Twitching caterpillars. Why?

[shonkarko]

I found about 20 or more cabbage white caterpillars on a brocolli leaf and every 5 seconds or so (intervals varied) they would all lift one end of their bodies in the air in a quick twitch almost completely in unison. Some of the caterpillars were in a bunch but even the ones that weren't touching each other, 5cm away, still twitched in unison with the main bunch. They were doing it as long as I watched without me touching them or even passing my shadow over them. What is this behaviour?

[Meta menardi]

Sometimes you just have to accept that you have seen something that has not been recorded or explained before. Quite possibly, someone will come along with a well known explanation, but if no one does, write it down in as much detail as you can and keep on looking.

[charlieb]

It's some form of defensive action; perhaps it makes the 20 or so larvae look like a larger organism or maybe the twitching resembles leaves blowing in the wind. Or perhaps they are releasing some form of chemical to disuade predators?

[svensol]

hi all,

I've recorded this! I found this thread hoping that someone knew what it was:



Here is the link to the video.

http://www.stephensolberg.co.uk/wildlife/twitch.m4v

Let me know if you can see it, I'm playing around with a video converter, and I'm not sure who can see it

(Woo-hoo! My first posting!! )

[charlieb]

That's a Large White, Pieris brassicae

(vid works for me )

[Lance Morgan]

Originally Posted by charlieb

It's some form of defensive action; perhaps it makes the 20 or so larvae look like a larger organism or maybe the twitching resembles leaves blowing in the wind. Or perhaps they are releasing some form of chemical to disuade predators?

Agree with charlieb here. When they are small they do this to look bigger.
Later on when they become solitary the secrete a green smelly liquid from their mouths as a form of defence.

[Action_Man]

I have seen Aphids do this regularly, and its usually when there a threat present, like an Ichneumon Wasp ...