Spined Micrathena, Castleback Orbweaver
Micrathena gracilis
Araneidae (Orb-weavers), Order Araneae
Physical Description
A highly distinctive spider with a hard, leathery, and bulbous abdomen featuring five pairs of sharp spines or tubercles. The cephalothorax is relatively small and dark. Females have a complex, 'castle-like' abdominal silhouette, while males are significantly smaller and lack the prominent spines.
Size Estimate
Females: 7-10mm (body length). Males: 2-5mm. This specimen appears to be an adult female based on the prominent spines.
Coloration & Markings
Commonly marble-white, cream, or bright yellow with black or dark brown mottled patterns. Some individuals, like the one pictured, can appear reddish-brown or darker. The pattern typically mimics bird droppings or lichen to provide camouflage against woodland backgrounds.
Web Type
Large, vertical, circular orb webs with very tight spiral silk. They often build at head-height between trees or shrubs in forested areas and typically feature a circular hole in the center (hub).
Eye Arrangement
Standard Araneid pattern with eight eyes: four median eyes forming a trapezoid and two lateral pairs on each side, though they are difficult to see without high magnification.
Leg Characteristics
Relatively short and slender legs compared to other orb-weavers, typically dark brown or black. They are often tucked in close to the body when the spider is resting.
Venom & Safety Information
Venom Information
The venom is a mild neurotoxin design to paralyze small insects. It is not considered toxic or dangerous to humans.
Medical Significance
Harmless. Bites are extremely rare as the spider is not aggressive and lacks the jaw strength to easily pierce human skin. Symptoms, if bitten, are usually less severe than a bee sting, involving minor localized redness.
Safety Recommendations
No special safety measures are needed. If found on patio furniture (as in the photo), simply relocate it to a nearby bush or tree using a piece of paper or a cup. They are beneficial garden residents.
Habitat & Behavior
Habitat & Range
Common across North America, ranging from the eastern United States to the Great Plains. They prefer deciduous forests, wooded gardens, and shaded areas with dense vegetation.
Behavior & Temperament
Passive and non-aggressive. They spend most of their time in the center of their web. If disturbed, they may drop from the web on a dragline or vibrate the web rapidly to confuse predators.
Diet & Prey
Small flying insects like gnats, flies, mosquitoes, and small moths trapped in their sticky orb webs.
Seasonal Activity
Most active and visible during late summer and early autumn (July through September) when females have reached full size.
Description
The Spined Micrathena is one of the most unusual looking spiders in North America. Its hard, spiny abdomen protects it from predators like birds and wasps, making it difficult to swallow. It is a common sight for hikers, who often inadvertently walk into its high-tension webs in late summer.
Key Features
Ten distinct spines on a hardened abdomen, 'castle-like' appearance, vertical orb web with a hole in the center, and mottled white/black or reddish/black coloration.
Similar Species
Micrathena mitrata (Mitered Spider) which has fewer spines, and Micrathena sagittata (Arrow-shaped Micrathena) which has a distinctly triangular abdomen and bright yellow coloring.
Life Cycle
Females lay eggs in silk-covered sacs during late summer or autumn. Spiderlings hatch and overwinter within the sac, emerging in spring. They live for approximately one year.
Ecological Importance
Significant natural pest control, consuming large quantities of mosquitoes and nuisance flies in woodland ecosystems.
Conservation Status
Not evaluated by IUCN, but considered common and stable throughout its range.
Interesting Facts
The name 'gracilis' means slender or graceful. Its spines are an anti-predator adaptation; if a bird tries to eat the spider, the sharp spines make it a painful and difficult meal to swallow.
Photography Tips
Focus on the side profile to capture the unique shape of the abdominal spines. Use a macro lens if possible and try to shoot with backlighting to highlight the intricate silk of the web.
Identified: 2026-07-16T10:33:10.025348