Defensive Froths From Diverse Glands

A surprising diversity of defensive secretions has been converted to froths that may literally bathe small adversaries with compounds that seem to adversely stimulate the olfactory and gustatory receptors of their predators. The independent evolution of deterrent froths by moths, grasshoppers, and ants demonstrates that this form of defensive discharge can be highly efficacious in adverserial contexts.

Species in moth genera in the families Arctiidae (apose-matic tiger moths), Hypsidae, and Zygaenidae secrete froths, the production of which is often accompanied by a hissing sound and a pungent odor. The aposematism of these moths is enhanced by secretions discharged from brightly colored areas on or near the prothorax. These secretions do not seem to contain plant natural products but rather, toxic de novo synthesized compounds such as pharmacologically active choline esters. Some arctiid froths contain blood, but its importance is unknown.

Frothing is highly adaptive in the ant genus Crematogaster. Workers in this very successful myrmicine genus do not possess a hypodermic penetrating sting, but rather, a spatulate sting that is enlarged at the tip. Venom accumulates at the tip and can be smeared onto small adversaries such as ants as if with a paintbrush. This mode of administration of venom is obviously identified with a topical toxicant that can penetrate the insect cuticle much as an insecticide does. There is no indication that tracheal air is added to the venom to generate the discharged froth.

Two grasshopper species produce froths that are derived from a mixture of tracheal air and glandular secretion. Both species are eminently aposematic, and this warning coloration is enhanced by a powerful odor emanating from the froths of the pyrgomorphid Poekilocerus bufonius and the acridid Romalea guttata.

P. bufonius, a specialized milkweed feeder, is brilliantly colored, exhibiting a dark bluish gray background with contrasting yellow spots and orange hind wings. From a bilobed gland opening between the first two abdominal tergites, disturbed grasshoppers discharge a viscous secretion that is converted to a froth when it mixes with air while passing over the second abdominal spiracle. The froth enhances the aposematism of P. bufonius by appearing to be rainbow tinted in contrast to the dark background. This grasshopper is well protected from predation because its exudate contains de novo synthesized toxins and sequestered plant natural products that are strongly emetic.

In contrast to P. bufonius, R. guttata is a generalist feeder but as with P. bufonius, its defensive exudate is discharged as a froth that contains plant natural products as well as compounds synthesized by the grasshopper. If R. guttata temporarily specializes on a plant species rich in allelochemicals (Allium spp.), its defensive froth can be highly repellent. The aposematism of this acridid rivals that of the pyrgomorphid, and the warning coloration of the former is considerably enhanced by a loud hissing that accompanies the very odoriferous secretory froth.

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