Correlation between Pupal Color Dimorphism and Gender Ratios of Papilio polytes romulus (Lepidoptera: Papilionidae) in Sri Lanka

Abstract

Papilio polytes romulus (Common Mormon) is the most common swallowtail species widely distributed throughout Sri Lanka with females being polymorphic. Their larval food plants include several Citrus and Murraya species and they also show pupal color dimorphism. To study the correlation between pupal color dimorphism and gender ratios their life cycle was studied in the month July of 2021. A free ranging female butterfly was observed laying eggs on Citrus aurantiifolia (lime) in a home garden in Battaramulla.16 eggs were found and 3 days later the eggs hatched and the larvae were collected with leaves and reared in the lab. The duration and measurements of each stages were recorded. After eclosion the gender and wingspan of all adult butterflies were recorded and released back to the environment. The complete life cycle of Common Mormon is known to be about 32-36 days with adult stage about 8-10 days. The immature stages (N=16) duration was recorded as follows: Egg 3.5±0.5 days, 1st instar 3.5±0.5 days, 2nd instar 2.5±0.5 days, 3rd instar 2.5±0.5 days, 4th instar 2.5±0.5 days, 5th instar 5.5±0.5 days, pre-pupa 1.0±0.0 day and pupa 11.5±0.5 days. The length of immature stages (N=16) were recorded as follows: Egg 1.0±0.2 mm, 1st instar 4.0±1.0 mm, 2nd instar 8.0±2.0 mm, 3rd instar 14.5±1.5 mm, 4th instar 23.0±3.0 mm, 5th instar 38.0±8.0 mm, pre-pupa 27.5±2.5 mm and pupa 31.5±0.5 mm. Adult butterfly wingspan ranged from 80- 115mm. Pupae were observed in 2 colors: Green and Brown. Out of 16 pupae, 4 were brown (25%) and 12 were green (75%). 5 pupae (4 brown and 1 green) were made on sticks (rough surface) and the rest (11 green) were made on the wall of the glass container (smooth surface). This showed a strong relationship between pupal color and substrate texture. From the 4 brown pupae emerged 2 males (50%) and 2 females (50%) butterflies. From the 12 green pupae emerged 10 males (83%) and 2 females (17%) butterflies. The gender ratios regardless of the pupal color showed male dominance with 12 males (75%) and 4 females (25%). All four females were of form romulus. Further investigation into environmental factors influencing pupal color plasticity such as season, type of larval food plant, captive vs. non-captive setting, temperature, relative humidity, pupation on plant vs. off plant, substrate texture and background color needs to be studied to fully understand the basis behind this relationship. However, in this study no relationship between pupal color, gender and gender ratios was seen.

Keywords: Common mormon, Pupal color, Gender ratios, Sri Lanka