Gasteracantha

Females are unmistakable due to the combination of six prominent abdominal spines, hardened shell-like abdomen, and bright coloration. No other North American orb-weaver combines these traits. Males are tiny, lack spines, and are difficult to identify without examination. Gasteracantha cancriformis specifically can be distinguished from the unrelated Micrathena orb-weavers by its broader, more flattened abdomen with fewer but stouter spines, and its more open web hub. Micrathena species typically have more numerous, finer spines and tighter spiral webs.

Females possess a broad, flattened, hardened abdomen with six prominent spines projecting outward, resembling a crab shell or medieval mace. The abdominal exoskeleton is thick and shell-like, not soft-bodied. Females range 5-13 mm in width (wider than long) and exhibit bright color polymorphism including ivory and black, yellow with black or red spines, or red with black spines. The underside is black with yellow or white mottled spots, with spinnerets arising from a central turret. Males are diminutive (2-3 mm body length), lack prominent spines, and are drably colored.

Tropical and subtropical woodland edges, citrus groves, gardens, and rural areas with adequate vegetation for web support. In the southern United States, found in low shrubs and bushes, often 2-4 feet above ground. Webs are constructed in open areas between branches or vegetation where flying insects are abundant.

Worldwide in tropical and subtropical climates. Most diverse in tropical Asia from India through Indonesia. Gasteracantha cancriformis occurs from the southern United States (Maryland, Arkansas, Oklahoma southward) through Central America to Argentina, and west to southern California. Other species occur in Africa, Asia, Australia, and Pacific islands.

Gasteracantha cancriformis is most abundant from October through January in Florida (late fall to early winter). Activity patterns vary by species and latitude; generally active year-round in tropical regions, with peak abundance during warmer months in subtropical areas.

Flying insects intercepted in orb webs, including flies, beetles, small moths, bees, and leafhoppers. Gasteracantha mammosa has been observed capturing bees despite being stung, and rejects pentatomids (stink bugs) apparently due to their defensive secretions.

Females deposit eggs in distinctive sacs attached to leaf undersides; the egg sac of G. cancriformis is covered in coarse yellow silk with a central green stripe. Spiderlings overwinter in egg sacs and emerge in spring. In G. minax, aggregated egg sacs experience higher parasitism rates than solitary ones.

Females construct circular orb webs 30-60 cm in diameter with large open hubs where they sit. Webs often contain white silk tufts on radial threads near the hub, hypothesized to function as warning beacons to prevent bird collisions. Gasteracantha mammosa immobilizes prey exclusively by biting rather than wrapping with silk, and pulls large prey to the hub along a radius rather than carrying suspended. G. minax exhibits facultative aggregating behavior where females join groups, attach web support threads to existing webs to reduce silk investment, suffer lower predation rates, and attract more males, though without increased prey capture success.

Predator of flying insects; contributes to insect population regulation. May serve as prey for spider-hunting wasps, though the hardened exoskeleton may provide defense. Egg sacs are parasitized by wasps. Adults visit flowers for nectar and may contribute to pollination.

Generally harmless to humans; bites are not medically significant. Sometimes called "crab spiders" colloquially but unrelated to true crab spiders (Thomisidae). Occasionally encountered in gardens and ornamental plantings where they may be mistaken for inanimate objects due to their crouched posture.

• MicrathenaAlso has spiny, hardened abdomens and constructs orb webs, but typically has more numerous finer spines, tighter web spirals, and is not closely related within Araneidae. Micrathena species are primarily New World.
• IsoxyaAnother genus of spiny orb-weavers with superficially similar abdominal armature, but distinct in spine arrangement and genitalic morphology.
• Macracantha, Actinacantha, ThelacanthaRelated genera that may be nested within Gasteracantha based on molecular phylogenetics; these share the spiny abdomen trait but differ in spine number and arrangement.

Despite the common name "crab spider" applied to some species, Gasteracantha is not closely related to true crab spiders (family Thomisidae). The hardened abdominal spines were long assumed to be defensive against predators, but this has not been rigorously tested; the thick exoskeleton itself may be more significant for defense than the spines.

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Name That Spider Species | Bug Squad
• Destination Big Cypress National Preserve to meet a smiley face spider: Spiny orbweaver, Gasteracantha cancriformis — Bug of the Week
• Bug Eric: Spinybacked Orbweaver
• Bug Eric: Arrowshaped Micrathena spider
• Bug Eric: February 2014
• Bug Eric: Services
• Predatory Behavior of Gasteracantha mammosa C. KOCH (Araneae: Araneidae)
• Biological and Behavioral Notes on Gasteracantha cancriformis (Arachnida: Araneidae)
• Dissecting a Geographical Colourful Tapestry: Phylogeography of the Colour Polymorphic Spider Gasteracantha cancriformis
• Copulation and sperm release in Gasteracantha cancriformis (Araneae: Araneidae): differential male behaviour based on female mating history
• Gene flow and Andean uplift shape the diversification of Gasteracantha cancriformis (Araneae: Araneidae) in Northern South America
• Costs and benefits of facultative aggregating behaviour in the orb‐spinning spider Gasteracantha minax Thorell (Araneae: Araneidae)
• The Genetic Basis of Color Polymorphism in the Orb‐Web Spider Gasteracantha cancriformis