Why Mothra Doesn’t Exist

Scientists in Sweden asked why insects such as butterflies that have a very high capacity for rapid growth during the larval stage do not continue growing just a few more days. This question is prompted by the common laboratory observation that larger bodied females have the capacity to produce many more eggs than smaller females. Since growing larger seems to be relatively easy for these butterflies, it is difficult to see what keeping them from doing so!

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In the paper the researchers report on experiments performed on a Swedish population of the Speckled Wood Butterfly, Pararge aegeria. They show that large females lay more eggs than smaller females if they are allowed to lay eggs throughout each day, but in a situation where only part of the day was suitable for egg-laying the size of the female did not matter for the number of eggs laid.

This exercise suggests that growing to a much larger size typically doesn’t pay off in more eggs laid, and the optimal female size predicted by the model is relatively close to what is actually observed.

The paper concludes that one important reason why insects with a high capacity of larval growth do not evolve towards larger sizes may be that the fecundity [# of offspring produced] benefit is in fact relatively limited under natural conditions.

Mothra To the Rescue!

What Keeps Insects Small? Time Limitation during Oviposition Reduces the Fecundity Benefit of Female Size in a Butterfly. 2007. K. Gotthard, D. Berger, and R. Walters. The American Naturalist, volume 169 (2007), upcoming paper.