I wonder about strange things sometimes. This is one of them.
For the first creature, imagine a bat-like creature in which the hind limbs rather than the fore limbs have evolved into wings. I have two questions:
[ol]
[li]Could such a creature even produce lift? I am under the impression that bats and birds have their wing-bones at the leading edge of their wings, and rely on the difference in behaviour of the leading and trailing edges to produce lift. Is it possible for this creature to produce lift without a tail to support the wing behind the wing-bones?[/li][li]Is there any way such a creature would be capable of stable flight? I am under the impression that unless the wing is brought forwards to the centre of gravity, it’s not really going to have any easy way to steer.[/li][/ol]
The second creature is more traditional: a human with four arms.
[ol]
[li]I’m assuming that placing the second pair below and behind the first pair would provide the most useful range of movement. Do others agree?[/li][li]Would the structure required to anchor the second pair of arms to the body, a second clavicle and scapula and associated muscles, interfere with the creature’s ability to breathe?[/li][/ol]
The second creature is more traditional: a human with four arms.
[LIST=1]
[li]I’m assuming that placing the second pair below and behind the first pair would provide the most useful range of movement. Do others agree? [/li][/QUOTE]
Some photographic reference of (almost) the real thing might help you
There was a prehistoric reptile called Sharovipteryx that was basically this (though it had a tail). A recent study found that it could produce lift, but was a glider unlike actual bats.
Dyke et al, 2006. Flight of Sharovipteryx mirabilis: the world’s first delta-winged glider. Journal of Evolutionary Biology 19: 1040-1043.
Abstract: The 225 million-year-old reptile Sharovipteryx mirabilis was the world’s first delta-winged glider; this remarkable animal had a flight surface composed entirely of a hind-limb membrane. We use standard delta-wing aerodynamics to reconstruct the flight of S. mirabilis demonstrating that wing shape could have been controlled simply by protraction of the femora at the knees, and by variation in incidence of a small forelimb canard. Our method has allowed us to address the question of how identifying realistic glide performance can be used to set limits on aerodynamic design in this small animal. Our novel interpretation of the bizarre flight mode of S. mirabilis is the first based directly on interpretation of the fossil itself and the first grounded in aerodynamics.
Thanks for the responses so far. I’d never heard of that child before now.
According to the Wikipedia article, they came to the same conclusion I did about such a creature’s ability to control its flight without additional control surfaces.
The second creature is more traditional: a human with four arms.
[li]I’m assuming that placing the second pair below and behind the first pair would provide the most useful range of movement. Do others agree?[/li][/quote]
Wouldn’t being placed at the hip be more sensible?
Now that you mention it, I think you’re right. The legs already have a restricted range of movement, so putting the second pair of arms there wouldn’t interfere much.
The article on that baby with three arms says the doctors are planning on amputating one. Man, if I ever learned that I’d been born with three arms but that doctors had amputated one, I’d be pissed! I mean, how many times would an extra hand have come in really handy?
As for the assumption that the second pair would be below the first pair, doesn’t that depend on how one defines which pair is the “first” and which is the “second”?
Without adding additional skeletal structures, extra arms would work best attached to the hips or the shoulders. I’m not sure why you would think the new pair would be placed behind the current version at the shoulder. Directly below, above or forward makes more sense. Range of motion behind the back wouldn’t be much use without neck or eye adaptations. However, it might be easier to scratch those itches on my back that way.
As far as flight with hind legs, as long as the animal had a tail to maintain balance, rear limb wings could work as well as the foreward versions. The pterosaurs all seemed to use the weight of their heads as a control mechanism. Forward loading adds stability to aircraft by keeping the nose down allowing recovery from stall conditions, so it would be beneficial to flying animals also. Most birds keep their weight close to the CG which gives them better maneuverability with greater efficiency. Birds, pterosaurs, and bats take advantage of wing beating action to reduce the energy required for breathing and heart beats, this might not work out well with an actual flying animal using rear wings, as opposed to gliding.
It’s a bit off topic, but here’s an article from a week later, saying the surgery was a success.
Consider it a naming convention. I just called the one closest to the position of the human shoulder the “first”.
My reasoning wasn’t about the range of motion behind the back, but so that the second pair did not prevent the upper arm of the first pair from pointing straight towards the ground. I was thinking of a desired arc of motion something like this: