A spider with a bright red dot often uses this vibrant coloration as a warning signal to potential predators, a phenomenon known as aposematism. This distinct marking immediately directs attention to a specific group of spiders known for their potent venom. The query about a spider with a red dot overwhelmingly points to the most well-known and medically significant species in the genus Latrodectus. Their distinguishing red mark is a primary feature used for identification.
Identifying the Black Widow Spider
The spider most likely described as having a red dot is the Black Widow (Latrodectus). The adult female is typically glossy, jet-black with a large, spherical abdomen, measuring up to 13 millimeters long (excluding legs). The characteristic red marking is usually an hourglass shape located on the underside of her abdomen, often visible when she hangs in her web.
The marking can vary, sometimes appearing orange or yellowish-red. In some species, the hourglass is broken into two separate spots, or, as with the Northern Black Widow (L. variolus), a row of red dots may appear along the top of the abdomen. Female Black Widows are significantly larger than males (sexual dimorphism), and only adult females possess venom considered a risk to humans. Male Black Widows are smaller, may have red and white stripes, and their bite is not medically significant.
Habitat and Web Characteristics
Black Widow spiders prefer dark, secluded, and undisturbed locations, often near ground level. Common outdoor habitats include:
- Woodpiles
- Under stones or debris
- In hollow stumps
- Around utility boxes or garden furniture
They are nocturnal, staying hidden during the day and actively spinning and maintaining their webs at night.
The webs spun by Black Widows are not the neat, geometric spirals of orb-weaving spiders. Instead, they are characteristically irregular, messy, and tangled, often described as “cobwebs.” These webs are made of strong, sticky silk designed to ensnare crawling insects. The spider frequently hangs upside down within this messy structure.
Assessing the Danger of a Bite
The venom of the female Black Widow contains alpha-latrotoxin, a neurotoxin that affects the nervous system and causes latrodectism. Bites are rare and usually occur only when the spider feels threatened or is accidentally pressed against the skin. Symptoms often begin with sharp, localized pain, sometimes described as a pinprick, followed by swelling and redness at the site.
If significant venom is injected, systemic symptoms can develop within 30 to 60 minutes. These include severe muscle cramps and spasms that may spread to the abdomen, chest, or back. Other signs of latrodectism are nausea, vomiting, sweating, headache, and elevated blood pressure. While fatalities are extremely rare in healthy adults, the elderly, young children, and individuals with underlying medical conditions may experience more severe reactions.
Immediate first aid involves cleaning the bite area with soap and water and applying a cool compress or ice pack. Seek professional medical attention immediately if a Black Widow bite is suspected, especially if systemic symptoms appear. Medical treatment is typically supportive, focusing on pain management with analgesics and muscle relaxers. Antivenom is available for severe cases and is considered for at-risk groups.
Spiders Mistaken for the Black Widow
Several other spiders are confused with the Black Widow because they share a dark coloration and may possess red or similar markings. The False Black Widow (Steatoda grossa) has a glossy, dark brown to black body with a rounded abdomen, but it lacks the red hourglass marking. Its abdomen is more oval, and any markings are usually faint, pale, or cream-colored.
The Brown Widow (Latrodectus geometricus) is in the same genus but is typically tan or brown and has a less prominent, often orange or yellow, hourglass on the underside. The Red-Backed Jumping Spider (Phidippus johnsoni) is a black spider with a bright red stripe or dot on its back. However, it is visually distinct due to its shorter legs and unique eye arrangement, and it is a harmless species.