The larva problem: homology, evolvability, terminology

Larva' is one of those biological terms that seem to grant unambiguous communication between researchers despite the lack of agreement on a satisfactory definition.Problems crop up however, when one wants to fix their meaning.To address this 'larva problem', I suggest a soft notion of larva, which takes on a different meaning depending on the research project in which it is applied.This allows to address semantic issues such as the distinction between larva as developmental condition vs. larva as developmental stage and the entrenched conceptual problem of adultocentrism (treating early developmental stages as if the adult, and only it, were 'the true animal').Three popular criteria to recognize a larva (the larva is a non-reproductive developmental stage; the larva is fundamentally different from the adult, from which it is separated by a metamorphosis; the Hox genes of the zootype are not expressed in the larva) are critically discussed.The evolvability of larvae is also briefly discussed from an evo-devo perspective, pointing to intraspecific variation in early developmental processes and in larval forms in the absence of genetic diversity, or despite minimum genetic diversity.As there are many sensible ways to dissect ontogenies into comparable units and these do not necessarily correspond to conventionally named stages, I defend the legitimacy of a pluralistic treatment of larvae.To provide materials for a desirable streamlined communication about these issues, I discuss in the final section a number of terms that cannot be used without precise qualification, and others that have undergone an injustified drift in spelling in recent times.