Giant arthropods, then and now

October 25, 2012 • 7:43 am

by Greg Mayer

The Naturmuseum Senckenberg in Frankfurt am Main, one of Europe’s great natural history museums, has announced the discovery in Laos of one of the world’s largest known daddy longlegs by Senckenberg researcher Peter Jager. The apparently new species is now being studied by Jager and his Senckenberg colleague, Ana Lucia Tourinho. Daddy longlegs are also called harvestmen, although I grew up with lots of daddy longlegs, and never knew them to be called harvestmen except in books.

(The current incarnation of the Senckenbergischen Naturforschenden Gesellschaft incorporates the museum in Frankfurt and several other German natural history museums and research institutes.)

Many people mistake daddy longlegs for spiders. While both spiders and daddy longlegs are eight-legged arachnids, daddy longlegs have a more compact body with the abdomen and cephalothorax not separated by a constriction as in spiders, and their legs are invariably thread-like. The new giant form from Laos, which has not yet been formally described as a new species, has a leg span of over 33 cm. The record is 34 cm for a South American species.

Although science fiction films abound in giant arthropods or arthropod-like aliens (my favorite: Attack of the Crab Monsters), it is in fact hard for an

Attack of the Crab Monsters (1957), original poster from Wikipedia.

arthropod to get very large. The size limitation of arthropods is due to constraints on their chitinous exoskeletons and their ability to breathe. The University of California, Berkeley, website Understanding Evolution has great animations explaining these size constraints. Both kinds of constraints arise from the disproportion of size and shape as animals get bigger. Basically, many of the functions of animals (including their weight bearing skeleton and their respiratory surfaces) increase as the square of their increase in size, whereas their need for the functions increases as the cube of their size. So, if you double the size of an animal, without changing its shape, you will quadruple its surface area, but its volume will increase by a factor of eight. Thus if you have a physiologically important surface area (say the lining of your respiratory system), you are going to be lacking by a factor of two, as your oxygen needs are set by your volume. The “square/cube problem” in biology has long been known, and J.B.S. Haldane, one of the founders of modern evolutionary theory, wrote an influential popular article on the subject, “On being the right size“, in 1926. (Haldane, by the way, a geneticist-physiologist-soldier-pacifist-communist-Hindu-atheist-patriot-expatriate, was the original most interesting man in the world, as we’ve noted before here at WEIT.)

The largest insects today are not very big. They grew larger in the distant past, and this is thought to be related to a higher concentration of oxygen in the atmosphere at that time (35% during the Carboniferous period, compared to 21 % today). The largest known arthropod ever, also an arachnid, but an aquatic one, a sea scorpion or eurypterid from the Devonian, was discovered by Simon Braddy and colleagues a few years ago (pdf).

Giant arthropods from the fossil record compared with the average height of a (British) human male; (a) the eurypterid Jaekelopterus rhenaniae, Early Devonian, Germany; (b) the trilobite Isotelus rex, Late Ordovician, Manitoba, Canada; (c) the dragonfly Meganeura monyi, Late Carboniferous, France; (d) the millipede Arthropleura armata, Late Carboniferous, Europe. Scale bar (a–d), 50 cm. (e) Chelicera of the giant eurypterid J. rhenaniae from the Early Devonian of Willwerath, Germany, PWL 2007/1-LS. Photograph, the disarticulated fixed (above) and (rotated) free ramus (below). Scale bar, 10 cm.

They were 2.5 m long, and longer if you stretched out their claws (and imagine the size of the turds produced by d!). This is, as shown in the figure above somewhat larger that the typical British male (who, I must say, appears admirably buff and well-muscled in this outline drawing; perhaps it’s all that wrist-wrestling, or is it elbow-bending, down at the pub).

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Braddy, S.J., M. Poschmann, and O.E. Tetlie. 2008. Giant claw reveals the largest ever arthropod. Biology Letters 4:106-109  (pdf)

Haldane, J.B.S. 1926. On being the right size. Harper’s Magazine (March) 424-427. (retyped pdf)

h/t Andrew Sullivan

33 thoughts on “Giant arthropods, then and now

    1. Another UK voice: daddy long legs = crane fly, harvestman = neckless spiderlike thing with eight long thin legs.

  1. I generally like spiders and other creepy crawlies, but I would definitely not like to meet (d) walking through the forest.

    1. (d) is okay, it’s only a millipede, eats plants and suchlike. But my worst nightmare would be meeting a CENTIpede that size.

