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For Insects, Spiders & Their Kin
For the United States & Canada
Micromalthus debilis  - Micromalthus debilis

Micromalthus debilis - Micromalthus debilis
Wisconsin, USA
Micromalthus has perhaps the most convoluted life cycle of any animal, involving adults that sometimes look like larvae, female forms that either give live birth or lay a single egg, larvae that start life as highly motile speedsters and grow into legless sluggish grubs (shown here), and males that must cannibalize their own mothers in order to mature.

Paedogenesis - not adults that look like larvae.
This is what we learned in advanced taxonomy of coleoptera here at the University of Wisconsin:

The adults are actually no longer able to reproduce, and the species has evolved to utilize paedogenesis, which is reproduction in the larval stage. They have all the characteristics of larvae other than that one tiny detail, and it occurs before a pupal stage, the adults are still typical beetles occuring after a pupal stage with truncate elytra and dead-end genitalia.

Adults are rare, only emerging after a spike in temperature (Prairie fires perhaps?) and the females actually have a grasping mechanism that will destroy the male adeagus in their efforts to mate despite the fact that it'll be fruitless because they are sterile in the adult stage.

They really do have the most convoluted life cycle of any animal in my opinion. Amazing research being done right now to figure them out. Including the male larvae that cannibalize their own mothers.