Thomisidae

Thomisidae

Family: Thomisidae — Crab Spiders

Crab Spiders (Thomisidae) are compact, often flattened spiders best known for their sideways, crab-like stance and sit-and-wait hunting style. Many are found on flowers, where they remain still among petals and wait for flies, bees, butterflies, and other visiting insects. Others live on bark, leaves, stems, grasses, or the ground, where mottled browns and grays help them disappear into the background.

For representative images and visual context, see the page: Crab Spider in the Gallery section.

Description

Thomisidae are generally recognized by:

• Overall form: Compact, broad-bodied, and often somewhat flattened; many have a wide, angular outline compared with web-building spiders.
• Body length: Usually small to medium-sized spiders; many North American examples are only a few millimeters long, with larger females reaching roughly 1 cm.
• Cephalothorax: Often broad and low, with the front of the body giving the spider a squared or crab-like appearance.
• Abdomen: Variable; rounded, oval, triangular, angular, or flattened depending on genus and sex.
• Color pattern: Highly variable. Flower-dwelling forms may be white, yellow, greenish, or pinkish; bark and ground forms are often brown, gray, tan, or mottled.
• Eye arrangement: Eight eyes, often with the lateral eyes placed on small raised tubercles. Eye arrangement can support family-level identification but usually is not enough for species-level identification from casual photos.
• Chelicerae and mouthparts: Usually not the most visible field mark; these spiders subdue prey at close range rather than by web entanglement.
• Pedipalps: Mature males have enlarged palps that are important for genus or species confirmation. They are usually not visible enough in wide field photographs.
• Legs: The first two pairs are commonly longer, thicker, and held outward or forward like grasping arms; the rear legs are often shorter.
• Leg banding, spines, or setae: Some genera show banded, bristly, or spined legs; these details are useful when photographed sharply.
• Spinnerets: Usually not prominent in field views.
• Web, retreat, or silk structures: Thomisids do not make prey-capture webs. Silk may be used for draglines, egg sacs, retreats, or tying leaves/petals together.
• Sex differences: Females are often larger and heavier-bodied. Males may be smaller, darker, more slender, and more mobile.
• Juvenile or immature differences: Immatures are common on flowers and foliage but can be difficult to identify beyond family or genus.

The most useful field impression is the combination of flattened body, sideways leg posture, enlarged front legs, and still ambush behavior. Color alone is unreliable because many species vary by sex, age, molt stage, background, and lighting.

Habitat and Behavior

Typical habitats include:

• prairie flowers, garden flowers, meadow edges, roadsides, old fields, and sunny herbaceous vegetation
• shrubs, saplings, leaf surfaces, grass stems, seed heads, bark, logs, and low vegetation
• ground-level leaf litter or dry, open sites for mottled brown genera such as Xysticus
• prey-rich flower heads where pollinators repeatedly land

Behavioral notes:

• Activity: Often active by day, especially flower-inhabiting forms that hunt visiting insects.
• Resting posture: Legs are held laterally or forward, giving the spider a crab-like stance; the spider may sit motionless for long periods.
• Web type or hunting style: Ambush hunter; does not use an orb web, sheet web, or cobweb to trap prey.
• Retreat or shelter: Some use folded leaves, petals, stems, or hidden vegetation as retreats; egg sacs may be guarded on leaves or among plant structures.
• Movement: Capable of quick sideways or backward movement when disturbed; otherwise usually still.
• Feeding or prey capture: Uses the enlarged front legs to seize insects at close range. Flower crab spiders may capture prey larger than themselves.
• Mating or courtship behavior: Males wander more than females and may be seen searching vegetation for mates.
• Egg sacs or maternal behavior: Females often attach and guard silk egg sacs in folded leaves, under petals, or among protected plant parts.
• Seasonality: Most noticeable from late spring through early fall in Minnesota and the Upper Midwest, with adults and immatures most often observed during warm months.
• Substrate or microhabitat association: Flower-dwelling species are often found where pollinators concentrate; brown, mottled species may be found on bark, stems, soil, or leaf litter.

