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Above: Amerila astreus with two large globs of defensive froth

Moths of the genus Amerila are often described as frother moths, because they're able to secrete a frothy, foul-tasting substance that helps to deter predators. The fluid is produced by the prothoracic glands, which are located near the base of each wing (just behind the moth's head) and a distinctive "sizzling" or "hissing" sound is emitted as the frothy substance bubbles out.

Above: Amerila astreus

As this article explains:

If molested, resting adults produce quantities of a frothy, orange fluid from their prothoracic glands, accompanied by a sizzling sound. The froth not only has an aversive odor to humans but also contains PAs (pyrrolizidine alkaloids) which are taste-repelling.

Above: a frother moth producing its defensive secretion

The adult moths of this genus are pharmacophagous, which means that they acquire the chemicals that are used to create their defensive froth by ingesting plants that contain toxic or noxious compounds; those chemicals are then sequestered within the moth's own body, where they are repurposed and transformed into a frothy secretion.

Above: genus Amerila

Frother moths also use aposematic markings to signal their toxicity and/or unpalatability. Their legs, thorax, and abdomen are all decorated with reddish-pink markings, and they have large black spots that stand out against their mostly-white bodies.

Above: Amerila crokeri and Amerila rubripes

The genus Amerila contains dozens of species, and they are all known to have this defense mechanism. They are widely distributed throughout many different parts of the world; depending on the species, they may be found in the Himalayas, Indochina, Southeast Asia, Melanesia, Australia, or Africa.

Above: Amerila crokeri

Several other moths from the subfamily Arctiinae can produce a similar defensive secretion when threatened, but the color, consistency, and composition of the substance differs greatly from one species to the next.

Sources & More Info:

iNaturalist: Genus Amerila

Metamorphosis Australia: Australian Arctiid Moths

Australian Lepidoptera: Amerila crokeri

Metamorphosis Australia: Weird and Wonderful Moths

Entomo Brasilis: Defensive Froth in Arctiidae Species in the Rio Grande do Sul State, Brazil (PDF)

Moths of Australia: Adult Adaptations for Survival