The other night, I met Mr Curly on my way to the Indian restaurant. He was hiding under a fig-leaf by the footpath, trying to look inconspicuous. But it was the shape of the figleaf that gave him away. Or what was left of it. You see, Mr Curly eats highly poisonous leaves that ooze latex when damaged, and he gets away with this dietary quirk by attacking these leaves in a very special way: he messes around with the plant plumbing.
Mr Curly is the caterpillar of Euploea core, the Common Crow butterfly (also known as ‘Ricey’ because of the small white spots on its black wings)¹. The Common Crow is found all the way from Australia to India and even Russia (or in my case, between my house and Traditional India²). Here’s the figleaf it was eating – note that only the leaf tip is missing, and there’s a curious little notch in the midvein:
And here’s Mr Curly in his full glory:
I don’t think I need to explain how Mr Curly got his name. But what about that notch? Mr Curly bit that little notch in the leaf before he ate the rest of the leaf. In this way, he cut off the main flow of latex to the tip of the leaf.
Latex is a milky fluid found in over 20,000 plant species, including figs, dandelions, sweet potatoes, papayas and lettuce. It’s usually white, but can be clear, yellow or even red (e.g. in cannabis). Latex has been used to make rubber, chewing gum, and those weird all-over suits that are currently all the rage in Japan.³ It’s usually very sticky, and often contains bioactive chemicals (e.g. morphine in poppies and cardenolides in cotton bush). Since latex is found in so many plants, it’s often encountered by the numerous insect species that eat plants. And some insects like the Common Crow have found very interesting ways to deal with it.
Latex is produced and stored in elongated, branching plant cells called laticifers, which are often associated with leaf veins. Sometimes these laticifers branch like a tree and don’t loop back on themselves. These were probably the sort of laticifers Mr Curly found in the fig leaf, as all you need to do to stop the flow of latex in this type of leaf is to bite a little notch in the midvein:
But some leaves have articulated laticifers (try saying that with a mouth full of latex) which form a network – if only one part is cut, the latex can still flow in the rest of the network. But caterpillars have a solution for this type of laticifer too – to stop the flow of latex they make little trenches around the section of leaf they want to eat. This type of ‘trenching’ or ‘sabotaging’ has been observed for the Common Crow feeding on Oleander and Monkey Rope vine, but also for the Monarch or Wanderer butterfly Danaus plexippus, feeding on plants like the cotton bush Gomphocarpus or Milkweed Asclepias.
Even though these caterpillars alter the flow of latex in the leaf, they still end up eating some of it. And this may be the reason why Common Crow butterflies are highly toxic to many animals (but not to other insects): they somehow store the plant’s poisonous chemicals inside their bodies without poisoning themselves.
That’s another very clever insect trick, and another story. Maybe to spin out over our next dinner at the Indian restaurant.
- Jordan, F. and H. Schwencke (2005) Create more butterflies: A guide to 48 butterflies and their hostplants for south-east Queensland and northern New South Wales. Earthling Enterprises, West End.
- Traditional India, Sandgate. I can never go past the Mattar paneer….
- Ooops, those weird suits are actually made of lycra not latex. I wonder if cyclists have ever made that mistake?
Dussourd, D. E., & Denno, R. F.. (1991). Deactivation of Plant Defense: Correspondence Between Insect Behavior and Secretory Canal Architecture. Ecology, 72(4), 1383–1396. http://doi.org/10.2307/1941110