Man of Steel: Armor, not weapons, protects harvestmen from certain doom

A lot of people mistake harvestmen for spiders, but there are two big differences between the two orders of arachnids. One, harvestmen do not scare the living shit out of me and I do not need to my girlfriend to kill any that wander into our house. Two, the eight-legged freaks commonly called daddy longlegs are awesome beyond your wildest imagination, whereas spiders are demons from Hell and are not awesome.

Among the 6,400 known species of havestmen, there are females who can give birth without the need for a male to fertilize thier eggs. There are males who mate with multiple females and then guard all the eggs, sometimes from egg-eating females they’ve recently mated with. There are harvestmen who enjoy each other’s company so much that they live together in groups of 70,000+ individuals. Then, there’s the granddaddy of wieners, willies, dongs and johnsons, the 400-million-year-old fossilized harvestman that possesses the world’s first known penis.*

Harvestmen are unique among arachnids in that they have a pair of exocrine glands that secrete a variety of chemical compounds that they use for communication, defense and even as antibacterial agents. Producing these chemicals is costly, though, and can deplete a harvestman’s energy reserves and affect their adult size, fitness and reproductive success.

For animals that produce chemical defenses, but also employ alternative defenses that are less costly, it makes sense use the cheaper options first and reserve the chemical weapons until they’re absolutely necessary. Harvestmen have a number of defensive options besides their chemical secretions, like running away, playing dead, pinching attackers with their mouthparts or leg spines and shedding legs as a distraction. They also have a hard exoskeleton that can protect them from injury in an attack.

If it can take a bit of punishment and keep a harvestman alive long enough for a predator to get frustrated and give up, relying on the exoskeleton would probably be the cheapest and best defense option. However, harvestmen are not a group that’s gotten a lot of attention from scientists (about one third of today’s known species were described by one guy, Carl Friedrich Roewer), so no one knew if this was actually the case. To find out, Elene da Silva Souza and Rodrigo H. Willemart, from the University of Sao Paulo, arranged a five-round arachnid vs. arachnid cage match to look at the aggressive and defensive behaviors of the harvestman Discocyrtus invalidus and the spider Enoploctenus cyclothorax.

E. cyclothorax is a large ambush hunter. Prior to this study, no one knew if it preyed on D. invalidus, but the spider’s penchant for dining on large roaches, crickets, other spiders and other species of harvestmen made it a sensible choice to play the role of predator. The first experiment tested whether or not the spider was actually up for the task and would go after the harvestman. Thirty-two spiders were collected from the wild and starved for one month to ensure they were hungry. Sixteen spiders were each paired with one harvestman and left together in the same tank for five days, while the other 16 spiders were each left in a tank with some crickets for the same amount of time.

The tanks were monitored once a day and inspected at the end of the five days. Thirty percent of the crickets were preyed upon within an hour of being placed in the tank and at the end of the experiment, less than a quarter of them were still alive. On the other hand, every last one of the harvestmen was still alive at the end of the fifth day and no injuries were noted on any of them.

Previous research by Willemart focused on starving E. cyclothorax and isolating it with another harvestman species, M. cuspidatus. In that study, only two out of nine spiders attacked and fed on the harvestman. Each of these spiders waited a full week in the tank before feeding and each fed on only one harvestman. The remaining seven spiders did not feed on the harvestmen even after 68 days in the tank (plus 21 days of starvation before even being placed in there). Every one of those spiders starved to death. Some of these spiders did attack, bite or touch the harvestmen, though, and strictly avoided them after that, suggesting that in close quarters combat, the harvestmen stood up for themselves somehow.

The second experiment focused on the details of the spiders’ and harvestmen’s interactions. Thirty-two spiders were starved and then each exposed to either a harvestman or a cricket and individually monitored the whole time. Eighty-one percent of the spiders attacked the harvestmen, but did not consume them, and ignored or avoided them after the first attack. Of the 13 harvestmen that were attacked, seven walked away from the spiders, five remained stationary and one was consumed. None of them attempted to defend themselves by pinching or biting the spiders, playing dead or releasing chemical defenses that the researchers could see or smell.

