Despite my childhood fears, I’m confident that Mothra will never try to take over the world. Mothra first appeared in the novel The Luminous Fairies and Mothra (Takehiko Fukunaga), but she has since appeared in several films. A fictional Lepidopteran, Mothra has characteristics of both moths and butterflies, and while I cannot find any references as to how large she would have been, you can see her next to Godzilla at the right, suggesting she was quite large.
So, how can I be confident a moth this large could really never occur? Moths and butterflies are arthropods and arthropods have a unique method of growth. Because they have a rigid exoskeleton, they essentially are wearing a suit of armor that will not grow with them. You and I have an endoskeleton that grows with us. For arthropods, growth occurs in several steps:
- First the existing exoskeleton is shed, or molted
- The animal plumps itself up with water, causing it to swell
- A new exoskeleton is secreted around the swollen animal; within a few hours (or days) this new skeleton will harden
- The swelling goes down, and the animal now has an exoskeleton it can grow into.
During the period between steps 2 and 3 above, the animal is not only helpless, laying as a ‘blob’, but it also has nothing to support its weight. Its likely that a bug the size of Mothra would have been crushed and killed under her own weight.
So, how big can arthropods get? The largest terrestrial arthropods are probably the Giant Weta (Deinacrida spp.), found in New Zealand; the largest one documented was ~70g! However, because water is more buoyant (and supportive) than air, aquatic arthropods can get much larger. The Japanese Spider Crab (Macrocheira kaempferi) can reach a width of up to 13 feet, and a mass of 20kg (44 lb)! I had a chance to photograph this crab last week at the Aquarium of the Pacific in Long Beach.
Japanese Spider Crab (Macrocheira kaempferi) Aquarium of the Pacific, October 2009
Despite the fact there was a lot of glare on the glass, I rather like this photo. Somehow I can’t help but think these crabs are planning a Mothra-like reve….