The Island of Giant Killer Insects! – Gigantism and dwarfism

A dear friend of mine, Tim Hanse, asked me why insects on small islands become larger than on the mainland. This question immediately calls into mind images of gigantic killer insects on an island far away which are the stars of nightmares or inspiration for horror movies like: “Empire of the Ants”. And although insects can be terrifying, in general, insects are not slayers of men. Furthermore, restraints to their maximum size makes the appearance of giant killer insects most unlikely. This does of course not mean that there are no large insects, but do insects on smaller islands reach larger size than on the mainland?

The island rule

This question relates to the island rule which states that small animals isolated on islands will become bigger (island gigantism) and large animals become smaller (island dwarfism). Take for example the Giant Fijian long-horned beetle (photo below). This beetle is native to the island of Viti Levu in Fiji and can reach a size of up to 15 cm. Seeing as the average size of beetles is about 0.5 cm that is pretty big and an good example of island gigantism. To the contrary, the Flores Man (Homo floresiensis), is closely related to us (Homo sapiens). Discovered on the island of Flores in Indonesia, this human species reached approximately 1 meter in length. Because they are far smaller than any modern human on the mainland, they seem an good example of island dwarfism (although no clear consensus about this subject has been reached in the scientific community).

But what causes small animals to grow bigger and big animals to stay smaller? It is thought that small animals are better at escaping predation, islands freshly inhabited lack predators, allowing for larger body size to be reached without succumbing to predation. Furthermore, the lack of competition on new terrain provides more resources (food) and also allows for larger body size. On the contrary, large animals that colonize an island will have trouble finding enough food in such a small place. The largest will die from starvation while the smallest individuals survive and thus small body size is selected.

The island rule and insects

Does the island rule hold for insects? That question is difficult to answer due to the lack of proof. Although it cannot be denied that large insect species can be found on islands, there are also large insect species on the mainland. In order to find any indication that the island rule applies to insects, one would need to measure insects on islands and mainland and compare the size distributions. However, any such experiments remain unknown to me and so we cannot be sure what the answer would be.

Why are there no man-sized insects?

For those that are still afraid that one day they will come across an insect the size of a human, let me take your worries away. There are several reasons why insects cannot become so large.
First, they do not actively breathe, air flows passively through their trachea (comparable to our lungs). Would they be as large as a human, their trachea would take up so much space that there would be no room for any other tissues, like muscles.
Second, they do not possess a circulatory system like we do (veins, arteries and such). Instead they have an open system in which the hearth pumps the hemolymph (blood) around in the main body cavity. This provides enough movement for small insects to get the hemolymph to all tissues, but when insects reach the size of a human this system is not nearly sufficient.
Finally, insects shed their skin to grow. We have an internal skeleton, which grows steadily as we grow. Insects do not possess such a skeleton, but rely on their exoskeleton (skin) for support. When they shed their old skin, the fresh skin is still soft and lacks strength. Would a man-sized insect shed its skin, it would collapse into a formless pancake of soft tissue. Not very convenient if you ask me.

All in all, insects can grow pretty big, but an island of giant killer insects does not exist and we do not need to worry of finding man-sized insects any time soon. Tim, thanks for your question.