Dimorphic Jumping (Maevia inclemens)

Physical description and identification

Maevia inclemens is a small, alert, brightly patterned member of the family Salticidae and the best known species in the genus Maevia. It is instantly recognisable as a jumping spider — squared face, two enormous forward-facing eyes, short bursts of movement with long pauses to look around — and the males are recognisable as this species because they come in two forms most people would take for different animals. Dimorphic here refers to the males only: not the usual difference between the sexes, but two genetically distinct male types within one sex, at roughly equal frequency across the range.

Adults

• Size: females are 6.5–8.0 mm (0.26–0.31 in) in body length, males smaller at 4.75–6.5 mm (0.19–0.26 in). This is a spider that sits comfortably on a fingernail.
• The tufted male morph: body and pedipalps black, three conspicuous black tufts of bristles standing up across the front of the carapace, and pale, almost translucent legs.
• The grey male morph: boldly striped black and white along the whole body, no tufts, and bright orange pedipalps that flash during display.
• Females: light brown carapace and a chalky white to rust-coloured abdomen with a row of dark chevrons down the centre, pale unmarked legs, and a black band along each side dusted with fine orange scales.
• Eyes and legs: eight eyes in four pairs — the anterior median pair very large and forward-facing for the sharp image, the small lateral eyes giving near-panoramic motion detection (see spider eyes). The front legs are longer and thicker and are waved in display; the short, muscular rear legs deliver the jump.

Eggs and egg sac

The female spins a closed silk nest — in a curled leaf, under loose bark or in a sheltered crevice on a fence — and lays small, round, pale eggs inside a thick, fluffy white sac. She stays in the nest and guards the sac rather than carrying it, and can produce more than one batch in a good season.

Spiderlings

Hatchlings stay inside the mother's nest until after their first moult, then leave and live independently, hunting exactly as the adults do. Dispersal happens at this very early stage, not at maturity as some older accounts state, and the young grow through a series of moults, each inside a fresh silk chamber.

Hunting, and why there is no capture web

The dimorphic jumper builds no snare of any kind — no jumping spider does. The small side eyes pick up movement anywhere around the spider; it swivels to bring the big main eyes onto the target, then stalks in short freezes until the prey is within a few body lengths. Just before launching it touches its spinnerets to the surface and anchors a silk dragline, which catches it if the leap misses and also lets it adjust its body angle in mid-air.

The leap itself is hydraulic: spiders have no extensor muscles at certain leg joints, so the animal drives body fluid into the hind legs under sudden pressure and they snap straight. Silk is used constantly — for that dragline, for moulting chambers, for the egg nest and for overnight and winter retreats — but never to catch anything. For the contrast with true web-builders, see spider webs.

Where the dimorphic jumping spider lives

Maevia inclemens is a spider of eastern and central North America, recorded from the Atlantic states across to the Great Plains, north into southern Canada — Ontario, Quebec and Manitoba — and south to the Gulf. State records include New York, Massachusetts, New Jersey, Pennsylvania, Virginia, Michigan, Indiana, Illinois, Wisconsin, Kansas, Alabama and Louisiana.

The habitat is vegetation and vertical surfaces in the sun: ivy and other climbers, shrubs and low foliage, woodland edges and hedgerows, old fields and gardens, and very commonly the things people build — fence rails, deck posts, siding, window frames and sheds. It is a frequent visitor to porches and window sills in summer, which is how most sightings happen.

Activity is strictly diurnal and weather-dependent. On a warm bright morning a hedge can hold several; on a dull cool day the same hedge looks empty, because the spiders are in silk retreats. In the north of the range juveniles pass the winter in those retreats under bark or in curled dead leaves and mature the following spring.

Are dimorphic jumping spiders venomous?

Yes, in the ordinary sense that almost all spiders are venomous — Maevia inclemens uses venom to subdue the flies, leafhoppers and small moths it catches. The venom is not medically significant to humans or pets, and there is no evidence that it ever has been. No antivenom exists for any jumping spider because none is needed.

The venom is tuned to small invertebrate nervous systems, the volume delivered is tiny, and the fangs of a 6 mm spider are barely able to engage human skin. The Salticidae as a whole are among the safest spiders to have around: they are not related to the recluse spiders or the widow spiders, the groups responsible for genuinely serious spider bites.

Can dimorphic jumping spiders bite?

They can, but almost never do. The reaction of a dimorphic jumper to an approaching hand is to turn and face it, then retreat, drop on its dragline or jump clear. A bite requires the spider to be physically trapped against skin — caught in a glove, pressed under a sleeve or squeezed while being flicked away.

When it happens the effect is trivial: a momentary pinprick, a small red spot and occasionally a little itching, gone within hours. Wash the area with soap and water and apply a cold pack if needed; no creams, home remedies or medical treatment are required. See a clinician only if redness or swelling spreads well beyond the bite, which would indicate a rare allergic reaction or an ordinary skin infection rather than the venom.

Ecological importance and behavior of the dimorphic jumping spider

This is a daylight predator of small insects in exactly the places webs cannot be built — the open upper surfaces of leaves, rails and walls. Through a summer a garden population takes a continuous supply of flies, leafhoppers, aphids, plant bugs, small moths and mites, without a single strand of capture silk.

Courtship and the two morphs: because females choose by sight, males perform, and the two forms perform differently. The tufted morph displays from further back, rearing up tall so the three tufts stand out against the sky; the grey morph works in close, low to the surface, flashing its orange pedipalps. Video-playback experiments found no preference for either morph — females respond to the movement of the display, not the costume. The striped form catches attention from a greater distance, the tufted form performs better once close, so both persist at roughly equal numbers.

Natural predators: birds are the main threat in the open, along with lizards, mantises, assassin bugs and robber flies. Mud-dauber and spider wasps hunt small salticids to stock their nest cells, and larger spiders take them too.

Prey-predator dynamics: the eyesight that lets the spider measure a leap also spots an approaching predator, and the dragline is the escape hatch: a startled Maevia drops instantly and hangs beneath the leaf until the threat has passed.

Seasonal behaviour: the cycle is annual. Juveniles overwinter, mature in late spring, breed through early and mid summer, and adults are gone by autumn — roughly a year in total, most of it spent as a juvenile.

Relationship with humans: entirely benign. These spiders do not colonise houses, leave no webs to clear away and damage nothing. One found indoors has wandered in by accident; cup it and release it on a plant outside.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species name inclemens is Latin for harsh, rough or unmerciful — a surprising label for a 7 mm spider harmless to everything larger than a housefly.
• Male dimorphism at this scale is genuinely rare. In most spiders every male looks essentially the same; here two distinct male types with two distinct courtship dances coexist at roughly equal frequency, and video-playback experiments show that females respond to movement rather than to morph — neither costume wins, which is why both survive.
• Jumping spiders have the sharpest vision known for animals of their size — finer than a cat's — thanks to a tube-shaped main eye with a layered retina that the spider swings behind the lens to scan a scene.
• Every leap begins with a silk safety line that arrests a missed jump and acts as a rudder in mid-air.

In summary, the dimorphic jumping spider is a small, harmless, sharp-eyed hunter of eastern North American gardens and hedges, remarkable for a piece of biology you can watch on your own fence rail: two kinds of male, two kinds of dance, one species.