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Re: insects - not! (OT)

Andrew Simpson wrote:

There is some reason that insects can't grow beyond a
certain size. (thank god) About sparrow size. I can't
remember if it has to do with breathing issues or
exosceliton issues. Anybody?

Both. Arthropods wear their skeletons are on the outside of their bodies ('exoskeleton"), rather than on the inside ("endoskeleton"). The bigger an animal gets, the bigger its skeleton has to be to support it. By and large, as an animal gets longer, surface area increases to the square and volume/mass to the cube. So a 2-fold increase in body length results in a 4-fold increase in surface area and an 8-fold increase in volume/mass. At some point as an insect became larger the mass of muscles needed for the insect to function would not be able to fit inside the exoskeleton.

The breathing issue is also connected (although a little more tenuosly) to this length-volume relationship. In the absence of a centralized respiratory system in insects, the oxygen has to get to all the tissues in the body from the outside via a network of tiny branching tubes (tracheae --> tracheoles). Again, as an insect becomes larger it becomes more difficult for the airflow to penetrate to the deeper tissues. However, insect respiration is not as simple as passive diffusion, and insects can direct and increase the rate of airflow (ventilation; e.g., air sac pumps, abdominal pumping). Also, some flying insects in the Paleozoic got pretty damn big, so obviously they were doing something right back then to get enough O2 into the high-demand flight muscles (perhaps helped by the hypothetical "oxygen spike" in the atmosphere).


Airflow through

Spiders must have the same limitation.

The centapede, being neither insect nor spider, once
grew to six feet. I don't even want to think about the
sounds it made when walking.


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