Halfway up a woody heather stem, I spied what appeared to be a caddisfly case. Nowhere near a pond, my suspicions were roused as this was too big a case to belong to a land caddis. I placed it on a stone and waited.
A few minutes later, a small black head peeked out of the open end of the tube. It belonged to a caterpillar. It turns out that I had bagged a bagworm, an insect with a very interesting and surprising life-cycle, particularly given its uninspiring name.
What are bagworms?
The dusky sweep (Acanthopsyche atra) is one of 20 species of micro-moth that belong to the Psychidae family, or bagworms. These creatures, as their name implies, live for at least part of their life in a ‘bag’ or case.
It’s very much like the case of a caddisfly larva: pieces of plant material – usually lichen, bark fragments, stems or seeds – are sewn together to form a protective shroud in which the vulnerable caterpillar lives.
The dusky sweep is one of the larger species, the case of a fully grown specimen reaching about 20mm long. Despite this it’s never easy to find, as it is both well camouflaged and rare – its bag decorated with long sections of grass, rush and heather that are lined up along its axis. Added to this, the adult moths are small and rarely draw attention to themselves.
Bagworm moth lifecycle
Bagworm caterpillar
The caterpillars remain inside this silk-lined, hand-crafted bivvy bag, dragging it along with them and extending it as they grow. After a season of feeding, they pupate. Not that you’d know it, as nothing really changes in the outward appearance of the case, other than that it stops moving and the caterpillar seals itself in while it undergoes its transformation.
Bagworm moth
When it’s time to emerge, it’s only the males that do so, at least at first. Keep an eye out for a small fluffy moth with dark grey wings. The females stay sitting inside their cases and produce a pheromone – or chemical signal – that sends a whisper of sexual promise into the air and lures the flighted males to mate. Where do they mate? You’ve got it – in the bag.
After this, things get really weird. She emerges, not looking like a moth at all, but rather a peculiar pale thing with reduced legs and antennae that you could easily mistake for a maggot. She even impersonates one by writhing and moving her body in a manner that bears an uncanny resemblance to a fly or beetle grub.
A tasty one at that. You see, this is the plan: having spent her whole life impersonating something inanimate she now changes things up. Stepping into the light, she wriggles around in an attempt to catch the eye of a bird. And you can probably guess what happens next.
Studies where the mated females were deliberately fed to robins didn’t end as badly as you might imagine. Well, they do for the individual female, but in sacrificing herself she ensures her young are dispersed. The eggs within her body survive the journey through the bird’s digestive tract and are pooed out the other end. Like seeds in berries, these eggs in a moth are scattered further afield than she could ever have travelled. If you lack wings of your own, the next best thing is to borrow someone else’s!
More by Nick Baker
- Meet the moon snail, a killer predator who likes to paralyse and liquidise its prey
- Oil beetle lifecycle: How bees are crucial to the rare oil beetle’s survival
- Bladderwort guide: how this carnivorous pond plant catches its unsuspecting prey – fast
- Mayfly lifecycle, from naiad to its second moult
- Tardigrades: meet these fascinating tiny water bears
Main illustration © Peter David Scott/The Art Agency
