Grass Spider (Agelenopsis)

Physical description and identification

Grass spiders are the North American members of the genus Agelenopsis, in the funnel weaver family Agelenidae. Fourteen species are described, and they are close enough in appearance that most are separated only by examining the genitalia of an adult. As a group they are unmistakable in the field: a brown, long-legged spider sitting head-down at the mouth of a funnel in a horizontal sheet of silk.

Adults

• Size: females typically reach 10–20 mm in body length, with the largest around 19 mm (about 0.75 in). Males are smaller and distinctly slimmer, with proportionally longer legs and swollen pedipalp tips.
• Colour: brown is the norm, with variations into grey, beige, ivory and near-black across the fourteen species. The carapace carries two strong dark longitudinal bands with a pale stripe between them, and the abdomen bears a pattern of darker chevrons on a lighter ground. The legs are faintly banded.
• Eyes: eight eyes in three rows — two on top, four in the middle and two below. This three-row arrangement is a genuinely useful mark, because most other funnel weavers show two rows. See our guide to spider eyes.
• Distinctive features: the spinnerets. Grass spiders have exceptionally long posterior spinnerets that project well beyond the tip of the abdomen and are visible without magnification — the single quickest way to confirm the genus.

Eggs and egg sacs

Mating takes place in late summer and early autumn. The female then builds a flattened, disc-shaped silk sac holding large round eggs and hides it under loose bark, beneath a stone, in leaf litter or inside dense vegetation rather than on the web itself. She usually dies within a few weeks of laying, and the eggs overwinter in place.

Spiderlings

Spiderlings emerge in spring and pass through a series of moults before maturity, growing only when they shed the old cuticle. Many disperse by ballooning on a thread of silk, which is how a mown lawn fills with fresh funnels within weeks. Our pages on spider moulting and the spider life cycle cover both stages.

The web

The web is a horizontal, slightly concave sheet slung across grass, low shrubs, ground cover, woodpiles or the angle where a wall meets a foundation, with a funnel-shaped retreat running off one edge into the vegetation. It is not sticky. There is no glue anywhere in it, and the sheet works instead as a trampoline and an alarm — an insect that lands on it is slowed by the tangle of scaffolding threads above, and the spider crosses the sheet fast enough to catch it before it recovers. Speed is what makes the design work. Webs become obvious on autumn mornings when dew collects on the sheets and a whole lawn appears to be covered in silk plates. For a comparison with sticky webs, see our guide to spider webs.

Where grass spiders live

Agelenopsis is a strictly New World genus. Species occur throughout the United States and across southern Canada, and the range continues south into Mexico and Central America. Some species are wide-ranging — A. naevia and A. pennsylvanica in the east, A. potteri in the north — while others are regional, such as A. oregonensis in the Pacific Northwest and the desert grass spider (A. aperta) in the arid south-west, in Arizona, New Mexico and western Texas.

The habitat is open, sunlit and structural: lawns, meadow edges, tall grass, low shrubbery, juniper and yew hedges, ivy, ground cover, brush piles and the outside of buildings where a sheet can be anchored between stems or between a wall and a bush. These are outdoor spiders. They occasionally wander indoors in autumn, usually as males looking for females, but they do not establish themselves inside the way the introduced Eratigena funnel weavers do.

Are grass spiders venomous?

Yes, as almost all spiders are — and no, not in a way that matters to people. Grass spider venom is built for insects and is not medically significant to humans or pets. No antivenom exists because none is needed. Interestingly, the venom of one species has been studied intensively for reasons that have nothing to do with danger: the desert grass spider, Agelenopsis aperta, produces a family of peptides called agatoxins that block specific ion channels with great precision, and these have become standard research tools in neuroscience laboratories worldwide. A compound useful in a laboratory at microgram doses is not a hazard on a lawn.

Can grass spiders bite?

They can, but it takes real effort to make one do it. Grass spiders are shy, extremely fast and permanently within a stride of their funnel, so their response to a hand or a lawnmower is to vanish. The rare recorded bites involve a spider trapped against skin — pressed under a hand in vegetation, or caught in clothing.

A bite produces brief local pain, a small red mark and sometimes mild swelling or itching that fades within a day or two. Wash it with soap and water, apply a cold pack and keep the area clean; seek medical advice only if pain and redness spread over the following days, which suggests a skin infection rather than venom. Their fangs are also small relative to their body, and many attempted bites do not break the skin at all.

Ecological importance and behavior of grass spiders

Grass spiders are ambush predators that hold a single web site and defend it. The spider rests in the funnel with its legs on the silk and reacts to vibration rather than sight, sprinting out, biting and dragging prey back into the retreat. Prey is mostly small insects — flies, moths, leafhoppers, ants, beetles, mosquitoes and grasshoppers — with other spiders taken opportunistically. A lawn with many funnels is a dense, free layer of pest control.

Natural predators: birds, especially ground foragers, along with spider-hunting wasps and mud daubers that stock their nests with paralysed spiders, toads, lizards, shrews, centipedes and larger spiders. Older accounts listing chameleons as predators are describing animals that do not live anywhere in this spider's range.

Prey-predator dynamics: web sites are a limited resource, and grass spiders compete for them. Work on A. aperta in the desert south-west made the species a classic subject in behavioural ecology: individuals assess a site for insect traffic and shelter, hold the good ones, and fight over them, with territorial contests settled largely by size.

Seasonal behaviour: in most of the range the life cycle is annual rather than the one to two years sometimes quoted. Spiderlings hatch in spring, grow through summer, mature in late summer, mate in early autumn, and the adults die with the first hard frosts while the eggs overwinter in their sacs. In the warm south some individuals survive the winter and live longer.

Relationship with humans: entirely benign. The webs are conspicuous but harmless, and they vanish after the first heavy frost. Nothing needs to be done about them; a grass spider that gets indoors can be carried out under a glass.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The genus was described by the German zoologist Christoph Gottfried Giebel in 1869. The name combines Agelena, the Eurasian grass spider genus, with the Greek opsis, "appearance" — literally "looks like Agelena".
• Venom from the desert grass spider gave neuroscience the agatoxins, peptides that block particular calcium and calcium-channel subtypes so specifically that they are sold as laboratory reagents.
• One species, Agelenopsis riechertae, is named for Susan Riechert, whose long-running field studies of A. aperta turned grass spiders into a textbook example of animal territoriality.
• Because the silk carries no glue, a grass spider is essentially running its prey down on a silk platform. Members of the genus are among the fastest spiders in North America over a short dash.
• The sheets that suddenly "appear" across a lawn in September were mostly there all summer. They only become visible when autumn dew collects on them.

In summary, the grass spider is a harmless, fast and hard-working neighbour that quietly thins the insect population of a lawn all summer. Eyes in three rows and two long spinnerets identify it, and its conspicuous funnel sheets are a sign of a healthy garden rather than a problem.