Posted by: obscuripes | October 26, 2010

Giant ‘insects’ (arthropods) can in fact roam the Earth?

Part 1
I recently had a conversation with a friend who informed me she was attempting to design hypothetical alien flora and fauna for a high oxygen alien planet.

Phyllis cited a desire to hear about “strange insects from across the globe” and was inspired to cultivate giant-sized insects the size of mastodons that would roam the wide stretches of continents. She felt talking to a scientist might make her creations more believable or realistic.

I attempted to burst her bubble and informed her that even with the high oxygen content known to be present in the Carboniferous period, the giant dragonfly Meganeura was not actually all that tremendous (2.5 foot wingspan, and probably shorter body),
Meganeura monyi
and like the modern stick insect, was likely able to evolve an abnormal size due to its elongate and extended body still providing for a high surface area to volume ratio. Furthermore, I cited what I had learned in Entomology class: an insect breathing apparatus (spiracle system) would be unable to pull in the large amounts of air necessary to aerate a human or mastodon-sized body directly. Also, I rattled off the structurally fragile quality of an exoskeleton when increased in size by orders of magnitude and the high improbability of giant-sized insectoids producing anywhere near as many offspring as their miniscule cousins do. I tried to explain something about the energy resource to mass ratio, and adaptive trade-offs of maternal parenting vs. high reproductive output.

I then cited the curious case of the large man-eating social arachnids found in the Robert A. Heinlein novel Starship Troopers, and popularized by a movie. In the image below, these ten-foot tall arachnids pour out by the thousands from an underground nest onto a desolate landscape. We are led to understand the entire landscape of the planet is in fact as barren and uninviting as it looks. The arachnids are spread from planet to planet by interstellar meteors that somehow spread insect “spores” (not the right term for insects). What’s wrong with this picture?
too big to breathe?

Giant arachnids aside (perhaps with real pseudo-lungs or reinforced skeletons) I expressed my sudden disbelief that organisms of such size could reproduce at such a rapid rate and create such large numbers of fighting warriors to combat the would-be human space marines depicted in the movie. For the main reason: the planet is absolutely desolate and there appears to be no plant life or any other kind of food material on the lower level of a food-chain needed to support such a massive number of apparent carnivores (with no apparent true mouths).
Even with verdant savannahs where giant elephants roam, hundreds but not thousands of animals can exist together in a herd, and are in fact made of smaller family structures made by an animal with a long gestation period and only one offspring at a time.
At this point I began to question, other than high maternal care and energy needs, what would prevent an elephant from evolving and somehow producing many smaller young with little maternal care. But I did not delve deeply into the idea…

No! You can't eat me! obscuripes says your surface area to volume ratio  is too small!

"No! You can't eat me! obscuripes says your surface area to volume ratio is too small!"

Part 2: later this week…

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  1. Do the Starship Trooper arachnids actually have eight legs?

    Also, do you think high maternal care and energy needs are ENOUGH to prevent an elephant from evolving a different reproduction plan?

  2. Technically the book calls them Pseudo-arachnids (wiki said so).
    I believe the ‘arachnids’ in the Starship Troopers movie are not of that taxonomic status. Given the xenophobia surrounding the creatures (“The only good bug is a dead bug!”) clearly not classifying the aliens into a grouping worthy of scientific analysis, I arrive at the assumption that arachnid is another blatant misnomer, despite its attempt at scientific accuracy.

    One argument for their attribution to that terrestrial group is that the terrestrial arachnids are usually classified as having eight thoracic appendages. There is no visible thorax present. However, if one were to make a stretch, one can see four appendages adapted for mobility, and two for sword-like attacking thrusts, though on clearly different segments of the body. The remaining two I imagine are vestigial: either the two apparently nonsensical non-jointed appendages hanging off the posterior side of the body, or the small mandible like manipulators anterior to the main body column and eyes. In some pictures of the creatures, a mouth is now present; in others, it’s hard to make out anything.

    However, all terrestrial arthropods follow a body organization prescribed by activation and inactivation of hox genes present in many complex metazoan organisms. The linear segments of centipedes, arachnids, insects, and even crabs all follow this genetic rule. Centipedes have a large number of the potential segments activated by the hox genes, arachnids and insects have many of these segments combined through tagmatization. Crabs also have the same body organization, though hidden: the homologous abdomen is in fact shrunken and curled underneath their bodies and still contain the reproductive apparatus as found in the ancient and distantly related Horseshoe Crab.

    The life-form described in the picture in part 1 does not appear to have the same body plan as any Earth-bound arthropod, making it clearly abnormal (unlike many of the other organisms presented in the movie, which are more generic [eg. Tanker bug]. The crawling legs are part of a segment that extends –below- the singular segment which has eye, mandible and other appendages. This lack of linearity is uncharacteristic of arthropods.

    Elephants are real. They require more thought. Perhaps more soon.

  3. [...] Part 1 [...]

  4. [...] hematophage has a rant about how giant fictional arthropods shouldn’t be allowed to roam the Earth?. There’s more to come, too. More rants, not 60m wide dragonflies, I [...]

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