Developmental morphometrics of drone honey bees (Apis mellifera L.): A comparative analysis across lowland and highland agro-climatic zones

Aim: The present study aimed to evaluate altitude-driven ontogenetic variation in drone honey bees (Apis mellifera L.) by comparing morphometric traits across all developmental stages (egg, larva, pupa and adult) from two contrasting agro-climatic regions of Northern India: Kangra, Himachal Pradesh (high altitude) and Ludhiana, Punjab (low altitude). Methodology: A total of 320 drone specimens were collected from healthy colonies maintained under standardized apicultural conditions, with equal representation from both regions. Morphometric analyses were conducted across successive developmental stages, including egg dimensions and volume, larval body length, width, weight and cephalic capsule width (L1–L5), pupal structural traits, and detailed adult anatomical measurements. Measurements were taken using calibrated stereomicroscopy, digital calipers and image analysis software, and data were expressed as mean ± SD for inter-regional comparison. Results: Across all developmental stages, drones from high-altitude Kangra region consistently exhibited marginally larger morphometric values than those from Ludhiana. Eggs showed slightly higher volume at higher altitude. Larval growth parameters, pupal body dimensions, wing pad diameter, and adult traits including body length, compound eye size, antennal and proboscis length, leg segments, and pigmentation intensity were consistently greater in Kangra drones. These differences became more pronounced during pupal and adult stages. Interpretation: The morphometric augmentation in high-altitude drones reflects altitude-associated developmental plasticity, likely driven by cooler temperatures, extended developmental duration and floral resource diversity, consistent with established ecological size-temperature relationships. These findings provide valuable baseline data for altitude-specific breeding, conservation, and sustainable management of A. mellifera drones.