The folly of making assumptions

On my recent trip to Brandon Marsh I happened along what I thought was a caterpillar crawling along the path.  I took a picture, and couldn’t find any butterfly caterpillars that looked anything like it.


I assumed therefore that it was a moth caterpillar – it even moved along like a caterpillar (see my very poor attempt at video below) so I put a request for an ID out on twitter.

Anyway, I was mainly ignored except by a friend who correctly ID’d it from his insect guide book.  It is in fact the larva of a glow worm, Lampyris noctiluca.  Neither of us had ever seen one before but I had assumed that they weren’t found round here (in fact I blogged about them 6 years ago) so it never would have occurred to me that this could have been a glow worm in the making.  My level of excitement about this discovery would be considered by some to be a little bit sad or over the top perhaps.

So, obviously, more research was needed and I came across an excellent website all about glow worms and discovered that the larvae are just as interesting as the adults (all the information I have included here along with so much more is included on this site – please take the time to visit and learn more).  For a start, glow worms spend only about two weeks as an adult, but an average of 15 months as a larva.  Adults never eat, they have no mouthparts, so they have to get sufficient food reserves before they pupate.  They do this by eating snails and slugs so they tend to hang about in the same place as small snails and slugs (surprising they are not in my garden then!) and prefer damp conditions in the dark (their eyesight is very poor).  They find their food via their very sensitive antennae and the palps on the front of their mouth that they wave about almost constantly.

When the larva finds the snail (or slug) it bites it and delivers a toxin that starts to digest the victim from the inside.  Larger snails require several bites and apparently the larva often rides on the snail’s shell whilst waiting for it to die.  The larva has to be careful not to get stuck to the snail as its defence mechanism is to cover itself with sticky mucus.    Once the snail has died the larva sucks up the molluscian broth, and even has little hairs in its mouth to filter out any bits that are too large and pointy blades on its mandibles to cut them into smaller pieces.  These larvae are so cool!

As previously mentioned, slugs and snails are pretty sticky, so after polishing off its dinner the glow worm larva has to clean itself up – it has a special attachment on its tail end with lots of hooks for this purpose and scours itself clean.  This is also handy for hanging from plants if necessary.

All this eating snails means that the larvae get too big for their skin, and therefore, like some other invertebrates, they shed their skins several times before they make the transition to full adulthood, a process which takes a couple of weeks and for which the larvae tend to group together.  The glow worms now have two weeks to find a mate or die trying.

Oh, and one further cool fact, even the larvae can glow a little, although as yet, no one seems to know why they do – there being several reasons put forward, some more plausible than others.  I just like to think that it’s so they can find each other.

Anyway, the moral of the story is, never make assumptions when trying to ID something new.