okay so. to begin with, you have holometabolous insects, that is to say ones with grub-like immature stages rather than ones whose immatures are basically smaller versions of the adults, so things like flies and beetles and moths as opposed to grasshoppers and true bugs which are hemimetabolous insects. the advantage of the holometabolous lifestyle is that the larvae exploit entirely different ecological niches from the adults and thus don’t compete with them for space and resources. like a maggot and an adult fly live in different places, eat different things, and so on. additionally, when your body is basically a big flexible sac with a mouth, you can eat a lot more and grow a lot faster, while immature hemimetabolous insects have to go through the whole molting rigamarole every time they outgrow their chitin.
the problem of the holometabolous lifestyle is how the hell do you transform from a grub or a maggot or a caterpillar to an adult insect? they’re entirely different body types built for different things; again a simple sac with a mouth vs a complicated adult with wings and legs and genitals and all that fancy stuff. when a larva pupates, it doesn’t just completely liquify and regrow all those parts from nothing, that would be ridiculous. so instead what happens is that the larva has these imaginal discs, which are clumps of special cells throughout its body that, as insect grows and nears pupation time, develop into the precursors to all the adult limbs and organs. imagine them as sort of deflated versions of the adult organs, tightly packed into little disc-shaped bundles that, when the larva pupates, evert and inflate, while all the other non-imaginal cells liquefy.
