Arrow-shaped Micrathena (Micrathena sagittata)

Physical description and identification

The arrow-shaped micrathena is a small, hard-bodied orb weaver named for the shape of the adult female: an abdomen that narrows to a point at the front, widens sharply at the rear and ends in two long, outward-flaring spines, so the whole animal reads as an arrowhead in mid-air. The colour combination — bright yellow, black and rust red — makes it unmistakable among North American spiders.

Adults

• Size: females are about 8–9 mm in body length; males roughly 4–5 mm. Even a large female is under a centimetre, so the spider is far smaller than photographs suggest.
• Colour: the female's abdomen is glossy canary to lemon yellow, marked with black spots and dark bands, and the rear spines are usually tipped black or red. The carapace and legs are reddish brown to rust, sometimes almost crimson. Males are drab by comparison: a small, elongated, mostly dark body edged in white.
• Spines: females carry three pairs of abdominal spines. The front two pairs are short cones; the rear pair is much longer, curving outward and backward like the barbs of an arrowhead. Males have no spines at all.
• Other features: eight small eyes in two rows, none enlarged; short legs held close to the body; a hard, shell-like abdomen that is stiff to the touch.

Eggs and egg sac

In autumn the female fastens her small, round, pale eggs in a silk sac placed at the edge of the web or on the underside of a nearby leaf, rather than in the middle of the orb. The sac overwinters in place and the female dies with the first hard frost. See our overview of the spider life cycle for how this annual pattern works.

Spiderlings

The spiderlings hatch the following spring. They start with a longer, narrower abdomen and only stubby spines, and the adult arrowhead outline develops gradually across successive moults through the summer.

The web

Both sexes spin a small vertical orb, typically 10–20 cm across, with a fine, closely spaced capture spiral and an open hub where the spider hangs head-down. Webs are strung between saplings, shrubs and low branches in the woodland understory, often at the edge of a clearing or a trail, and are rebuilt regularly as the sticky silk loses its grip.

One widely repeated claim needs correcting: the arrow-shaped micrathena is often credited with a zig-zag silk decoration, or stabilimentum, above the hub. That structure is a hallmark of the garden spiders of the genus Argiope. Micrathena webs are plain — the striking thing about them is the density of the spiral, not any added ornament. For how orb webs are built, see our guide to spider webs.

Where the arrow-shaped micrathena lives

Micrathena sagittata is an eastern North American species. It occurs from southern New England and the lower Great Lakes south to Georgia and Florida, and west to eastern Texas, Oklahoma, Kansas and Nebraska, continuing south through Mexico and Central America. It is generally less abundant than its close relative the spined micrathena, with which it frequently shares a wood.

The habitat is deciduous and mixed forest, forest edge, second growth and the scrubbier corners of large gardens and parks that back onto trees. The spider needs an open gap in low vegetation with enough traffic of small flying insects to make a web worth building, so clearings, path margins and streamside thickets hold the highest densities. It is a spider of the understory, rarely more than two metres from the ground.

Are arrow-shaped micrathena spiders venomous?

Yes, in the sense that applies to nearly all spiders: venom is how this species subdues the insects it catches. It carries no medical significance for people or pets. No serious envenomation has been documented, no antivenom exists because none is needed, and the spider is unrelated to the widow spiders or to recluses. Its fangs are short and fine, and on a spider this small they often fail to break human skin at all.

Can arrow-shaped micrathena spiders bite?

A bite is possible but very unlikely. The spider's defences are the orb itself and a fast escape: disturb the web and the animal drops on a dragline into the leaf litter or shakes the silk into a blur. It has no interest in confronting anything the size of a hand, and reported bites involve the spider being trapped against skin.

When a bite does happen it feels like a brief pinprick, with at most a small red mark that fades within a couple of hours. Clean the site with soap and water; a cold pack will settle any soreness. Nothing more is needed. A mark that instead spreads, blisters or grows more painful over days is a sign of secondary infection or of a different cause, and should be seen by a doctor.

Ecological importance and behavior of the arrow-shaped micrathena

The arrow-shaped micrathena is a diurnal sit-and-wait predator. It hangs head-down at the hub through the day, reads vibration through the radii, and reaches a struggling insect within a second or two, biting and then wrapping it in silk. The mesh is fine, so the catch is made up of small, weak-flying insects: gnats, midges, small flies, leafhoppers, tiny moths, parasitic wasps and the occasional beetle. Over a season a population removes a substantial number of them from the understory at no cost to anybody.

Natural predators: mud dauber wasps are the most consistent threat, paralysing orb weavers to stock their nest cells; spider-hunting wasps, birds, mantises and other web-raiding spiders take the rest. The abdominal spines and hard cuticle probably work as armour, making the spider difficult for a small bird to grip and swallow, and the bright yellow may add a warning signal that predators learn to avoid.

Prey–predator dynamics: because it works in daylight and low down in the forest, this species intercepts a stream of insects that nocturnal orb weavers never see, which is one reason several orb weaver species can share the same patch of wood without competing directly.

Seasonal behaviour: the cycle is annual. Spiderlings emerge in spring, grow through the summer and mature from July; adults are most obvious in August and September. Males stop building webs on maturing and wander, approaching a female's orb from the edge and plucking the silk in a measured rhythm before crossing to her.

Relationship with humans: harmless and unobtrusive. The spider does not enter houses and its webs are small enough to be a minor nuisance at most on a woodland path.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species name sagittata is Latin for “arrowed”, from sagitta, an arrow — a rare case of a scientific name that describes exactly what you see.
• Of the six abdominal spines, only the rear pair is long. Those two barbs are what turn a lumpy yellow abdomen into a recognisable arrowhead.
• Males and females look so different that the sexes were sometimes catalogued separately in early collections — the male is barely a quarter of the female's bulk and carries no spines.
• The spider hangs head-down, so the arrowhead points upward in the web. Photographs are often published upside down.
• Bright yellow on a hard, spiny body is a common combination among day-active orb weavers, and is thought to advertise how unrewarding the spider is to eat.

In summary, the arrow-shaped micrathena is a tiny, spectacular, completely harmless orb weaver of eastern woodlands. Look for the yellow arrowhead with red legs in a small web at knee to shoulder height between July and October, and leave the web where it is — the spider is working.