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I have read in biology text-books that one of the most important constraints on the size of an insect happens to be the importance of the surface-volume ratio. It simply states that if we assume constant oxygen levels and increase the size of an insect there would be a point where the insect's exoskeleton would no longer be able to support its weight.

Some textbooks go on to say that this is why we don't have insects the size of elephants. But, if this is true...is there a least upper-bound on the maximal body mass of an insect?

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I assume there must be a maximum size for insects, and I atmospheric oxygen probably has something to do with it. Insects do not breath like vertebrates, they have a series of tubes call trachea that run through their bodies and open to the surface at spiracles. Air passes through these tubes and oxygen and CO2 passively diffuse through the tissues. Insects can force air through the trachea to increase the diffusion rate, but they just aren't as efficient as vertebrates with lungs and hemoglobin to carry oxygen around the body.

In the past, oxygen levels were much higher. Higher amounts of oxygen could allow for larger invertebrates, and there were 6 foot long millipedes and giant dragonflies. Their size probably was not entirely caused by the extra oxygen, this was before they faced competition from vertebrates, so increased competition probably had more to do with their size reduction than loss of oxygen.

I don't know what the maximum size for an insect is at modern oxygen levels, but you'd have to eliminate the other selective pressures such as competition and limits on food before an insect could reach that size.

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  • $\begingroup$ Good point. But, in that case I wonder why text books sometimes deduce with confidence that insects can't be as large as elephants? Unless a biologist has approximated the maximal size of a terrestrial insect... $\endgroup$ – Aidan Rocke Oct 20 '14 at 22:40
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    $\begingroup$ Another limit on size is strength. An insect can't be directly scaled up to large sizes because an object's weight scales with the volume, but strength tends to scale with cross-sectional area. Since volume is cubic and area is squared, weight can overcome strength unless the legs are made thicker relative to the rest of the body. Additionally, insects have less efficient circulatory systems, where hemolymph escapes from the vessels and passes directly over the tissues, being collected at the other end of the vessels and passed back through the heart. $\endgroup$ – user137 Oct 21 '14 at 2:18

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