Life in a bag

Have you ever heard of bag moths? They are not literally moths in a bag, rather caterpillars in a bag, or a case. Until I stumbled on a specimen at Dunedin Botanic Gardens the other morning, I had never heard of them, let alone seen one. What at first looked like a twig was rustling on a shrub beside me. As I looked closely, the head of a fat caterpillar popped up from it.

The larva was simultaneously hanging on to a leaf and hooked onto its twig-like bag from the inside thanks to its tiny prolegs—3 on the thorax, 5 on the abdomen. Made of silk and materials from the insect’s habitat (e.g. leaves, tiny wood sticks, soil), the case is light, soft, but tough and provides superb camouflage—I was extremely lucky to spot it.

The larva starts building its protective bag immediately after hatching and more silk is spun and added to its front as the insect grows in size (bag moth caterpillars indeed only go through a single metamorphosis to become moth). Bags can reach up to 8cm in length. The caterpillar never abandons its bag, which it drags along like a snail pulls its shell.

During periods of inactivity, the larva generally closes the front of its bag tighly from the inside, but sometimes keeps it open as if to ventilate it.

Despite being blind, the larva is impressively agile and can climb branches with ease using its head, whiskers and prolegs to touch and feel its environment. As it moves, the insect leaves a silk trail behind, which helps it stabilize, twist and turn, and keeps it from falling off, somewhat like a spider.

Naturally, bag moth caterpillars find smooth surfaces rather unappealing because there is no grip!

Once ready to pupate, the larva seals its bag and what follows is awkward—more or less depending on the sex and the species.

If male, the larva will turn into a moth, grow wings and fly away. If female, the larva will only partly transform and never leave its bag. Female moths are wingless, generally eyeless and miss antennas, proper legs and mouth parts. Sadly for them, they are more like a maggot than a butterfly. They have to wait inside their bag for a male to find them. Reproduction occurs through the end of the bag, which has a small opening that also serves to excrete. Then females lay eggs inside the bag and die or jump out, fall to the ground and die. In some species of bag moths, eggs do not need to be fertilized, so mating does not even occur.

At this point, you may be thinking males are better off than females. Sure males can fly. But they cannot eat: they are moths without mouths and their only life purpose is to reproduce as quickly as they can before they die.

These insects definitely rival mantis in oddity.

The species I encountered is the common bag moth (Liothula omnivora), which is endemic to New Zealand. Here is a short clip that shows this very special caterpillar crawling around with its silk bag, spinning silk or just looking around, as it loves doing—or should I say “feeling around” since it cannot see without eyes!

All photos and videos on this blog were taken with a Nikon Coopix S8200 point-and-shoot camera.

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