Arranged around the edges of an opposite pair of silver birch leaves,
about an inch across at the widest, a number of pallid green
caterpillars about 15mm long with black heads and a near-transparent
final segment.

The arrangement was extraordinary. Each caterpillar had about 3mm of its
head end anchored right at the edge of the leaf surface, in line with
the circumference, and the rest of its body poked up in the air with the
last segment or two hooked over, making an S-shape. The next caterpillar
had its head snug up against the body of the one in front, with its tail
in the air, and so on all around the leaf, a bit like parked supermarket
trolleys. There was a slight twist to each caterpillar so that its
raised body stuck out beyond the edge of the leaf on more or less the
same plane as the leaf surface, making a kind of fringe to the leaf.
Approximately eight to the leaf, completely motionless, and on those two
leaves only among many surrounding birches.

Anyone know what species they are, and could anyone hazard a guess as to
the Darwinian explanation of such behaviour?

brian mitchell

brian mitchell wrote:
Arranged around the edges of an opposite pair of silver birch leaves,
about an inch across at the widest, a number of pallid green
caterpillars about 15mm long with black heads and a near-transparent
final segment.

The arrangement was extraordinary. Each caterpillar had about 3mm of its
head end anchored right at the edge of the leaf surface, in line with
the circumference, and the rest of its body poked up in the air with the
last segment or two hooked over, making an S-shape. The next caterpillar
had its head snug up against the body of the one in front, with its tail
in the air, and so on all around the leaf, a bit like parked supermarket
trolleys. There was a slight twist to each caterpillar so that its
raised body stuck out beyond the edge of the leaf on more or less the
same plane as the leaf surface, making a kind of fringe to the leaf.
Approximately eight to the leaf, completely motionless, and on those two
leaves only among many surrounding birches.

Anyone know what species they are, and could anyone hazard a guess as to
the Darwinian explanation of such behaviour?

brian mitchell

This is where cameras come in handy! I can't quite picture your
description. It sounds intriguing though.

--
David in Normandy.
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David in Normandy wrote:

This is where cameras come in handy!

http://www.flickr.com/photos/lyannewylde/2801894025

found it on google. Apparently sawfly larvae, though it doesn't say what
kind of sawfly. Suggestion here is that this is defensive behaviour but
I'm still mystified by it. I didn't see any eating going on, but I'll
keep a watch on them