Body size in holometabolous insects is determined by the size at which the juvenile
larva undergoes metamorphosis to the pupal stage. To undergo larva-pupa transition,
larva must reach a critical developmental checkpoint, the threshold size (TS); however,
the molecular mechanisms through which the TS cues this transition remain to be fully
characterized. Here, we use the flour beetle Tribolium castaneum to characterize the
molecular mechanisms underlying entry into metamorphosis. We found that T. castaneum
reaches a TS at the beginning of the last larval instar, which is associated with
the downregulation of TcKr-h1 and the upregulation of TcE93 and TcBr-C. Unexpectedly,
we found that while there is an association between TS and TcE93 upregulation, it
is the latter that constitutes the molecular trigger for metamorphosis initiation.
In light of our results, we evaluate the interactions that control the larva-pupa
transition and suggest alternative models.