Non-Invaginated Wing Primordia in Holometabolous Insects: First Report From Mecoptera.

This study presents the first detailed histological description of wing primordium development in Mecoptera and demonstrates a late-forming, non-invaginated developmental mode lacking a morphologically distinct peripodial epithelium. Wing primordium development in holometabolous insects follows two major modes, early-forming and late-forming. Late-forming primordia are epidermal cell populations that remain integrated within the larval epithelium, contribute to cuticle production during each larval molt, and begin morphogenesis only in the final instar. The order Mecoptera remains among the least studied groups of Holometabola with respect to wing primordium development. Using a newly established rearing method and histological analysis, we investigated wing development in the scorpionfly Mavropanorpa japonica and found the following: (1) wing primordia are absent in actively feeding larvae; (2) they originate as localized epidermal thickenings beneath the lateral pinacula approximately 20 days after cocoon formation; (3) thickening of wing primordia begins concurrently with apolysis, and the epithelium extends outward without pouch-like invagination; and (4) no morphologically distinct peripodial epithelium was observed at all stages. Our findings show that wing primordia in Mecoptera develop externally, without invagination and without formation of a morphologically distinct peripodial epithelium. This study provides new comparative insight into heterochronic shifts and evolutionary diversity in holometabolous wing development.