Image: after Eisner et al., PNAS

The tiny whirligig beetle has an effective defense against the fish it regularly encounters in ponds and slow streams: from two glands opening at the tip of its stomach, this surface swimmer secretes a combination of chemicals, including gyrinidal, that apparently make it taste bad. New research shows that largemouth black bass in particular will either avoid this beetle juice or try to rinse it off before eating a whirligig. Cornell University scientists Thomas Eisner and Daniel Aneshansley describe the finding in the October 10th issue of the Proceedings of the National Academy of Sciences.

Eisner and Aneshansley conducted a number of experiments on the beetles and bass they collected from ponds near Cornell. When they fed the bass both beetles and their larvae, mealworms, they discovered quickly that the secretion-free mealworms were clearly preferred (see chart). Whereas the bass ate all of the mealworms, they ignored some 80 percent of the beetles. The three that were eaten--all by one fish--were flushed first: the bass went through an elaborate process of spitting the bug out and reingesting it repeatedly. Other bass spit out for good the whirligigs they flushed, and in all cases the beetles were no worse for the wear.

In another experiment, Eisner and Aneshansley coated the mealworms with the beetle juice and found the larvae were also heavily flushed, although eventually eaten. The mealworms' ultimate fates, the authors suggest, is because the secretion can be completely rinsed off. The beetles, in contrast, emit this chemical slowly and steadily. The researchers further demonstrated that the amount of flushing the fish did was dependent on the dose of the secretion--and that the fishes' tolerance for it rose with their hunger.