Numerous true spider species (Order Araneae) display red coloration, either fully or as distinct markings. The striking appearance of these brightly colored arachnids often leads to the question of whether they pose a threat. The term “red spider” is frequently applied to a diverse group of creatures, including harmless spiders and other non-spider arachnids, which explains the confusion surrounding their identity and potential danger. Understanding the specific species involved is the first step in accurately assessing the concern.
Confirmed Species of Red Spiders
Many genuine spiders exhibit prominent red hues, often using this coloration for communication or defense. One notable group is the jumping spiders, particularly species within the genus Phidippus, common throughout North America. The Red-backed Jumping Spider, Phidippus johnsoni, is a prime example. Adult females reach about one centimeter in length and display a bright red abdomen, sometimes marked with a black stripe.
Another true spider with red markings is the Australian Redback Spider, Latrodectus hasselti, a member of the widow family. The female is easily recognized by her spherical black body and the distinctive, vivid red stripe running along her abdomen. Certain crab spiders (Thomisidae) are also red, such as the Misumena vatia, which can change its body color to match the flowers it ambushes prey on, sometimes exhibiting a red stripe.
Red Arachnids Commonly Misidentified
The tiny, bright red creatures often seen crawling on sidewalks, walls, or plants are not spiders, but mites, which belong to the subclass Acari, a separate group of arachnids. The most commonly encountered example is the Clover Mite (Bryobia praetiosa), which measures less than a millimeter in length and is a plant feeder. Mites are frequently mistaken for small spiders because they possess eight legs, the defining characteristic of the class Arachnida.
A key physical difference separates mites from true spiders: body structure. Spiders have two distinct body segments—the cephalothorax and the abdomen—joined by a narrow stalk called a pedicel. Mites, including the Clover Mite and the Red Spider Mite (Tetranychidae), have a single, fused body segment, giving them a more oval or globular appearance. Furthermore, true spiders spin geometric webs to catch prey, while the webbing produced by red spider mites on plants is a dense silk used for protection and to create a microenvironment.
Assessing the Danger of Red Spiders
The danger associated with red spiders varies dramatically by species, though the vast majority encountered are harmless to humans. The most medically significant red spiders belong to the Latrodectus genus, the widow spiders, which possess neurotoxic venom. The female Australian Redback Spider (Latrodectus hasselti) is the most clinically important spider in Australia. Her bite can cause latrodectism, characterized by severe pain, sweating, and muscle spasms.
The American Red Widow (Latrodectus bishopi), found primarily in Florida, also carries a neurotoxin used to subdue prey. Fortunately, fatalities from widow spider bites are extremely rare due to the availability of antivenom and modern medical care, but prompt treatment is advised. Red-marked jumping spiders like Phidippus johnsoni are not considered dangerous; their venom is mild, and their fangs are typically too small to pierce human skin effectively. The tiny red mites commonly mistaken for spiders are herbivores that pose no threat to human health.
Why Spiders Are Red
The presence of red pigmentation in spiders is primarily driven by two evolutionary pressures: warning predators or serving as camouflage. The brilliant red markings on species like the Redback Spider function as aposematism, or warning coloration. This signals to potential predators that the spider is toxic or unpalatable, helping them quickly learn to avoid these venomous arachnids.
In other species, the red color provides crypsis or camouflage. Certain crab spiders that are red or yellow can blend seamlessly into the flowers where they ambush pollinating insects. The color is produced by pigment molecules, such as ommochromes and carotenoids, deposited in the spider’s hypodermis or specialized cells. Carotenoids, which are often responsible for yellow and red colors, must usually be acquired through the spider’s diet, linking the vibrancy of the color to the spider’s foraging success and health.