Identifying Venomous Spiders by Their Red Abdominal Markings

Spider Red Butts: Identifying Venomous Spiders by Their Abdominal Markings

Many spiders havedistinctive red markings on their abdomens, which can help identify venomous species. While most spiders are not dangerous to humans, some common red-bottomed spiders can inflict painful, toxic bites. Learn how to recognize spiders with red abdomens and whether their bites are a concern.

Why Some Spiders Have Red Butts

The reddish or orange markings on some spiders' abdomens come from pigments called carotenoids. Many insects and arachnids contain carotenoids to help attract mates.

In spiders, the vivid red pigments are thought to serve as a warning signal to potential predators by advertising toxicity. This phenomenon is called aposematismessentially, flashing bright colors to say "don't eat me, I'm poisonous!"

Some harmless spiders mimic the red warning colors of their venomous cousins for protection. However, most spiders with distinctively red-marked abdomens do possess toxic venom.

Venom Toxicity Varies Among Red-Marked Spiders

While the red abdominal design indicates venom, the toxicity varies greatly between different spider species. Some pack an extremely potent, medically significant bite. Others have mild venom that may only cause short-lived pain and itching.

Only two types of spiders with red butts have highly toxic venom that can cause severe reactions: the black widow and redback spiders. However, several other spiders with reddish markings can deliver moderately painful bites.

Widow Spiders (Latrodectus)

Widow spiders belong to the genus Latrodectus and are distinguished by the red or orange hourglass shape on their abdomens. They are found on every continent except Antarctica.

The three widow spiders of most medical concern are:

• Black widow (Latrodectus mactans) - Common across North America
• Redback spider (Latrodectus hasselti) - Found in Australia
• Brown widow (Latrodectus geometricus) - Native to Africa, now spread globally in warm regions

Female widows are the most venomous, delivering neurotoxic bites that can cause severe muscle cramping, abdominal pain, nausea, and excessive sweating. However, antivenom makes death extremely rare.

Redback Spider (Latrodectus hasselti)

The redback spider is one of the most dangerous spiders in Australia. It is a close relative of the black widow native to the Americas. Around 250 redback bites are reported annually.

Distinctive features include:

• Small black body with red stripe on the abdomen
• Around 1 cm body length
• Prefers habitats close to humans

Redback venom contains neurotoxins called latrotoxins similar to black widows. Bites cause severe pain, sweating, muscular weakness, vomiting, and headache. Antivenom is very effective in treating symptoms.

Brown Widow Spider (Latrodectus geometricus)

Brown widows are rapidly spreading from their native Africa to warm climates worldwide. They now inhabit parts of the southern and western U.S., Australia, South America, and the Caribbean islands.

Identification features include:

• Base body color varies from light tan to very dark brown or black
• Orange hourglass marking on underside of abdomen
• Spiky egg sacs with small tufts sticking out

Brown widow venom can produce pain, swelling, and numbness but is less toxic than black widows. Their bites are rarely medically significant.

Hobo Spider (Eratigena agrestis)

The hobo spider is a mildly venomous spider found across the northwestern U.S. and western Canada. They are medium-sized spiders identified by:

• Pale brown body covered in small hairs
• Darker brown violin-shaped marking on the cephalothorax (front body section)
• Abdomen with a series of chevron shapes

Hobo spiders have more potent venom than most other funnel web spiders. Their bites can cause minor skin lesions, though severe reactions are rare.

Redback Spider (Latrodectus hasselti)

The redback spider is one of the most dangerous spiders in Australia. It is a close relative of the black widow native to the Americas. Around 250 redback bites are reported annually.

Distinctive features include:

• Small black body with red stripe on the abdomen
• Around 1 cm body length
• Prefers habitats close to humans

Redback venom contains neurotoxins called latrotoxins similar to black widows. Bites cause severe pain, sweating, muscular weakness, vomiting, and headache. Antivenom is very effective in treating symptoms.

Brown Widow Spider (Latrodectus geometricus)

Brown widows are rapidly spreading from their native Africa to warm climates worldwide. They now inhabit parts of the southern and western U.S., Australia, South America, and the Caribbean islands.

Identification features include:

• Base body color varies from light tan to very dark brown or black
• Orange hourglass marking on underside of abdomen
• Spiky egg sacs with small tufts sticking out

Brown widow venom can produce pain, swelling, and numbness but is less toxic than black widows. Their bites are rarely medically significant.

