My Favorite Life Cycles: Green-Banded Broodsac

Rob Lang
3 min readJul 25, 2018


If you read my comics, you know I’m a big fan of interesting life cycles. The joy of learning about how animals develop from birth to death started when I saw a documentary about parasites. It featured a snail with eyestalks that looked like they were about to blast off to the moon. The film was narrated by David Attenborough, no less.

The memory of the snail with pulsating eyes stuck with me. And it’s a good thing too, because I was commissioned to write a book for 4th graders about parasites. I knew that the perfect opening to the book was the story that I’m about to tell you.

Even though that quick bit was published in a small book for fourth graders, I’ve always wanted to expand on this story with my own commentary and illustrations to go along with it. So hold on to your antennae:

It all starts with a shy snail and bird droppings. Instinctively photophobic, the amber snail (family Succineidae) usually stays in the shadows or on the underside of leaves. But you know, who can resist an easy meal of delicious excrement? Little does the snail know, the poop is mixed with parasitic worm (Leucochloridium paradoxum) eggs.

After the eggs of the parasite are digested and hatch, groups of larvae make their way into the eyestalks of the snail and form long sacs. The sacs cause the tentacles to swell and pulsate, which gets the attention of hungry birds. What’s more, when the snail has the infection, they make their way to the tops of plants so birds can see them. The parasite controls the snail’s mind—making them photophilic—they actually want to be in the sun. In fact, the parasite stops throbbing when it’s dark out!

Seeing this delicious-looking pulsating, caterpillar-like piece of meat, a bird can’t resist chowing down. This parasitic behavior is called aggressive mimicry.

Once inside, the main mission for the worm is to become an adult living in a bird’s butt. There it will feed on feces and mate inside in the warm rectum. What could be more romantic? Any bird will do really: crows, jays, sparrows or finches are all drawn to the succulent tentacles.

Eventually the snail will grow its tentacles back, but while they’re
growing, it gets to dine on more tasty bird feces — many of which
will be infected with the eggs of the green-banded broodsac!

This doesn’t seem to harm snails very much (other than getting its eyes torn off) — because the worm wants to keep making more parasitic worms! What a fun life.



Rob Lang

I make a comic strip about nature called Underdone. The best place to see it is on Instagram/UnderdoneComics