Identification

Key features for field diagnosis:

• Size and build: Small to medium-sized, compact, broad, and often flattened.
• Color and markings: White, yellow, green, tan, gray, brown, or mottled; color may match the flower, bark, leaf, or ground surface.
• Cephalothorax pattern: May be plain, banded, or mottled; sharp dorsal photos help.
• Abdomen shape and pattern: Rounded, angular, flattened, or triangular; patterns may include side bands, spots, central markings, or mottling.
• Eye arrangement: Eight eyes, often with lateral eyes raised on tubercles; a front-facing macro view can be helpful.
• Leg length, banding, and spination: The first two pairs are usually longer and more robust; leg banding and spines can help separate genera.
• Pedipalps or epigyne, when visible: Mature palps or epigyne are often needed for confident genus/species confirmation.
• Spinnerets or posterior features: Usually secondary field marks for this family.
• Web type, retreat, or hunting behavior: Lack of a prey-capture web and patient ambush behavior support Thomisidae.
• Habitat or seasonal clues: Flower-dwellers, bark/ground dwellers, and grassland species may belong to different genera.
• Photo-identification limits: Many thomisids cannot be identified to species from a single dorsal photo. Multiple views are best: dorsal body, face/eyes, side profile, leg spination, and habitat context.

For field photography, try to capture:

• a dorsal view showing abdomen shape and markings
• a front view showing the eye row and raised lateral eyes
• the front legs and their resting posture
• the flower, bark, leaf, or ground surface where the spider was found
• scale, prey, egg sac, or retreat if present

Similar Species

Use this section to distinguish Thomisidae from likely lookalikes.

• Running crab spiders (Philodromidae): Also crab-like and laterigrade, but generally flatter and faster-moving, with legs more evenly spread for running. They often rest on bark, walls, leaves, or vegetation rather than sitting with enlarged forelegs poised on flowers.
• Huntsman or giant crab spiders (Sparassidae): Much larger, long-legged spiders with a broad, flattened stance. These are not the small flower ambush spiders usually meant by Thomisidae.
• Wall crab spiders (Selenopidae): Extremely flattened, fast wall or bark runners. Their body is more disk-like, and they do not match the typical compact flower-crab form.
• Lynx spiders (Oxyopidae): Active foliage hunters with long, spiny legs and a more alert, elevated posture. They usually do not have the broad, flattened thomisid body or heavy front-leg ambush stance.
• Jumping spiders (Salticidae): Compact hunters with large forward-facing principal eyes. They stalk and jump rather than waiting with crab-like forelegs spread.
• Orb-weavers sometimes called “crab spiders”: Some unrelated orb-weavers have “crab” in a common name, but they build orb webs and lack the Thomisidae hunting posture.

Focus on:

• body shape and proportions
• whether the front two leg pairs are noticeably stronger
• laterigrade, crab-like resting posture
• eye arrangement and face shape
• flower, bark, ground, or foliage association
• hunting behavior versus web-building behavior
• whether the spider runs, jumps, or waits in ambush

Ecology and Notes

Crab spiders are important small predators in flower and foliage communities. Flower-dwelling species often sit directly on petals or near the flower center, where pollinators land. Their camouflage can make them difficult to notice until they move or until prey is already captured.

In Minnesota and the Upper Midwest, thomisids are most likely to be noticed on wildflowers, garden flowers, tall prairie vegetation, shrubs, and sunny field edges. Yellow and white individuals are especially conspicuous once noticed, but mottled brown species on bark or leaf litter may be overlooked.

Photo-based identification should be treated carefully:

• family-level identification is often possible from posture and body form
• genus-level identification may be possible with good dorsal and frontal views
• species-level identification may require mature specimens and close views of reproductive structures
• juveniles, females in variable color forms, and worn individuals may remain uncertain
• flower color match is useful natural-history context but not a reliable species character by itself

Most crab spiders are harmless to people and are best treated as beneficial predators. They may bite if handled or pressed, but they are not considered medically significant in normal field encounters.

References

• iNaturalist taxon page
  https://www.inaturalist.org/taxa/47866-Thomisidae

• BugGuide
  https://bugguide.net/node/view/1957

• World Spider Catalog
  https://wsc.nmbe.ch/family/100/Thomisidae

• Regional field guide
  Bradley, R. A. Common Spiders of North America. University of California Press.

• Identification manual
  Ubick, D., Paquin, P., Cushing, P. E., & Roth, V. (eds.). Spiders of North America: An Identification Manual. American Arachnological Society.