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.

Have you seen any strange, green lights recently?

I saw a tweet at the weekend stating that it was high season for glow worms.  So, I hear you ask, what about them?  Exactly, I realised that I knew absolutely nothing about glow worms, so I decided to do a bit of research.

There are two species of glow worms in the UK, but one is very rare and restricted to only a couple of sites.  The common glow worm, Lampyris Noctiluca, (in common with other glow worms) is a member of the beetle family, and is dark coloured with three pairs of legs at the head end of the body.  The green bioluminescent light is emitted by the flightless lady glow worm as a way of attracting any male glow worms flying by and wearing their light-sensitive goggles.  (Male and larvae are only faintly luminescent.)  As is so often the case in the insect world, the female is larger than the male coming in at between 15 and 25 mm.

(As they say on the TV, now for the science bit, for those of you interested, the glow is caused by the oxidation of luciferin.)

Once the green light has worked its magic the female glow worm turns off her light, lays her eggs and dies.  The larvae look like the female, but have small light coloured spots.  They feed on small slugs and snails, hanging about under rocks and bark for a couple of years before the adult emerges which then has a lifespan of only 14 days (this is cut tragically short by the adults’ inability to eat).

Glow worms are mainly found in the south of England, and favour chalky or limestone areas.  (I am therefore not surprised that I haven’t seen any in Daventry because a) this is the land of claggy clay and b) I have never looked for one and, until researching this article, did not know what one looked like.)  Disused railway lines are apparently cool places for them to hang out, although they are often seen in gardens, June and July being the optimum months.

There is quite a lot of information out there for those of you interested in learning more about glow worms, the most useful that I found being:

So, have you ever seen a glow worm?