Giant arthropods, then and now ? Why Evolution Is True

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

Source: http://whyevolutionistrue.wordpress.com/2012/10/25/giant-arthropods-then-and-now/

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