Desert Grass (Agelenopsis aperta)

Physical description and identification

The desert grass spider is a long-legged, fast-moving funnel weaver in shades of grey, tan and brown — colours that vanish against dry grass and gravel. It is built for a single explosive sprint across its own sheet web, and almost everything about its shape serves that: a low body, slim legs and an unusually long pair of spinnerets at the rear.

Adults

• Size: body length ranges from about 10 to 19 mm, with females at the larger end (roughly 13–19 mm) and males slimmer at 10–13 mm. Legs add a great deal to the apparent size, so a large female on a lawn looks bigger than she is.
• Color: grey-brown to yellowish tan. The carapace carries two dark longitudinal bands with a paler stripe between them, and the abdomen has a row of darker chevrons and pale flecks along a lighter mid-line. Legs are faintly banded and covered in fine hairs.
• Eyes: eight eyes of similar size in two rows, the usual funnel weaver arrangement. They give only crude images; the spider works almost entirely from vibration.
• Distinctive features: the long, finger-like posterior spinnerets projecting beyond the abdomen are the most reliable field mark and separate grass spiders at a glance from similar-sized wolf spiders. Males have swollen pedipalps and longer legs. Separating Agelenopsis aperta from the other North American grass spiders requires a microscope, so range and habitat do much of the work.

Eggs and egg sacs

Females mate in late summer and autumn and then build lens-shaped egg sacs of white silk, hiding them under rocks, bark, dead vegetation or deep in the funnel retreat. Each sac holds dozens of eggs. In most of the range the adults die with the first hard frosts and the eggs overwinter inside the sac.

Spiderlings

Spiderlings emerge in spring as pale miniatures of the adults, disperse quickly and build tiny sheet webs of their own within days. They darken and grow through a series of molts over the summer and reach maturity by late summer, completing the cycle in a single year. Our guide to the spider life cycle covers the pattern in detail.

The web

The web is the classic agelenid design: a dense, flat or slightly dished sheet of dry silk, often the size of a dinner plate, drawn out over grass, cactus, low shrubs, rock ledges or a lawn, with one corner tapering into a tubular funnel where the spider waits. The silk is not sticky. It functions as a sounding board — an insect landing anywhere on the sheet sends vibrations to the funnel, and the spider crosses the sheet in a fraction of a second, bites, and drags the prey back. Where the sheet meets vegetation, extra threads act as knock-down lines that drop flying insects onto it. See our overview of spider webs for how this compares with a sticky orb.

Where desert grass spiders live

Agelenopsis aperta is a spider of the arid and semi-arid West. It is common across Arizona, New Mexico, western Texas, Utah, Colorado, Nevada and southern California, and it extends south into northern Mexico.

Within that range it occupies two very different kinds of site, and the difference matters. In open desert grassland, webs are scattered and spaced far apart: sun exposure limits how long a spider can sit at the funnel mouth, and good anchor points are scarce. Along desert streams and in irrigated land — riparian scrub, orchards, garden borders and watered lawns — insects are far more abundant and webs can be packed close together. Long-term studies in New Mexico showed that spiders from these two habitats differ measurably in how boldly they behave, and that the difference is partly inherited rather than simply learned.

Around houses the species turns up on lawn edges, in ground-cover plantings, on stone walls and in the corners of porches and window frames, which is where most people meet it.

Are desert grass spiders venomous?

Yes, but the venom is built for insects. Agelenopsis aperta is in fact the source of the agatoxins, a family of venom peptides named after the spider itself. Some of them block invertebrate calcium and glutamate channels so precisely that pharmacologists adopted them as laboratory tools: ω-agatoxin IVA, isolated from this species, is the standard reagent used worldwide to identify P/Q-type calcium channels in nerve cells.

That specificity is the point. The molecules that make the venom so useful in the laboratory act on ion channels that are far more important in insect physiology than in ours, and the quantity a spider this size can inject is tiny. There is no evidence of medically significant envenomation by grass spiders in people or pets.

Can desert grass spiders bite?

They can, but it almost never happens. A desert grass spider's entire strategy is built around a web it does not want to leave, and its first response to a looming hand is to bolt down the funnel. Bites are recorded mainly when a spider is picked up, trapped in clothing or pressed while someone is weeding or moving rocks.

The reported effect is minor: a brief pinprick, some redness, occasional mild swelling and itching that fade within a day. The fangs are small and often fail to break skin at all. Wash the site with soap and water, apply a cold pack if it is sore, and leave it alone. As with any wound, watch for spreading heat, pus or fever, which point to a bacterial infection rather than to spider venom and need medical attention.

Ecological importance and behavior of the desert grass spider

This species has been a model organism for the study of animal conflict. In arid grassland a web site that is both shaded and near insect traffic is a scarce, valuable resource, and spiders compete for it. Contests begin with signals — vibrating the sheet, posturing, approach and retreat — and escalate only when the two spiders are closely matched in weight. The heavier animal almost always wins, so a mismatched contest ends before either spider risks injury. The resulting pattern of threat, assessment and settlement made Agelenopsis aperta one of the most cited examples in the literature on territorial behaviour.

Natural predators: hunting wasps in the family Pompilidae are the most specialised enemy, paralysing the spider and using it to provision a burrow. Birds, lizards, larger spiders, centipedes and parasitoid wasps also take a share.

Prey-predator dynamics: the sheet intercepts grasshoppers, flies, beetles, moths, ants and leafhoppers — a large slice of the local insect biomass. Because webs stay in one place for weeks, a population of grass spiders is a steady, non-selective drain on insect numbers across a field or garden.

Seasonal behavior: spiderlings appear in spring, mature by late summer, and mature males abandon their webs to wander in search of females. Females remain, produce their egg sacs and die with the cold; the population overwinters as eggs.

Relationship with humans: entirely benign, and the sheets that appear across lawns on dewy autumn mornings are a sign of a healthy insect-eating population rather than an infestation.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The agatoxins are named after this spider's genus. ω-Agatoxin IVA, purified from Agelenopsis aperta venom, became the defining pharmacological tool for P/Q-type calcium channels and appears in thousands of neuroscience papers.
• Grass spiders are among the fastest sprinters on a web. The sprint is short and explosive, and because the silk is dry the spider never has to detach itself from glue to make it.
• Desert and riparian populations differ in temperament — riparian spiders are quicker to attack prey and slower to flee — and part of that difference is heritable, making the species an unusual natural experiment in behavioural genetics.
• The species was described in 1934 by Willis J. Gertsch, an American arachnologist who named a large share of the continent's spider fauna.

In summary, the desert grass spider is a harmless, highly effective insect predator that happens to have taught biologists a great deal about territorial contests and ion channels. Look for the two dark carapace stripes and the long trailing spinnerets at the mouth of a funnel, and you have found one.