Life Cycles

Passages on this topic usually hand you a diagram or a short description of stages and ask you to read it correctly, since the test does not expect you to have memorized every organism's cycle beforehand. Complete metamorphosis is the clearest case: a moth larva has one job, eating and growing, while the adult has a different job, flying and reproducing. Inside the pupa, most larval tissue breaks down and is rebuilt from clusters of set-aside cells called imaginal discs, so the two stages can have completely different bodies suited to completely different lives.

Not every cycle is a clean loop from egg to adult and back. Plants that show alternation of generations switch between a haploid stage, the gametophyte, which makes eggs and sperm directly without any division that halves chromosome number, and a diploid stage, the sporophyte, produced when two gametes fuse. The two stages can look nothing alike and even live independently, so a diagram of a fern or moss cycle will show two distinct body forms, not one organism maturing. Parasite cycles add another twist: some parasites require two different host species to complete development, one where they mature and one where they reproduce or get transmitted further.

Read any life-cycle diagram stage by stage and ask what job each stage does and what body or host it needs for that job. If a required host or stage is removed, as when snails are cleared from a pasture to stop a liver fluke, the whole cycle breaks at that point and transmission stops. That same logic connects life cycles to population and ecosystem questions elsewhere on the test, where removing one species from a food web or habitat can cut off a cycle that other species depend on.