In the heat of the night: an assassin bug's sensory journey

Natural History, July, 2005 by Graciela Flores

The tetrahedral shape also has a clever purpose. Because three edges come together at each vertex, whenever the insect walks up to a vertex, it faces a choice of two ways to proceed, left or right. If the experimenter presents a stimulus on the right different from that on the left, and notes which way the insect turns, the animal's behaviors can be studied in detail.

I inherited my spangenglobuses from Lazzari: ten hand-carved hollow Styrofoam balls. I guarded the little treasures with zeal and kept them together with ten custom-made acrylic boxes, ten diminutive copper pincers, a miniature brush made of my own hair, and a few solid aluminum plates that served as heat sources. With this basic homemade tool kit I studied the vinchucas' responses to heat. We later acquired more sophisticated gadgets, but the first experiments, the ones conducted with the most primitive tools, were by far the most interesting.

With the spangenglobus, Lazzari had tested the behavior of blinded vinchucas--their eyes had been covered over with paint--when he confronted them with pairs of metal plates at different temperatures. He found that the insects walked toward temperatures close to that of a warm-blooded animal, away from plates that were hotter, and were unaffected by the ones at room temperature. This result might seem obvious, but it is not. It has an important implication: the insects can estimate the temperature of a distant object. Furthermore, in Lazzari's experiments the vinchucas never raised their proboscises in an attempt to bite the warm metal plate: they "knew" it wasn't close enough, even though their eyes were covered.

In another experiment, this time with unrestrained, freely walking--but still blind--vinchucas, I observed that once the insects reached the heat source, they repeatedly tried to bite and pierce the metal; they all extended their proboscises when they were about half an inch from the source, even though they could not see it. Did that imply that they could estimate distance as well as temperature? Abundant evidence suggested they could; it was time to ask how.

To answer that question, think about what happens to the thermal sensilla. When confronted with a heat source, the sensilla receive a given amount of heat, which fires neurons. But that information, by itself, is not enough to discriminate sources at different temperatures, because a distant--or small--hot source could produce the same local warming as a closer--or larger--cool source. Thermal sensilla alone cannot distinguish those two conditions, but vinchucas certainly can.

The insects might be recognizing two characteristics of the heat source: distance and size, and "computing" a third characteristic, its actual surface temperature. Vinchucas could gauge distance--we had ample evidence of that. But I was determined to find out about their ability to discern size. If they could estimate the first two of these variables, then, given the basic physical and geometric circumstances, maybe they could "calculate" the third.