial inhabitants of any garden, ecosystem, or home because of their important contributions to biological control of pest insects. Spiders are considered to be the most important terrestrial predators, eating tons of pest insects or other small arthropods every year.  Spiders are generalist predators that are willing to eat almost any insect they can catch.  They are abundant and found in most habitats.  They only need to be left alone!

Facts About Spiders
Spiders' Role in Biocontrol of Insect Pests
How to Increase the Number of Spiders in the Garden
Common Garden Spiders

Facts About Spiders

How do spiders differ from insects? Spiders have two body parts, eight legs, chelicerae or fangs, pedipalps, no wings or antennae. Insects have three body parts, six legs, mandibles, wings, and antennae.

What traits are typical of spiders? Production of spider silk, pedipalps used in mating, venom, and external digestion.

How long do spiders live? Most spiders live for 1 to 2 years. Tarantulas and trapdoor spiders live for 6 to 20 years. In temperate regions, spiders overwinter as eggs or adults.

What do spiders eat? All spiders are predators. They do not eat plants, but only other living animals. Insects, spiders, invertebrates, some small vertebrates are the typical prey items.

How do they eat? Spiders use venom to kill or paralyze their prey. Venom is transported through a duct in the fangs. Digestive enzymes are regurgitated to liquefy prey externally. No solid food is eaten. Prey may be crushed by the chelicerae or wrapped.

How do they grow? The exoskeleton (external skeleton) of the spider must be shed or molted. Molting is necessary for growth, but a risky part of a spider's life as it leaves him vulnerable to other spiders.

Where are spiders found? Everywhere but in oceans and Antarctica. They are common in gardens.

How does silk work? Silk is extremely strong and flexible. Webs are enlargements of the spider's sensory system. Webs slow down and entangle prey. Orb webs are designed to capture prey with little silk, and translate the force of a flying insect throughout the web so it does not bounce out.

How do spiders reproduce? Carefully! Females will sometimes eat the male. The male's anatomy and courtship behavior are adapted to surviving mating.

Are many spiders poisonous? Although there are more spiders than all vertebrate species combined, spiders are relatively poorly known and needlessly feared. Actually, there are VERY few spiders whose bites require medical attention and these species are very rare in Colorado.    Most spiders do not have fangs that are strong enough to pierce human skin or venom which can affect us.  Of the 38,000 spider species described, there are only four species in the USA which are poisonous (black widow, brown recluse, hobo, and yellow sac spiders).  Only the ranges of the black widow and brown recluse may be uncommonly found in Colorado.

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The Role of the Spider in Biocontrol

There are many advantages to allowing spiders to biologically control insects in your garden or farm plot.  They are voracious predators found in most habitats, eat almost anything, and they are abundant.  They can rapidly colonize a suitable habitat and they will eat enormous numbers of insects.  Since many spiders overwinter as adults, they can reduce prey numbers early in the season before other biocontrol agents are active. 

On the down side, there are also disadvantages to using spiders as the sole method of biocontrol. Because spiders are generalist predators, they do not limit their diet to pest species but will happily consume non-pest insects (such as honey bees, butterflies), other spiders and other beneficial predators (eg. preying mantises, some beetles).  Many spiders are sit-and-wait predators so insect pests must come to them.  Spiders have long generation times, so they cannot increase rapidly in response to an outbreak of insects.  

The bottom line is that spiders are important and beneficial predators in gardens, but they are rarely successful in controlling a large outbreak of a single pest insect.

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How to Increase the Number of Spiders in Your Garden

  • Use mulch. It provides protection and humidity.
  • Provide places for web attachment or homes: Crates, tall plants, bundles of hay.
  • Leave areas untilled or leave plant stalks for overwintering habitats.
  • Grow flowers that bring in prey.
  • If spraying pesticides, spray at the times the spiders are less active or use a pesticide that has fewer effects on the spiders.

