The nature walk in question happened mere hours ago, at Kettle Moraine State Forest (Southern Unit), where we had occasion to notice that the local conditions support a much larger population of flying (and biting) insects than the Free-Ride family is accustomed to.
Younger offspring: What happens if a mosquito bites your eye?
Dr. Free-Ride's better half: It probably flies away disappointed. Eyes don't have very much blood.
Younger offspring: Why do mosquitos drink blood anyway? Are they related to vampires?
Dr. Free-Ride's better half: No. They drink blood because it's pretty nutritious.
Younger offspring: But they're bugs! They don't know anything about nutrition.
Your offspring are so brillant. Such an insightful question... my bet is some pregancy-chemical tells the mosquito it's time for blood. But I have no idea what it might be! And I'm a malaria researcher, so it's quasi-relevant to my work!
Actually, come to think of it, there might be a lot of pregnancy-chemicals that help organisms that "don't know anything about nutrition" figure out what to eat (and do pickles and ice cream count?)
Answer that! You can't!
You brought the nice hot weather up here to Wisconsin. Be glad the bugs are actually pretty low this year. Sometimes just hiking the trails I had to wear a net - that generally was early morning though. You could have looked for Emerald Ash Borers, found one, and became famous! Not that many ashes in SKM area though. Those are good ski trais when we (used to) have the snow.
It's actually rather incredible that animals go for nutritious foods they actually need and rich, developed world humans are more attracted towards chocolate and nasty fast-food type of things.
Also, I was supposed to list 7 awesome bloggers so I've nominated you for a blogging award: http://unsubstantialbubbles.net/post/2008/08/23/Awarded (do whatever you want with it!).
One answer to Younger Offspring (and Brian) would be:
It isn't about knowing ahead of time. But those that have a preference for some things rather than others will be more prosperous if their preference turns out to be more nutritious.
The different preferences are due to random gene mutations and the different nutrition leads to natural selection for that preference.
Also, in answer to Citronella:
Hi C, ta for the link. I don't have the cite to hand but there is research to the effect that the high fat and sugar content of junk food attracts us in a similar way to how mosquitos are attracted to blood.