Insect embryos develop inside eggs that contain food (yolk) needed for development. The embryo also needs oxygen for respiration. Eggs lack spiracles that are found in insect larvae. Instead, eggs have tiny openings called aeropyles to allow exchange of carbon dioxide and oxygen.
In eggs that are laid in areas subject to flooding, water can cover the egg, block the respiratory channels and drown the embryo. Eggs of the caster butterfly have respiratory filaments, tubular structures that extend above the egg surface*.
Aeropyles (ap in figure) are located near the tips of these filaments. The posterior end of the egg (not shown; see paper) is glued to the surface of a leaf. The glue prevents the egg from being washed off the leaf. The glue also maintains the egg position such that the respiratory filaments always extend upward. During floods, the leaves typically float to the surface.The respiratory filaments extend above the surface of the water and maintain access to the atmosphere.
*Asutosh Kumar Srivastava & Krishna Kumar. Ultrastructure of egg chorion of castor butterfly Ariadne merione (Crammer) (Lepidoptera: Nymphalidae). Zoologischer Anzeiger 263 (2016) 1–5