Flight muscle structure and flight capacity of females of the long-distance migratory armyworm, Spodoptera frugiperda: effect of aging and reproduction, and trade-offs between flight and fecundity

Insects in flight achieve the highest metabolism rates and wing beat frequency known in the animal kingdom, but how they do this is not fully understood. Here, we demonstrate the structure and arrangement of flight muscles in the thorax of Spodoptera frugiperda Smith & Abbot (Lepidoptera: Noctuidae). Dorsoventral direct flight muscles are stronger and more complicated than dorsoventral indirect flight muscles, which may be important for the moth to swiftly control the trajectory. The abundance of giant mitochondria and the extensive tracheolar system in flight muscle are two prominent features that are well adapted to the special requirements of flight. Parameters of flight muscle and flight capacity were lower in young females, and then increased with time and peaked in middle-aged females. Mating accelerated the deterioration of the structure and function of flight muscles. Virgin females showed lower ovarian development rate but higher flight capacity than mated females. Further analysis found that female flight capacity was significantly negatively correlated to her fecundity, suggesting a trade-off between flight and reproduction. The resource allocation to flight and egg production may be the result of a balance between the intensity of natural and sexual selection on both traits.