Utetheisa, like many other Lepidoptera of the family Arctiidae, is aposematic. White, with pink hindwings and black and yellow markings (Figure 1A), it is highly conspicuous on the wing. It flies as readily in daytime as at night. We suspected the moth to be unpalatable, and we were able to prove this in experiments with orb-weaving spiders.

We knew from previous work that moths are protected from entanglement in spider webs by their investiture of scales. Instead of sticking to webs as "naked" insects typically do, they simply lose scales to points of contact with the orb and flutter loose (4). Utetheisa, in contrast, becomes instantly quiescent when it flies into a web. The spider converges on the moth and inspects it, but then, almost invariably, sets it free. During the inspection the moth sometimes emits its defensive froth (Figure 1C), but it is released even if it withholds the effluent. Spiders such as Nephila clavipes cut the moth from the web by snipping the entangling threads with their fangs (5, 6) (Figure 2A), while others, such as Argiope florida, free the moth by pulling it from the web (T.E., unpublished observations).

We knew Utetheisa to feed on poisonous plants as a larva (Figure 1B). The plants, of the genus Crotalaria (family Leguminosae), were known to contain pyrrolizidine alkaloids (henceforth abbreviated as PAs), intensely bitter compounds potently hepatotoxic to mammals (7). Other species of Utetheisa were known to sequester PAs (8). We found this to be true for U. ornatrix as well. Adult Utetheisa raised on Crotalaria spectabilis, one of the principal foodplants available to the moth in the United States, contain on average about 700 µg of monocrotaline (1), the principal PA in that plant (9, 10).

In the laboratory, we succeeded in raising Utetheisa on two alternative artificial diets, one made up with Crotalaria seeds and containing the PA monocrotaline (CS diet), the other based on pinto beans and devoid of PA (PB diet). On the assumption that the PB diet-raised moths, which we proved to be PA-free, would be vulnerable to predation, we took moths

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