Skeletal economy in certain herbivorous beetles as an adaptation to a poor dietary supply of nitrogen

ABSTRACT.

• 1 Flying Coleoptera were sampled with light traps from the tree canopy of lowland alluvial rain forest in Brunei, Borneo.
• 2 Adult chrysomelid beetles were found to carry significantly less skeleton relative to their overall mass than those of any family other than herbivorous scarabaeids (subfamily: Rutelinae). Xylophagous lucanids and cerambycids and dung-feeding scarabaeids (subfamilies: Aphodiinae and Scarabaeinae) carried as large a proportion of skeleton by mass as predatory carabids and cicindelids.
• 3 Skeletal production requires the commitment of assimilated nitrogen to chitin and proteins. Adequate nitrogen is more readily available to predators, carrion or dung-feeders, and xylophages (given enough time) than to leaf-chewing chrysomelids, which are also environmentally exposed to predatory attack. Female chrysomelids were found to carry a larger number of mature eggs/insect than those of any other family. A larger proportion of their mass was represented by eggs than skeleton, as reproductive compensation for reduced skeletal protection.
• 4 It was estimated that a chrysomelid beetle required approximately 43% of the nitrogen incorporated by a predatory carabid of the same mass, whereas xylophagous cerambycids and lucanids would need between 84% and 98% of that in the predator.