Garden Allies: Spinning & Weaving Spiders

From Folklore to Biological Control Agents
Despite their relatively small size, spiders loom large in the folklore and mythology of many cultures. Arachnida, the name of the arthropod class that includes spiders, stems from the myth of the Greek weaver, Arachne, who challenged the goddess Athena to a contest and was subsequently turned into a spider. The Navajo learned to weave blankets from a girl taught by Spider Woman. In West Africa, we find Anansi, the spidery trickster who later traveled with the slave trade to the Caribbean. In the British Isles, we hear the tale of Little Miss Muffet and, more recently, of Boris the Spider, the ill-fated arachnid featured in a song by The Who. In the United States, we sing to our children about itsy-bitsy spiders climbing up waterspouts, but also instill in them a fear of hairy, scary spiders.

The relatively sedentary snare builders (spinning and weaving spiders), and many of the wandering and hunting spiders (see Pacific Horticulture, October, ’08), belong to the true spiders (Araneomorphae). These are distinguished by chelicerae (fangs) that can move apart sideways before striking, allowing a greater biting span than for spiders in the Mygalomorphae, which can only strike straight downwards. Each fang has a tiny hole near the end, connected to venom sacks (absent in only a few spider species). Once the victim has been immobilized, a pair of accessory jaws, the maxillae, is used to assist in prey manipulation. In addition to the chelicerae and maxillae, spiders have a pair of pedipalps (palps) between the chelicerae and the first pair of legs. Palps are used to sense objects, and to aid in feeding, web building, and prey capture; males employ them for sperm storage in reproduction.

The Magic of Silk
Several families of spiders use silk to build snares for their prey. Silk is composed of an unusual protein, and uses a lot of energy to produce; some spiders can conserve by consuming their old web before spinning a new one. Orb webs, the familiar symmetrical round webs that catch flying insects, are created principally by the Araneidae (eg, common garden spiders, Argiope and Araneus) and, to a lesser degree, two other families (Theridiosomatidae and Tetragnathidae). The small Agelenidae (funnel and grass spiders) create funnel-shaped webs, running out to capture prey and drag it back to the shelter of their hideaway. The Linyphiidae (sheet or dwarf spiders), the dominant family in the north temperate region, produce sheet webs. Theridiidae (black widows and other comb-footed spiders) make irregular meshes, as do the Pholcidae (daddy-long-legs). Some spider species use a combination of all four types of webs. Spiders are generalist predators, but webs can act as selective filters. Some insects are able to avoid webs; for instance, beneficial syrphid flies are rarely caught in webs. Theridiidae and Linyphiidae webs catch far more plant-sucking insects than pollinators, and are best suited to capturing hopping or walking insects.

Curious Reproduction
Spider reproduction is unique. Once a male reaches maturity, he stops eating and becomes a vagabond, wandering about until he finds a mate. He then weaves a small silken web, onto which he taps a few drops of sperm. The sperm is absorbed and stored in receptacles located on the palps, which he uses to insert the sperm into the female. Mature males are easily recognized, as the palps are enlarged, and often clubbed. Sperm webs may be produced several times before a male’s short life ends. In addition to being longer-lived, females are often larger than males. Finding a suitable mate can be a tricky business, as males may occasionally be mistaken for prey. In web-building species, males usually “pluck” the strings of the web to signal that they are not prey.

When the large and common garden spider (Argiope) appears in autumn gardens, we are surprised and wonder where they came from. The spiders have been in the garden all season, but, as fall approaches, the females increase dramatically in size. Not all spiders are as easily spotted; many species are masters of camouflage and mimicry. Some species resemble twigs, grasses, plant buds, or bird droppings; many hundreds of species mimic ants. Some spiders like to hide in damp, dark corners—one reason they are often featured in horror movies.

Spiders are an integral component of a healthy garden food-web and a critical element of successful garden biological control strategies. In agricultural systems such as vineyards, sheet-web spiders (Linyphiidae) have recently been discovered to be important in pest management. Fortunately, it is easy to integrate spider habitat into an attractive landscape. In addition to the groundcovers, mulches, and bunch grasses useful for attracting wandering and hunting spiders, a few additional components are helpful for spinners and weavers. Plant a variety of flowers to attract flying insects so that spiders will have an abundant food supply. Leave the long, sturdy stems holding decorative seedheads to act as scaffolding for webs. Make sure to include bushes, trees, and the crevices provided by stones, logs, and loose bark. Hot coffee in hand to brace against the autumn chill of early morning, I love to venture into the garden to enjoy the magic of sparkling, dew-spangled webs, strung like jewelry among the fall stems of salvias and grasses.

In a Nutshell

Popular Names:
Orb weavers, sheet-web weavers, funnel-web weavers, mesh weavers

Scientific Name:
Class: Arachnida. Order: Araneae. Suborders: Mygalomorphae and Araneomorphae. (Arachnida also includes harvestmen, scorpions, ticks, mites, whip scorpions and pseudoscorpions)

Common Families:
Orb-weavers: Araneidae (garden spiders), Theridiosomatidae, Tetragnathidae, and Uloboridae. Funnel weavers: Agelenidae (grass and funnel spiders). Sheet webs: Linyphiidae (sheet-web and dwarf spiders). Mesh weavers: Theridiidae (comb-footed spiders, eg. black widow) and Pholcidae (daddy-long-legs)

Distribution:
Worldwide, over 40,000 species in over 100 families. Pacific Coast, about 1,000 species. Spider taxonomy is continually undergoing additions and revisions.

Life Cycle:
Number of molts depends upon species and gender; males and smaller species undergo fewer molts.

Appearance:
Eggs: always in a silken cocoon of varying shapes (spheres, teardrops, discs) attached to a substrate. Juveniles: a smaller version of the adult form. Adults: females and males appear similar until the penultimate or last molt, when males manifest enlarged pedipalps. Spiders range from virtually microscopic to a body length of 3.5 inches and a spread of 11 inches across the legs.

Life Span:
In temperate zones, usually one year, sometimes two. Some live for many years, and can take up to eight or more years to reach maturity. Only females have a long life expectancy.

Diet:
Flies (primarily), beetles, grasshoppers, and butterflies; sometimes millipedes, sowbugs, and even other spiders. Rarely, large spiders may prey on small vertebrates.

Favorite plants:
Bunch grasses, plants with sturdy stems for web-building. Certain spiders are associated with specific plant species; careful observation will reveal many relationships.

Benefits:
Spiders prey on many pests, and are themselves food for other beneficial garden residents.

Problems:
Spiders prey on beneficial insects as well as pests. Some bite, and some are venomous. (Few Pacific Coast spiders are dangerous; black widows and hobo spiders are exceptions)

Interesting facts:
In an effort to change the habits of spiders to match his own diurnal habits, one researcher gave spiders amphetamine and saw immediate changes in web design. Experimenting with other drugs on spiders, he concluded that LSD actually improved web symmetry, but caffeine resulted in the most chaotic webs.

Spider hairs are sensory organs, and may also be used for mechanical tasks: for sensing wind direction, for adhesion to substrate, as combs for silk, and for cleaning.

Sources:
Easy to attract into gardens.

More information:
www.ipm.ucdavis.edu/PDF/PESTNOTES/pnspiders.pdf
Spiders of the World, Rod and Ken Preston-Mafham 1984. A British book on spider basics with beautiful photographs.
Biology of Spiders, Rainer F Foelix (1996). For every spider enthusiast’s shelf.
Spiders and their Kin, A Little Golden Guide. Wonderful, well-illustrated introduction.
[See also Pacific Horticulture, October ’08, pages 14-15, for non-weaving spiders]