Developmental Modularity and Pre-Eclosion Sclerotization of the Pronotum Facilitate Eclosion in the Ladybird Beetle, Harmonia axyridis

Developmental modularity allows specific tissues to adjust their developmental timing to meet stage-specific functional demands. During insect eclosion, the biomechanical requirements for emergence often conflict with adult defensive adaptations. This study investigates the evolutionary and molecular mechanisms of the enlarged pronotum in the ladybird beetle, Harmonia axyridis. We found that pronotal melanization and sclerotization are completed approximately 3 h before eclosion, asynchronous with the elytra, which mature post-emergence. RT-qPCR confirmed that the key tanning gene HaLac2 peaks early in the pronotum but is delayed in the elytra, allowing the latter to remain flexible for expanding. RNAi knockdown of HaLac2 resulted in a 72.5% eclosion failure rate due to a soft pronotum being unable to rupture the pupal cuticle, demonstrating its critical role in this process. Furthermore, larval knockdown of the Hox gene HaScr induced a homeotic transformation of the prothorax into a mesothorax-like identity, causing a complete loss of this accelerated developmental modularity program. Together, our findings provide functional and molecular evidence demonstrating how upstream Hox genes couple segmental identity with modified developmental schedules to optimize distinct, stage-specific survival strategies.