Abstract.The chorion of the egg in Callosobruchus maculatus (Coleoptera: Bruchidae) provides protection but also has to allow exchange of respiratory gases. A single opening in the chorion (the egg pore) allows diffusion of gases, and young larvae die if the egg pore is blocked. Comparison of respiration rates using a Gilson respirometer showed an increase in oxygen uptake from the first to the seventh day after the egg was laid; the increase was from 6.8 to 11.2 pl/egg/day in a Brazil strain and from 8.6 to 23.5 pl/egg/day in a Yemen strain. The Yemen strain had double the metabolic rate of Brazil‐strain insects in the larvae, which are exposed to seed toxins, but not in the non‐feeding adults. It is suggested that the higher metabolic rate in the Yemen larvae is related to their ability to develop in seeds that are toxic to the Brazil strain. The egg pore also differed in size and shape between the two strains. The egg pore was cylindrical in the Brazil strain, but shorter and funnel‐shaped with an increased external diameter in the Yemen strain. Calculation showed that the different shape and size of the Yemen egg pore would allow a 2–3‐fold increase in gas conductance compared with the Brazil strain, and this increase was consistent with the enhanced oxygen requirement of the Yemen larvae, which was a little over twice that of the Brazil larvae at 7 days. The anatomy of egg pores in Cmaculatus thus seems to represent a trade‐off between two functions, allowing adequate gas exchange while maintaining the protective function of