    1. The BBC did a series a few years ago on these – Walking with Monsters – search that & you will get some great clips.

  2. That’s quite a build you average British men got there 😉 (referring to the last picture)

    I love that name: ‘daddy longlegs’. Here in Germany, at least in some regions, they are called ‘Weberknecht’ which translates to something like ‘servant of a weaver’.
    Interestingly, as a child when I was quite afraid of spiders, these daddy longlegs never bothered me very much. Guess, my amygdala must have picked up some zoological knowledge early on.

    1. So Sweden didn’t get the folk name from Germany. We call them “loki [probably, unclear] spiders” or sometimes “longleg spiders” (sw: “lockespindlar”, “långbenspindlar”).

  3. Two comments

    The average British man is apparently a rugby player who could beat the crap out of me, and

    Where I grew up in East Tennessee, they called ’em granddaddy longlegs. My family being from Indiana called them just daddy longlegs.

  4. In Virginia we called ’em daddy longlegs; I too have never seen the term “harvestmen” except in books. Ditto for “water striders” (Gerridae) which we called “snake doctors”. Craneflies (and cranefly orchids, Tipularia discolor) we called by their “proper” names. And a sowbug is a “roly-poly”.

    There must be an anthropology/linguistics/entomology thesis project here somewhere.

    I recently put some daddy longlegs photos on my blog. The first post shows mysterious “hair” on a limestone outcrop:
    http://ironwing.wordpress.com/2012/10/07/whats-this/
    The second post has more photos of what turns out to be a dense cluster of daddy longlegs:
    http://ironwing.wordpress.com/2012/10/09/what-it-is/

  5. The largest known arthropod ever, also an arachnid, but an aquatic one, a sea scorpion or eurypterid from the Devonian

    taxonomic pedantry: Chelicerates, but not arachnids.
    Probably closer to arachnids than to horseshoe crabs, though. And who knows about pycnogonids.

  6. How, or why, did daddy long legs and some spiders evolve those amazing legs? They are really fragile and look to cause more problems. It seems (to me, uneducated, only read about a dozen books on evolution) that legs like that would be ‘less fit’ than shorter stronger legs.

  7. I love that essay. I’m surprised you didn’t quote what I think is the funniest and most famous passage from it:

    To the mouse and any smaller animal [gravity] presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes. For the resistance presented to movement by the air is proportional to the surface of the moving object. Divide an animal’s length, breadth, and height each by ten; its weight is reduced to a thousandth, but its surface only to a hundredth. So the resistance to falling in the case of the small animal is relatively ten times greater than the driving force.

    1. Sheep splash at 330 ft of free fall, if not less. Poor little lamb was in bits 10ft up the walls. It was quick though.
      Does anyone want to roll out the old standard parachute jokes? “Crossed legs?” “Grab hold tight!”

        1. Yes, and poor little Toby (why we decided that was it’s name … is lost in the mysteries of history) was not successful as a caving sheep (though they do keep on trying).
          “The Ballard of Toby, the ex-Sheep” was composed that day, and goes (in Sheep, with a Yorkshire accent)"Baaa? Baaaaah????Baaaaaaaah!Splat!(There is a lot of “hanging around” in caving. You’ve got to think about something.)
          De-tackling the hole was entertaining : standing with your face at boot level on a moor, about 2m from a natural bridge crossed by a popular footpath. I’m fiddling with bolts and trying to loosen knots that have had several hundred 100kilo “bounces” on them as we climbed back out. Along comes a couple of daylight tourists, asking the traditional “is it deep?” question. Which gets the equally traditional answer “get a big stone and drop it down here.” The tourists then can’t find any big stones … and eventually realise that it is deep, and all the big stones are already at the bottom!
          At least we didn’t have to climb through poor little Toby. Unlike the Skye cow. That brought new levels (depths) to the word “gross”.

          1. Okay, I’ll bite. (Possibly that’s an unfortunate term to use in this context…) I can see you’re just longing to tell us about the Skye cow…

