Nonglandular Discharges Of Plant Origin

Certain insects have evolved storage reservoirs for plant natural products that can be discharged in response to traumatic stimuli. This evolutionary development reflects the insect's appropriation of plant allelochemicals (defensive compounds) for subsequent utilization as defensive allomones. In essence, the insects have sequestered the plant's defenses and stored them in reservoirs, where they are available as defensive agents. This defensive system does not require the evolution of any biosynthetic pathways for the storage of compounds in nonglandular reservoirs.

Adults of hemipterous species in the family Lygaeidae possess dorsolateral (reservoirs) and abdominal spaces that contain a fluid very similar to that of the proteins in the blood. This fluid sequesters steroids (cardenolides) present in the milkweeds on which these species feed. The cardenolides are about 100-fold more concentrated in the dorsolateral fluid than they are the blood, and they thus constitute a formidable deterrent system.

Sequestration of plant natural products in nonglandular reservoirs also characterizes larvae of the European sawfly, Neodiprion sertifer. Feeding on pine (Pinus spp.), these larvae sequester both mono- and sesquiterpenes in capacious diver-ticular pouches of the foregut. Young larvae, feeding only on pine needles, sequester only three terpenes, whereas older larvae also ingest resin acids. These acids also serve to entangle would-be predators, thus providing a dual protective function.

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