When people encounter an arachnid that seems unusually large, strikingly colored, or simply unlike anything they have seen before, they sometimes describe it as an “alien spider.” This informal term refers to two distinct categories: invasive species that have spread to new regions, and spiders with physical characteristics that make them appear bizarre or otherworldly. This article explores examples from both categories.
The Invasive Joro Spider
The Joro spider, Trichonephila clavata, is often called “alien” due to its recent arrival and spread in new ecosystems. Native to East Asia, this large orb-weaver was first observed in the southeastern United States around 2013 or 2014, likely arriving via shipping containers. It has since rapidly expanded its range across Georgia, South Carolina, North Carolina, and Tennessee, with additional sightings reported in states like Alabama, Maryland, and West Virginia. The species adapts to diverse habitats, from forests to urban areas, and tolerates various climates.
Female Joro spiders are striking, reaching a body length of about 2.5 cm (1 inch) with legs extending up to 10 cm (4 inches), making them roughly the size of an adult human hand when outstretched. They display vibrant yellow, blue, and black coloration across their bodies and legs. Ballooning, where spiderlings release silk threads to catch the wind, enables them to travel considerable distances. This natural dispersal, alongside accidental human transport, aids their continued spread.
Despite their imposing size and bright colors, Joro spiders pose minimal threat to humans or pets. They are timid and non-aggressive, with fangs too small to easily pierce human skin. Their venom is not medically significant to humans, and bites are uncommon, typically occurring only if the spider feels directly threatened.
Spiders With Otherworldly Appearances
Some spiders are called “alien” due to their unusual physical forms, which diverge significantly from typical spider appearances. The spiny orb-weaver (Gasteracantha) is a prime example. These small spiders, with females measuring 5 to 10 millimeters, possess a hard, flattened abdomen adorned with six prominent, sharp spines. Their bodies come in various bright colors, including white, yellow, red, and black, often with contrasting black spots, giving them a distinct crab-like or mask-like look.
The wrap-around spider (Dolophones), found primarily in Australia and Oceania, possesses a flattened, concave body. This allows it to press tightly against tree branches. Its mottled brown coloration and unique body shape enable it to blend seamlessly with bark, making it nearly invisible to predators during the day. This specialized camouflage, known as mimesis, helps it hide until it builds its orb web at night.
The scorpion-tailed spider, Arachnura higginsi, is an Australian species with an elongated abdomen resembling a scorpion’s tail. This tail, which is soft and flexible, is present only in females and can be curled over their bodies when disturbed. While it mimics a scorpion’s appearance, this appendage is harmless and lacks a sting. The spider’s body color varies from cream to brown or black, sometimes with a bright yellow or red patch on the abdomen.
Debunking the Pseudoscorpion Myth
Pseudoscorpions are often mistaken for spiders or true scorpions. These small arachnids, typically less than 5 millimeters long, are related to spiders, scorpions, mites, and ticks. Their most distinctive features are their large, scorpion-like pincers, called pedipalps, located at the front of their bodies.
Despite their resemblance to scorpions, pseudoscorpions lack the elongated tail and venomous stinger of true scorpions. They also differ from spiders by not possessing spinnerets for producing silk or building webs. Pseudoscorpions use their pincer-like pedipalps to capture small prey and inject venom, which is not harmful to humans. These creatures are harmless and often beneficial indoors, preying on common household pests.