Would someone please explain...

[ian_g]

... something that has puzzled me for a long time?

I have read that 75-85% of a spider's body is actually water and I would imagine that an earthworm is relatively the same, possibly more.

Anyway, the question I have is why can spiders sit in the centre of their webs in direct sunlight and not dry up whereas an earthworm starts to have difficulties almost immediately?

The only thing I can think of is that their external 'skins' are made up differently, the earthworm's being significantly more moist.

Strange question, I know

[Wild-Woman]

I read that a certain beetle in Africa re-absorbs it's urine when it's hot.Perhaps a spider is able to do something like this. Don't know really but it's an interesting question.

[GRH]

Originally Posted by ian_g

... something that has puzzled me for a long time?

I have read that 75-85% of a spider's body is actually water and I would imagine that an earthworm is relatively the same, possibly more.

Anyway, the question I have is why can spiders sit in the centre of their webs in direct sunlight and not dry up whereas an earthworm starts to have difficulties almost immediately?

The only thing I can think of is that their external 'skins' are made up differently, the earthworm's being significantly more moist.

Strange question, I know

In a word: evolution.

Worms live in damp environments and have little need for water retention - a porous skin will even allow them to absorb water from the surrounding soil. A spider has evolved to sit in a web exposed to the elements and therefore has a relatively watertight exoskeleton to conserve water. (It's complex and has to do with calcium salts and chitin). Most water loss in insects and spiders will be through respiration (through spiracles/booklungs) or waste products.

Greg

[svenrufus]

Originally Posted by GRH

In a word: evolution.

Worms live in damp environments and have little need for water retention - a porous skin will even allow them to absorb water from the surrounding soil. A spider has evolved to sit in a web exposed to the elements and therefore has a relatively watertight exoskeleton to conserve water. (It's complex and has to do with calcium salts and chitin). Most water loss in insects and spiders will be through respiration (through spiracles/booklungs) or waste products.

Greg

The worm does not just absorb water through it's skin, but it breathes through it's skin as well. If it were not moist, it would suffocate. Whether it is that or the increase in body salt concentrations as it dries out that finally kills it - I don't know if any pathologists would ever bother to find out - but either way, it needs to be wet to breathe, and it's environment does not tend to lead to dessication. Did it choose it's habitat for that reason, or did it's habitat lead to it being a wet skinned organism - chicken and egg, probably bit of both, but in the word that Greg started with - that's evolution.

[DaveBinks]

This 75-85% water is pretty similar for all living organisms (including humans) - but it's in a fairly complex form incuding proteins and fats - forming "protoplasm" - so it's not just free to evaporate away - that's not to say that the animal can't become dehydrated with water loss in hot conditions.

[wyevilla]

One other problem for worms is that they have no defense against ultraviolet light. Once exposed they can recieve a fatal dose of sunburn in a very short space of time. They can therefore die even if in moist conditions.

I presume chitin offers the required protection for insects, spiders and the rest.

[ian_g]

Many thanks to all for being incredibly informative. I've been asking myself this question for some time now and knew that someone here would know.

I can sleep easy tonight!