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Common Garden Spiders

 funnel weaving grass spider (60299 bytes) Grass Spider (Agelenidae), funnel web weaver - Grass spiders are largish spiders running upright on sturdy sheet webs with a funnel-like retreat. They have long spinnerets. Their funnel webs stretch between vegetation and in windows.
Orb Web Weaver (Araneidae) Araneus, 'catface' spider (55617 bytes) Orb Web Weaver (Araneidae) Araneus - The lovely spiral webs produced by orb weavers are architectural wonders designed to extend the spider's sensory system, reduce the amount of silk used, and trap or slow down prey. The webs are characteristic of different species and may help capture different types of insect prey. Webs are typically rebuilt daily. Many day-active put white decorations in their webs to deter birds from flying through the webs. Orb weavers have poor vision but are extremely sensitive to vibrations in the web. Araneus is probably the most abundant nocturnal spider, while Argiope is commonly seen during the day.
spidweb3.jpg (62880 bytes) Elongate Long-jawed Spider (Tetragnathidae), The family Tetragnathidae is newly separated from the family Araneidae. Members in this family also build orb webs, but a little different from Araneidae. They build orb webs with open center while members in the Araneidae family build orb webs with a dense center. Their webs are usually horizontally inclined in sunlit areas,often near water. Species in this family are elongate spiders with long chelicerae and legs. Unlike the spiders in Araneidae family which first wrap their prey in silk after capture and then bites it, spiders in family Tetragnathidae bite first and then wrap.
Cobweb Weaver (Theridiidae) (34038 bytes)     Cobweb Weaver (Theridiidae) - Most cobweb weavers have a body shape like the black widow, with a round shiny abdomen, but most are not dangerous. Cobweaver spiders are found where their cobwebs have sufficient supports and protection from the elements. They hang upside down in the web. They are important predators in all ecosystems.
daring jumping spider (34744 bytes) Jumping Spider (Salticidae) - Jumping spiders do not build webs. They wander through the garden looking for insect or spider prey. They are found in retreats with egg sacs in goldenrod, windowsills, or mailboxes. These sturdy spiders have exceptionally good vision and the ability to jump impressive distances for their size. Jumping spiders are known for their intelligence. Males court females with bright chelicerae, contrast colored palps and leg tufts.
wolf spider (25758 bytes) Wolf Spider (Lycosidae) - Wolf spiders, non-web builders, are among the most abundant spiders in open fields and gardens. To get an idea of their abundance, try going out at night with a flashlight held at the same level as your eyes. The many eyes you see reflecting back at you are wolf spiders! Wolf spiders wander on the ground or in the lower parts of plants at night. They are typically shades of brown with sturdy legs and large eyes. Wolf spiders are the only spiders that carry their egg sacs attached to their spinnerets and their young spiderlings on their backs.
daddy longlegs (35552 bytes) Daddy Longlegs (Opilionid), Harvestmen - Harvestmen are not spiders, although they are close relatives. Harvestmen eat very small insects and scavenge.
Sac Spider (Cheiracanthium) (27810 bytes) Sac Spiders (Cheiracanthium) - These are wandering spiders that are found on vegetation and the ground. They are important predators of pests in gardens. They make their retreat sacs in folded leaves or grass blades. Sac spiders are light colored and have apparent spinnerets.
crab spider with bee (55021 bytes) Crab Spiders (Thomisdae) - Crab spiders catch prey in open flowers. They catch insects, such as bees, flies, and butterflies, which come to the flower for nectar. Crab spiders are capable of camouflage--changing color to match the color of the flower. They can change to white, yellow, or pink within several days. One of the best ways to find crab spiders is by looking in flowers for insects that seem to be at an odd angle - these are insects that are being eaten! Crab spiders are distinguished by their sideways crab-like walk.

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About the Author

Photo: Linda Cantrill (orbweb weaver)

Photos: Judy Sedbrook (grass spider, long-jawed spider, wolf spider, crab spider, sac spider, daddy longlegs, cobweb weaver)

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Date last revised: 01/05/2010