Tuesday, March 27, 2012

Spiders have their skeletons on the outside...

Since my journey into macro photography began, some years ago now, there have been quite a few asides to the actual business of taking photos.
One of these has been that my innate fear of spiders has lessened with the knowledge that has come from photographing and researching them.

I've only scratched the surface of course of what could be attained regards information on these critters, but all the same, I've come to realise that they are fascinating creatures and have an important place in our ecology.

I suppose that one of the most amazing revelations has been that spiders moult. I had always assumed that thing hanging motionless in the corner of the garden shed, just under the ceiling, was a dead spider, but no, not so.

I remember as a child seeing the remains of 'Pholcus phalangioides' the Cellar Spider hanging in the outside loo and thinking the same. It hadn't occurred to me that it was nothing of the kind but was actually a moult.

And so it came as quite a shock to me when I found and photographed my first spider-moult.

A Spider's Exoskeleton
The correct term for what you see here is an exoskeleton and put basically, it's a stiff support structure on the outside of the body.

Like we do, spiders move by contracting muscles attached to their skeleton. The way in which they differ from us is that their skeleton is on the outside. Exoskeleton segments are connected together with joints so that the spider can move them back and forth.

The skeleton is made up of several layers of cuticle,plus various proteins and chitin. These are arranged in long chains and layers a bit like plywood.

This structure makes the cuticle very strong, as well as highly effective at keeping the spider from drying out, but the material does have one serious drawback. Whilst it's flexible enough for movement, it can't expand like human bones and tissue:in other words it can't grow. In order to increase its size, the spider has to form a new, larger cuticle exoskeleton and shed its old one.

Moulting occurs frequently when a spider is young, and some spiders may continue to moult throughout their life. At the appropriate time, hormones tell the spider's body to absorb some of the lower cuticle layer in the exoskeleton and begin secreting cuticle material to form the new exoskeleton.
To shed the old exoskeleton, the spider has to burst out from the inside: it increases its heartrate to pump a lot of hemolymph (spider blood) from the abdomen into the cephalothorax. The pressure expands the cephalothorax, which pushes on the old exoskeleton until it cracks. The spider then flexes its muscles until the old exoskeleton falls away.

I'd like to apologise here for the amount of text in this particular blog-entry, and now ironically, I've added to it by typing this, and so I'd better apologise for doing that as well.
Sometimes it's required to tell a story though and I felt that it would be uninteresting and dull to just add photos with little or no explanation or reason.

Today I found a spider that had freshly moulted. In fact, it was so fresh that you can see in the picture below that the cephalothorax is still very soft and misshapen.

Click on any photo for a larger view.
An Orb-Weaver Spider moulting

This was an exciting find as I've never witnessed this before and it completes another piece of the jigsaw regards spider behaviour for me. Here's another view; this time from below. I was struck by the markings underneath the spider that make it appear to have eyes-it almost looks like a tiny octopus here, especially with the right amount of legs.

Typically, the spider does most of its growing immediately after losing the old exoskeleton, while the new one is highly flexible. The new exoskeleton is also very soft at this stage. making the spider venerable to attack. You can see from the photo above how lifeless the body looks at this stage.

Many species will lower themselves on a silk line during the moulting process, so they're out of reach of predators while the cuticle material hardens. This orb-weaver is just about to do that very thing in the photo above.

It was a real treat to see this today and I hope that I've not rambled on too much here. I wanted to share this event so that you too can marvel at nature's ingenuity as I have.

Util the next time then...


  1. My son and I found a spider molt and subsequently researched and found your blog. We LOVE what you taught us!!

    1. Thank-you so much Matt. Always nice to get positive feedback and great that your son is interested in nature...


Please feel free to comment on my blog. I am always grateful for any feedback, good or bad. Commenting should be fast and easy. Just enter your comment in the box, then click on the drop-down box beside 'Comment as'. You can use your Google ID if you have one, or just choose 'Name/URL and enter your name (URL is not needed). You can also just choose anonymous, if you would rather not be identified.

Regards 'JJ'.

If you do experience any difficulties, you can contact me directly from this blog and I will try to help.