Silva Souza and Willemart wondered if the harvestmen were indeed releasing defensive secretions, but in very small doses undetectable by the human eye and nose, a subtle chemical shield that could explain their lack of concern with being eaten. For the third experiment, forty-eight spiders were each isolated with either a harvestmen that had its glands obstructed with glue, a harvestmen with glue on its back, crickets with glue on their back or crickets with no glue. The unclogged harvestmen were attacked as often as the clogged ones (both types of cricket were attacked and eaten almost equally, so the glue seems to have had no effect on the spiders), suggesting that the harvestmen secreted no chemicals even when they were able to. Again, no other defensive behaviors were seen and 75% of the harvestmen simply walked away from the attacking spiders, 21% just stood there during the attack and one was eaten.

The harvestmen weren’t using a chemical defense, but if they did, would it even do any good? The researchers collected the chemical secretions of ten harvestmen for the fourth experiment. Several spiders were offered crickets, and as soon as the spiders captured their meal, the researchers applied the harvestman secretions to the some of spiders’ mouths and applied water to some of the others. None of the spiders released their crickets. While E. cyclothorax released captured crickets in other studies after a dose of secretions from the harvestman Acutisoma longipes, the chemicals from D. invalidus didn’t ruin its meal here.

Four rounds into things, the harvestmen had put up no resistance to attack. They hadn’t fought back, they hadn’t run and they hadn’t used their chemical defenses, which, it turns out, didn’t seem to bother the spiders anyway. The spiders kept giving up, though, and mostly steered clear of the harvestmen after a single attack or encounter. The harvestmen only had one more trick up their sleeves…

I am Harvestman!

The harvestman’s exoskeleton had to be the trick to fending off the spiders. To test the exoskeleton’s mettle, Silva Souza and Willemart took ten spiders and held harvestmen up to their mouths to be bitten. The bites were recorded on video, and later viewing revealed that only one of the spiders pierced the body of the harvestmen.

Photographing a harvestman with a scanning electron microscope revealed the chinks in the armor that allowed that single bite. The harvestman’s exterior is hardened on its back, bottom, sides and legs. The only soft, unprotected spots on D. invalidus are its mouth, the articulations of its appendages and the tips of the legs. There’s such extensive protection that the spiders, despite being much larger and stronger, rarely managed to find a spot their teeth could sink into. The harvestmen make the most of their armor with the way they walk, keeping their body close to the ground and forming a “fence” around it with their legs.

These experiments don’t rule out the use of defensive secretions by D. invalidus at all times and places, and its defensive chemicals might be its best bet against other predators. When staring down E. cyclothorax, though, a harvestman in shining armor is efficient and effective enough.

*There is one awesome harvestman fact that’s only a myth, though. Urban legend has it that the harvestman is the most venomous animals in the world, but possesses fangs too short or a mouth small to bite a human. However, no known species of harvestman has venom glands or fangs.

Reference: Souza, E., & Willemart, R. (2011). Harvest-ironman: heavy armature, and not its defensive secretions, protects a harvestman against a spider Animal Behaviour, 81 (1), 127-133 DOI: 10.1016/j.anbehav.2010.09.023

Willemart, R., & Pellegatti-Franco, F. (2006). The Spider Enoploctenus Cyclothorax Avoids Preying On the Harvestman Mischonyx Cuspidatus. Journal of Arachnology, 34 (3), 649-652 DOI: 10.1636/S05-70.1

Image: “Macro shot of Opiliones Harvestmen” by Mehran Moghtadai, used under a Creative Commons license


2 Comments, Comment or Ping

  1. Michelle

    I LOVE everything you’ve written here about Harvestmen and adding Black Sabbath in there is just brilliant :) I just recently found out about harvestmen and have been telling everyone i know about them, but nobody seems as interested in them as i was when i first found out about them haha.

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