The Bolas Spider, Charlotte's Cunning Cousin

How Bolas Spiders Trick, Then Trap Their Prey

Bolas spider are cunning hunters.
Bolas spider. Flickr user Judy Gallagher (CC license)

The fictional Charlotte of E. B. White's classic tale, Charlotte's Web, was known for her clever ability to weave words into her web. Were Charlotte real, she would have nothing on her cunning cousins, the bolas spiders. Although classified as orb weavers like Charlotte, bolas spiders don't bother building ornate webs. Instead, they use an entirely novel method of hunting.

How Bolas Spiders Got Their Name

Bolas spiders are so named for their remarkable use of silk weaponry to capture prey.

While other orb-weaving spiders build sticky, circular webs to ensnare insects passively, the female bolas spider takes matters into her own tarsi (spider feet).

A bolas is a simple yet effective throwing weapon, use most famously by South American gauchos to capture cattle on the run. The bolas consists of round weights suspended from interconnected cords. The gaucho swings the bolas rhythmically, building momentum, and then throws it at the feet of the fleeing animal, entangling its legs to stop it from running.

When hunting prey, the female bolas spider constructs similar weaponry using silk. She produces a silk line with a dense and sticky ball of silk at the end. Holding the tether, she launches the ball at her target, which sticks to the insect upon impact. William G. Eberhard, a scientist at the Smithsonian Tropical Research Institute who studies bolas spiders, notes that a more accurate name would be the "sticky yo-yo spider." Once it hits a target insect, the glue-like properties of the silk ball allow the spider to maintain hold of it.

Usually, the bolas spider will descend the line and wrap the prey with silk, to save it for a later meal.

Remarkably, the bolas spider doesn't prepare her bolas in advance. When she senses the vibration of an approaching insect (almost always a moth, her favorite), she quickly constructs a weapon. She doesn't deploy her bolas until another vibration triggers her to respond.

Bolas Spiders Use Scents to Lure Prey

But wait, there's more! Here's the extra cunning part of the bolas spider's hunting method. While having a sticky wad of silk to wing at passing insects is somewhat effective, it works even better if you can trick your prey into coming toward you first.

Bolas spiders use a strategy called aggressive chemical mimicry to lure their prey. Each species of bolas spider enjoys munching on a few particular moth species. To increase its chances of catching its preferred meal, the bolas spider produces pheromones that mimic those of a sexually receptive female moth. In fact, each bolas spider can produce several difference moth pheromones, to mimic the scents of the moth species it enjoys eating the most.

A male moth on the prowl for a sexual partner cannot resist the pheromone trail, thinking it's leading him to a female mate. Imagine his surprise when he follows the scent of a woman, closer and closer, only to be nailed with a sticky ball of silk and dragged to his death.

And just when you thought the bolas spider couldn't be more conniving in its quest for prey, consider this. A bolas spider may feed on several different species of moths, each of which is active at different times of the night.

The spider has figured out how to release the right pheromone at the right time of night to attract precisely the moth that is flying at that hour.

As an example, the American bolas spider (Mastophora hutchinsoni) likes to feed on both the smoky tetanolita moth (Tetanolita mynesalis) and the bristly cutworm moth (Lacinpolia renigera). The cutworm moths tend to fly earlier in the night, before 10:30 or so, while the smoky tetanolita is a late night breeder that doesn't become active until closer to midnight. Researchers at the University of Kentucky found the American bolas spider produces cutworm pheromone earlier in the night, and gradually reduces it as the night wears on. By late night, the moth has switched to the tetanolita pheromone.

What Do Bolas Spiders Look Like?

Bolas spiders look somewhat different from other orb weavers.

They usually have a shiny appearance, "like a fresh bird dropping" according to Bugguide. In many cases, female bolas spiders can be identified as such by the presence of two humps on the dorsal surface of the abdomen.

In the orb weaver spiders, it's common for the male to be notably smaller than the female of the same species. This is known as sexual dimorphism, a difference between males and females. In the bolas spiders, this size difference is taken to extremes. While females average between 10 and 15 mm in size, and some can be as long as 20 mm, the adult male bolas spiders are often a miniscule 2 mm long.

How Are Bolas Spiders Classified?

Although they look and behave differently than other spiders in their family, bolas spiders are orb weavers. They belong to their own genus.

Kingdom – Animalia
Phylum – Arthropoda
Class – Arachnida
Order – Araneae
Family – Araneidae

Genus - Mastophora

The Life Cycle of Bolas Spiders

In general terms (not accounting for differences in species and geographic region), bolas spiders mate at the end of summer. Females construct egg sacs in the fall, suspending them from silk lines near her retreat. Bolas spider egg sacs are quite large.

Where Do Bolas Spiders Live?

Bolas spiders (Mastophora genus) are known from Africa, Australasia, and the Americas. Most species inhabit South America. Just 15 species are known to live north of Mexico.

 

Sources:

"Genus Mastophora - Bolas Spiders," Bugguide.net. Accessed online April 7, 2017.

"Spider scents attract prey: Sex pheromone mimic lures two moths to their doom," by John Whitfield, Nature, June 24, 2002.

Accessed online April 7, 2017.

"Bolas spiders: masters of deception," by Catherine Scott, Spiderbytes.org, March 17, 2015. Accessed online April 7, 2017.

"The Natural History and Behavior of the Bolas Spider Mastophora Dizzydeani SP. n. (Araneidae)," (PDF) by William G. Eberhard, Psyche: A Journal of Entomology, December 22, 1980. Accessed online April 7, 2017.

"Mastophora – Bolas Spiders," Encyclopedia of Life. Accessed online April 7, 2017.

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Hadley, Debbie. "The Bolas Spider, Charlotte's Cunning Cousin." ThoughtCo, Apr. 8, 2017, thoughtco.com/bolas-spider-4136305. Hadley, Debbie. (2017, April 8). The Bolas Spider, Charlotte's Cunning Cousin. Retrieved from https://www.thoughtco.com/bolas-spider-4136305 Hadley, Debbie. "The Bolas Spider, Charlotte's Cunning Cousin." ThoughtCo. https://www.thoughtco.com/bolas-spider-4136305 (accessed January 19, 2018).