One of the classic examples of a mutualistic interaction is that between figs and the wasps that pollinate them.
1. A female wasp arrives at one of these structures and gathers pollen. She then enters and deposits pollen on the female flowers within and also deposits an egg in each of the ovules that she can reach. Then she dies.
2. The eggs hatch; the larvae feed on the developing seeds; the larvae pupate; and adult wasps emerge.
3. The males mate with females and chew a hole through which the females depart. The male promptly dies; lacking wings, he wasn't going anywhere anyway.
So how is it good for the plant to have eggs dumped inside it, its developing seeds eaten, and a hole chewed through it? As it turns out, the female flowers on the inside are different lengths. Some are too long for the wasp to reach down and deposit an egg on. As a result, some seeds escape predation by the developing wasps. So the wasp gets food for her offspring, and the fig gets a very reliable pollinator.
.«jMiEfl Even though in this case the plant and the insect are completely reliant on each other for survival, each species is acting in its own interests. The fig wasps don't pollinate the fig tree because they care about the fig; they do it because their offspring will feast on the developing seeds. And successful fig trees are the ones that produce seeds above and beyond the ones that get eaten. The wasp uses the fig; the fig uses the wasp; and the species are so tightly co-evolved that each would go extinct without the other (what scientists term an obligate mutualism) because each species of wasp pollinates only a single species of fig, and vice versa. But they're still not pals!
Oh, and if you've been keeping track of the dead wasps, you've noted that every time you eat a fig, you're eating a few dead wasps, too. Don't worry, though; they're very tiny — small enough to fit through the eye of a needle.