Growing up in Uruguay, Thomas Eisner was always fascinated by a curious arachnid, the harvestman, that `reacted fearlessly and gave out these juices'whenever he attempted to collect them. For years, Eisner wondered about these harvestmen: what are the yellow secretions and why are they secreted? He explains that part of this puzzle was solved in the 1950s when an international team of chemists isolated and identified three chemical compounds, previously unseen in nature, from the secretions of the harvestmen;benzoquinones. But no one knew for sure what the substance was for. For almost 50 years everyone assumed that it plays a defensive role and that assumption remained untested; until now(p. 1313).

Getting started is often the hardest part, and for Eisner, this proved true. Several decades elapsed before a window of opportunity opened when two graduate students from Uruguay joined his lab. Eisner enlisted their help in bringing live harvestman specimens, found only in South America, from Uruguay to Cornell. Clearing customs, it turned out, was the most difficult hurdle. Back in the lab, Eisner and his team performed some simple behavioral tests to see if the yellowish secretions succeed in defending the harvestman. In the first test, they placed individuals in a Petri dish with eight worker ants. The results were clear; in all four trials, when the ants attacked, the harvestman released its yellowish secretion and the ants backed off immediately. The ants then ran around, avoiding the harvestman and any smudges of the defensive fluid left on the dish.

Next, the team tested whether the substance is also effective against predatory Wolf spiders. First, they collected samples of the secretion by pinching the harvestman with forceps and quickly collecting the drops of the yellowish fluid emerging from the arachnid with capillary tubes. The secretion was then applied to the spot on a mealworm where a Wolf spider was dining. Unlike the ants, the spiders did not respond strongly to the harvestman secretions. Some of the spiders removed their fangs from the mealworm when the substance was introduced, but then continued to eat. The rest never even noticed! This was a great surprise to Eisner, who knew that substances that are noxious to ants are usually offensive to spiders as well.

Since the yellowish fluid provides little defence against Wolf spiders,what would happen when a harvestman encounters one? To test this, individual harvestmen were introduced into a hungry spider's cage. The results were another surprise; when a harvestman entered the cage, the spiders pounced on them, but then immediately released their hold and let the arachnids go. All thirteen harvestmen tested escaped encounters with the spiders uninjured. Eisner concluded that the harvestmen must have other chemical defenses that are repellent to spiders. In a complicated world, it seems that a single defense system is sometimes insufficient. `Multiple defenses may be more common than we realize. This is one example,' says Eisner. What this other defensive compound might be is still unknown. But for the moment, Dr. Eisner is savouring the satisfaction of a childhood mystery finally solved.

References

Eisner, T., Rossini, C., González, A. and Eisner, M.(
2004
). Chemical defense of an opilionid (Acanthopachylus aculeatus).
J. Exp. Biol.
207
,
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-1321.