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 – and, again, awesome – among arachnids in that they have a pair of exocrine glands that secrete a variety of compounds like quinones, ketones and phenols 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 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 68.6 days in the tank and 21.8 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 contact they recognized undesirable or dangerous prey through chemical signals.

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 an amount of 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 content and secretions of ten harvestmen’s scent glands for a 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 mechanical or chemical 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 their body 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

Mantis shrimp are, ounce for ounce, some of the most fearsome predators that you can pull out of the ocean. The marine crustaceans of the order Stomatopoda (neither shrimp nor mantids, they got the name because of their physical resemblance to both) are tiny and unassuming, but can use their front claws to attack with incredible speed and tremendous force. Stomatopods armed with “smasher” claws (there are also those armed with spearing claws) regularly crack open crabs and snails with cudgels that work on the same principal as crossbows: a spring-and-catch mechanism allows potential energy to be built up and stored and then released all at once. When all that power is unleashed, stomatopods can bludgeon prey with 45 mph strikes (the fastest known limb movement in the animal kingdom) and 340 pounds of force.

These war hammers aren’t just for hunting meals, though. Stomatopods use them on each other in territorial disputes, too. Given what these strikes can do, how have mantis shrimp not power-punched each other into extinction?

There’s two parts to the answer. One is the way they hit each other. When sparring over turf, two mantis shrimp will usually exchange a few strikes to each others’ tails as a way of sizing each other up before committing to a full-on and rumble and mutually assured destruction. The second part is where they hit each other. These ritual test blows are made to each other’s telsons, armored tail segments that are strong enough to take the punishment.

To find out just how strong telsons are and how they withstand such force, Sheila Patek from the University of Massachusetts Amherst and Jennifer Taylor, from the University of Indiana Purdue, took a few shots of their own at them. They got some mantis shrimp, let them live out a few final days eating grass shrimp in luxurious plastic cup accommodations and then put them in the freezer until they were dead, but not frozen solid. Then, they superglued the stomatopods to a strip of Plexiglass and dropped stainless steel balls on them.

The pair recorded the impacts with high-speed video cameras and used the data to calculate a coefficient of restitution, a valuerepresenting the elasticity of an object, for the tails. The coefficient is expressed as a ratio of and post- and pre-impact velocities, the basic principle being that the amount of elastic energy absorbed by an object can be measured by the loss of momentum of a colliding object suffers. Coefficients of restitution are often used to characterize and regulate products that take their fair share of blows, like automobiles, body armor, sports equipment and even fruits and veggies.

Patek and Taylor calculated the telson’s coefficient of restitution as 0.56. This is similar to a major league baseball, which has a coefficient of restitution between 0.45 and 0.50 when colliding with a bat. The telson dissipated a significant amount of energy, 69%, when it compressed during impact with the steel balls. The loss of energy implies that the telson absorbs impact inelastically, like a heavy punching bag does.

Patek and Taylor also used micro-Computed Tomography (a 3-D imaging method that uses penetrating waves) scans to examine mantis shrimp exoskeletons to see if they could find anything that might explain the telson’s resilience.

They found that the stomatopods’ tails are two times thicker than normal at three ridges called carinae that run along the telson. While the center area of the telson crumples inward upon impact, the carinae don’t deform. This provides a balance of stiffness and compliance that helps impact resistance by both absorbing energy and resisting penetration, a strategy human engineers have co-opted for designing armor.

Reference: Taylor JR, & Patek SN (2010). Ritualized fighting and biological armor: the impact mechanics of the mantis shrimp’s telson. The Journal of experimental biology, 213 (Pt 20), 3496-504 PMID: 20889830

Patek, S., Korff, W., & Caldwell, R. (2004). Biomechanics: Deadly strike mechanism of a mantis shrimp Nature, 428 (6985), 819-820 DOI: 10.1038/428819a

Images: Female Odontodactylus Scyllarus by Roy L. Caldwell, UC Berkeley, for the National Science Foundation. Odontodactylus Scyllarus by Flickr user prilfish, used under a Creative Commons license.