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It causes severe irritation.

Yes, but why?

The silk is impregnated with caterpillar hair, which is highly irritating when it touches the skin.

Indeed IB. IT.

Give the behavioural reason why you often find flying insects which have fallen in to large bodies of water.

A wild guess here.
a) They don't have stamina to cross large bodies of water and then drop into it,
b) Because they fly in erratic directions and not in a strait line (to avoid being caught) they land in the water and can't escape.

Excellent try ... but no, not correct.

I remember reading about this some time ago. Doesn't it have something to do with the reflection of light on the water, which confuses an insect into thinking that the sky is actually where the water is? If the sky is now in the wrong place, the poor little insect will flip itself upside-down to correct for this, and so fall into the water. Once it comes into contact with the water's surface tension, it is very difficult for it to extricate itself. I think that this can happen both at night and in the daytime, as long as there is a reasonable reflection on a body of water.

You are now IT!

Why do dung beetles roll their balls obsessively in a straight line, even pushing it over obstacles in the way (even though considerable energy may be wasted by so doing) rather than navigating around the obstacle?

If they don't, chances are that they could go in a circle and end up at the same dung pile where other dung beetles could then pirate his dung ball.

Thank you answering Roan. Yes, there is indeed the factor of getting as far away from the competition as quickly as possible. However, there is another reason why they move in a straight line; it doesn't make sense to struggle over an obstacle for a protracted period while the competition is catching up - why not just go around the obstacle?

No-one has got the answer yet ...

Not sure what you looking for here OWN, but it must have something to do with the fact that they actually "push" the dung ball backwards with their hind legs??

Rosemary, the pushing backwards does help to keep the ball in a straight line, as well as increase leverage, but there is one particular thing that the beetles use to keep themselves in a straight line.