Bad bugs

In her article on the assassin bug ["In The Heat of the Night" (7-8/05)], Graciela Flores falls into the inverse-square-law trap when she writes that the amount of heat received by a bug's antennae quadruples as the bug halves the distance to its prey. The inverse square law applies only when the heat source is a point.

When dealing with a distributed heat source, such as the heated plates used for the experiments in the article, the amount of heat received by the bug's sensors ceases to grow substantially once the distance from the insect to the heat source becomes smaller than the dimensions of the source. Perhaps it is this effect that the assassin bug uses to gauge its distance from a victim?

Philip L. Epstein

Monroe Township, New Jersey

GRACIELA FLORES REPLIES: Philip L. Epstein correctly points out that the inverse square law applies only to point sources. Although the thermal plates I used were certainly not points, they were smaller than the distance that separated them from the insects (though the illustration might have suggested otherwise). That makes the inverse square law a good working hypothesis.

The critical issue here is whether or not the increase in heat is inversely proportional to the distance to the heat source. Actually, the heat increases faster than the distance decreases, and vinchucas may be using this cue to learn their position relative to the heat source. It is also important to keep in mind that in the experiment described in the article, I was testing the ability of the vinchuca to estimate the area of the heat source, not its distance.

As Mr. Epstein suggests, it is theoretically possible that the heat received by the bugs' sensors becomes nearly constant as the distance from the insect to a massive heat source becomes very small. It is unlikely, however, that a vinchuca relies on that effect to gauge its distance from a host. Insects confronted with heat sources of different size always extend the proboscis at the same distance from the source.

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