This Spider Knows its Opponent’s Weaknesses

Philipe de Liz Pereira, Wikimedia Commons // CC BY-SA 3.0

You wouldn’t know it, being thousands of times their size and easily able to squish them under foot, but harvestmen, a.k.a. daddy longlegs, are built like tanks. Their hard exteriors keep them safe from most spiders that would want to eat them, but scientists have now found one species with a knack for getting past their defenses. 

(Harvest)man of Steel

A few years ago, researchers studying harvestmen’s defenses tossed a few of them into tanks with multiple hungry spiders for a couple of days. They found that the predators were hesitant to attack them and often starved instead. Those that did decide to rumble with a harvestman usually only attacked once and then steered clear of them after that.

The researchers noted that the harvestmen never attempted to defend themselves by biting their attackers, playing dead or using their chemical weapons. They either walked away from the spiders or simply stood there and put up with the assault. Their secret, the scientists figured out, is a good suit of armor. 

Harvestmen have a hardened exoskeleton covering their back, bottom, sides and legs. The only soft unprotected spots are their feet, leg joints, and mouths. These weak points are small, and the harvestmen move in a way that protects them, keeping their bodies close to the ground and their hard legs arrayed like a fence around them. Even when the scientists immobilized harvestmen and held them up to the spiders’ mouths to be bitten, only one spider out of 10 was able to find a spot it could sink its teeth into. The rest kept trying to bite the exoskeleton, but their fangs slipped on the hard surface and couldn’t pierce it.

Other experiments had similar results. Even spiders that are much larger than harvestmen or that can attack from afar by spitting venom show reluctance to attack the armored arachnids, and find very little success when they do attack.

Mini Martial Artist

Now, a team of team of researchers in Brazil reveal that not every spider has a such a hard time taking down a harvestman. Julio Segovia, a zoologist at the University of Sao Paulo, kept finding dead harvestmen in the webs of the recluse spider Loxosceles gaucho. While recluse spiders are known for their potent venom, Segovia says L. Gaucho has a “delicate” body and fangs that are no match for hard armor. Yet, the species makes a meal of harvestmen when so many other spiders fail. Segovia wondered how they overcame the harvestmen’s defenses, and began investigating. 

Segovia and his team figured that the spiders waited until harvestmen were stuck in their sheet-like webs before approaching and finding an unarmored place to bite. When they put spiders and harvestmen together in tanks, though, they found that this wasn’t the case. Some of the spiders were allowed to build webs and maintain them, while others had their webs removed right before the harvestmen were introduced. Both groups of spiders captured and killed a similar number of harvestmen, and there was no difference in the number of bites they made or the time it took them to begin attacking. The webs didn’t seem to make any difference, and the spiders could kill harvestmen just fine without them.

To see what was going on, the researchers recorded the spiders’ attacks and analyzed their hunting technique. They found that the spider approaches harvestmen slowly, keeping itself in front of its target so that it can’t escape. Then it begins touching the harvestman, which Segovia says probably allows it to find the prey’s weak points. When it finds a soft spot to attack, the spider executes a judo-like take down, grabbing the harvestman with two sets of legs and throwing it on its back. It then climbs on top and delivers several quick, venomous bites before finishing the harvestman off with a longer bite that can last a few minutes. 

The scientists saw the spiders deliver almost 200 bites during the experiments, and every one was aimed at the harvestmen’s unprotected leg joints. Unlike other spiders that were more indiscriminate with their bites, these spiders wasted no time trying to attack armored spots, and Segovia thinks that their initial probing gives them the cues they need to find the holes in the harvestman’s armor.

“It’s as if recluse spiders were war strategists that exploit their opponent’s weaknesses, and banana spiders were street fighters that attack harvestmen using the wrong technique,” the study’s senior author, Rodrigo Hirata Willemart, told Agência FAPESP.

Like in previous studies, the harvestmen here didn’t physically fight back or use their chemical weapons against their attackers. They seemed to rely on their armor as they do with other spiders, but were caught off guard by the recluse spiders’ fighting techniques and precise attacks.

The researchers think that even though these spiders can circumvent their armor, harvestmen don’t have to worry about them too much. A harvestman’s body size means that a spider has to spend more time and energy attacking it and use more venom to kill it. With plenty of easier prey around, harvestmen aren’t worth the effort—even if a spider does get to show off its cool moves.