Triangulate Cobweb Spider (Steatoda triangulosa)

The triangulate cobweb spider, also called the black triangle spider, can be found across the U.S. It has these identifying features:

• Medium-sized brown spider with lighter striping on legs
• Prominent black triangle shape on front of bulbous abdomen
• Builds messy cobwebs in garages, sheds, and woodpiles

Its venom is not considered medically important, though bites may cause mild pain, redness, and itching.

Harvestman (Order Opiliones)

Despite their spider-like appearance, harvestmen are not actually spiders. They comprise their own arachnid order called Opiliones.

Notable features include:

• Round, compact bodies
• Very long, thin legs
• Often reddish-orange coloring
• No silk-producing spinnerets

Harvestmen do not have venom glands and are completely harmless to humans.

Running Crab Spider (Philodromus spp.)

Running crab spiders belong to the large genus Philodromus. They can be found worldwide and get their name from their crab-like appearance.

They are identified by:

• Flattened, crablike body
• First two pairs of legs longer than hind legs
• Short, stout legs compared to many spiders
• Often pale with red abdomens

Running crab spiders prey on pest insects and are not considered medically threatening to people. Bites may cause temporary mild irritation.

A Look at Some Other Red-Marked Spiders

While widow spiders, redbacks, and hobos are the most notorious red-butted spiders, many other species share the distinctive abdominal marking. Most are not dangerous.

Jumping Spiders

Jumping spiders comprise over 600 diverse species worldwide. Many have iridescent scales or patterns on their abdomens, sometimes in red hues. Jumping spiders have excellent vision and pounce on prey.

Their bites are not toxic to humans and mainly cause only localized itching.

Orb Weavers

Orb weaver spiders construct large, intricate circular webs. Some tropical orb weavers have bright red, orange, or yellow abdominal markings used to attract prey and mates.

Their venom is not considered medically significant. Bites generally result in mild stinging with no lasting effects.

Lynx Spiders

Lynx spiders are prolific hunters that prowl plants in search of insect meals. They have excellent eyesight and do not build webs.

Some species have reddish dots or patterns on their abdomens. Lynx spiders rarely bite people. Their venom causes temporary mild pain.

Cellar Spiders

Cellar spiders build messy-looking tangle webs in dark corners of homes. They have elongated bodies with long, spindly legs.

Some have white bodies with bright red markings. Cellar spider bites are harmless, though the spiders can look intimidating.

Avoiding Bites from Red-Marked Spiders

Most spiders with red butts pose no threat, but black widows and their relatives can inflict severe reactions. Follow these tips to help avoid bites:

• Clear exterior clutter where spiders nest, like woodpiles.
• Carefully inspect or shake out shoes, gloves, and clothing before use.
• Wear gloves and protective clothing when cleaning sheds or garages.
• Keep beds pulled away from walls and tuck in sheets.
• Install insect screens on windows and doors.
• Use caution when reaching into dark small spaces or crevices.
• Keep homes clean and dry to deter infestations.
• Hire a professional exterminator if you have recurring spider issues inside.

Learning to identify potentially dangerous spiders by their markings helps avoid accidental contact. Always exercise caution around spiders with distinct red markings until you can confirm the species.

When to Seek Medical Care for Spider Bites

Most spider bites, including from red-marked species, can be treated at home with basic first aid and close monitoring. Seek medical evaluation right away if you experience:

• Severe pain or swelling at the bite site
• Expanding redness, lesions, or blistering of the skin
• Widespread itching or rash
• Signs of infection like pus
• Muscle cramping, difficulty breathing, vomiting, or other concerning symptoms
• Symptoms persisting longer than 48 hours

Black widow and redback bites in particular warrant urgent medical care due to risk of severe reactions. Quick antivenom treatment provides the best outcomes and fastest relief after these spider bites.

Key Takeaways on Red-Marked Spiders

Many spiders have red or orange markings on their abdomens to advertise toxicity and ward off predators. The red patterns serve as useful identification clues to determine if a spider is potentially dangerous.

The widow spiders and Australian redback cause the most concerning bites among red-butted species. Others like hobos can inflict moderately painful bites but are rarely medically significant.

Most small spiders with red markings pose little risk of serious injury. However, it is wise to avoid close contact with unfamiliar spiders boasting bright red butts until you can properly identify the species.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult with a healthcare professional before starting any new treatment regimen.