            1. Nothing terribly complicated. The Skye cow had fallen (or slid) more-or less intact into a hole in the ground. About 15ft into the ground. We noticed the smell when we were checking the exit from a cave system we were planning to traverse, which we did because the description warned of a long, low roofed section with a stream running through it. That section (and several other sections uphill and upstream) was an obvious trap if it rained while we were underground. With the nearest cave rescue team 250-odd miles away, it seemed prudent to check that we’d be able to get out. So we checked what we thought was the exit, and it was good. We then went a kilometre or so uphill to the entrance, kitted up and headed down into the system. Several “sporting” sections later (“sporting” is a euphemism for “airspace between water level and solid limestone roof of one nostril or less) we realised that the water level was indeed rising … and as “tail gunner charlie” I got the undesirable task of seeing if we could go back out the way we got in. No go – a sump of a couple of feet had extended to over 2m before I started to reverse my moves to come back out to an air surface. No way out that way ; we’ve got to go out the bottom (as per original plan), and we’d better hurry up about it, because it’s only going to get worse. We got through the low, sumpy crawls at the bottom, as expected, in rapidly rising water … started up the resurgence tube towards the surface. And met the cow. The very, very dead cow. Blocking the passage. Evidently the “exit” that we’d checked out was a different part of the resurgence complex. Oops!
              So, given a choice, we could be squeamish until the water flooded the tube and we died slow horrible deaths ; or we could climb through to the fading daylight. Not a difficult choice, made easier by the fact that we were all in wetsuits anyway (Scottish caves are definitely “sub-tropical ; a long way “sub-“). Smelly horrible slippery thrutching later, with a couple of the crew adding their breakfasts to the slime ; break a few bones to get through and everyone was back on surface with a story to “dine out on” for the rest of our lives.
              If you’ve not attended a caving club dinner … take heed. (The few American cavers I’ve met are not significantly different.
              Lots of North Atlantic scrubbing later, we were close enough to clean to risk going back to the minibus. Still a bit whiffy and getting offended looks from the seals on the shore. But good enough to get served at the Slig.
              Next morning, in driving rain and nursing industrial-scale hangovers some of us went back. There was a slimy dank horrible pool with a surface well above the level of the cave that we’d have been in if we hadn’t “Break on through to the other side” – which Doors track promptly went onto the mix tape for the hours in the minibus.
              Ironically, “Filthy Simon” had gone for a walk up Blaven that day as an alternative to going caving.
              Ah, youth and stupidity. How did we survive?
              And on that cue, Babushka-chik brings supper! Yummy!

              1. Thanks! Now I know why I never took up caving (aside from a little matter of being clustrophobic).

                Still, that which does not kill us – makes for a darn good two-pint yarn in the pub later.

  8. Many people mistake daddy longlegs for spiders.

    In parts of the US there are true spiders that resemble harvestmen and go by the common name of “daddy longlegs”. Rather than saying such usage is mistaken, I’d say that “daddy longlegs” is paraphyletic and is not synonymous with “harvestmen”.

  9. Actually, the giant millipede and eurypterids show that ecological, rather than mechanical, constraints determine the maximum size of living arthropods.

    The often-cited limitations of an exoskeleton, and for most terrestrial arthropods, respiration via a tracheal system, may confuse the optimal solution for very small animals with the evolutionary potential if large size is selected for.

    The largest living insects do not depend on diffusion alone to respire. They combine tracheae and tracheoles with a system of enlarged air-sacs, which are actively ventilated, with air-flow controlled by coordination of opening and closing spiracles. At the size of large beetles, moths, grasshoppers, you frequently see such respiratory movements at rest. In flight, it’s likely that most larger insects meet higher oxygen demands by enlisting flight muscle movements to increase ventilation.
    Like birds [and their small dinosaur cousins, and probably pterosaurs], the arthropod solution incorporates a flow-through rather than tidal system. Further, insects repeatedly evolve facultatively homeothermy.

    As to the size constraint imposed by exoskeleton vs. endoskeleton, consider: A chitin-based structural system is inherently than bone. Consider the performance of carbon-fieber vs. reinforced concrete.. Further, putting the structural elements on the outside of the body and limbs decreases the weight required for both static and active support.

    It’s true that the arthropod body plan imposes some penalties on evolving large size. But in my view, the key problem is for a truly large arthropod to manage its vulnerability to damage and predation at each molt, particularly in a world with large vertebrate predators.

    The Carboniferous was an oxygen-rich time. But given 300m years without serious competition from vertebrates, I’d bet that at least horse- and lion-sized bugs would roam the Serengeti, and at least crow-sized insects would circle the sky.

  10. I think the proper terms for the two divisions of a spider’s body are prosona and opistosoma. I don’t think they are exactly homologous to cephalothorax and abdomen.

  11. Haldane Senior did a lot of the original research work on “caisson disease,” from which the understanding of decompression sickness in divers developed.
    During WW2, Haldane Jr (JBS, beetle-fondler) continued work on gas physiology, in response to needs for submarine escape systems and decompression and asphyxiation problems for high-altitude pilots. And by all accounts I’ve heard, he carried out some truly insane-sounding experiments upon himself (and his research assistants).
    I’ve got a Haldane autobiography unread somewhere in the pile of stuff disrupted by moving house. I’ll have to get round to taking that out to sea one month.

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