I think those HIV/AIDS adverts work well to alert people to the risk of unsafe sex. I like the visual metaphor of the predatory 'animals' - it visually shocks people, thanks to our natural aversion to these creatures, but it also has the message that this disease is part of nature, and as such, can strike anyone at any time. It's a short, sharp shock, and is in direct contrast to the perhaps more moralising ways in which people are warned about HIV/AIDS.
I'm calling them 'animals' because 'arachnids' doesn't exactly trip off the tongue. Which is another vile image...