Some insects, like cecidomyian gall midges, reproduce in a weird way in which the offspring devour their mother. Theories of r- and K- selection can help explain this behavior.
Organisms adapt not just by altering their size and shape, but also by adjusting their "life history strategies".
If the environment is not stable, a good 'strategy' is to reproduce as many offspring as you can, as fast as possible, so that at least some will survive. This is r-selection (r- refers to "rate of increase")
Species living in stable environments, near the maximum population the environment can support, have no incentive to reproduce a lot. It's better for them to raise a few, finely tuned offspring. This is K-selection (K- is for "carrying capacity")
The gall midge, which is a tiny fly, mentioned above is an r-strategist.
They reproduce in two ways:
* normal - hatching from an egg after fertilization
* parthenogenetic - no male fertilization, females reproduce as larvae or pupae. Parthenogenetic females don't lay eggs. Their multiple offspring develop within their mother's body, devouring the mother from inside. Two days after the children emerge, their own children are developing to eat them in the same way. We can understand this behavior in terms of r-selection.
If these flies come across mushrooms, their food, they use it to inflate their population using the second method of reproduction. Similar methods of reproduction have independently evolved in several other species - Micromalthus debilis, Mycophila speyeri, Aphids for example.
In contrast, humans are consummate K-strategists.
Ever Since Darwin: Reflections in Natural History by Stephen Jay Gould
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