Excerpts from Jim Conrad's
Naturalist Newsletter

Caterpillar Hunter, CALOSOMA SCRUTATOR

from the July 1, 2012 Newsletter issued from the woods of the Loess Hill Region a few miles east of Natchez, Mississippi, USA

One morning this week a 1½-inch-long (4cm) beetle with shiny, green wing covers with strawberry-pink borders, and an almost-black, deep metallic blue back, or pronotum, with a golden margin -- in other words an unusually big and pretty bug -- was rescued from my rainwater tub. You can see how pretty he was above.

Bea in Ontario had no trouble figuring out that this was a member of the Ground Beetle Family, the Carabidae, and the Caterpillar Hunter genus, Calosoma. However, not having measurements it was hard to say which species. Since Bea got back to me before the beetle in the picture had finished drying, I returned to the twig, took measurements, and decided he could be no-one else but CALOSOMA SCRUTATOR, which most people seem to call Caterpillar Hunter, but others Fiery Searcher. The species is commonly distributed throughout most of the continental United States and southern Canada. At BugGuide.net his habitat is described as open areas such as fields, gardens, and orchards, but often near deciduous forests, which is precisely the situation of my woods-edge trailer with an orchard before it.

Caterpillar Hunters are important ecologically because they hunt caterpillars; they climb trees feeding on caterpillars, keeping their numbers under control. Adults survive the winter, living up to three years. From eggs laid in soil, larvae emerge, climb into trees and shrubs and also search for prey. When the larvae are ready to pupate they return to the ground and metamorphose in earthen cells.

A close-up of our beetle's pincer-like mouthparts is shown below:

Caterpillar Hunter, CALOSOMA SCRUTATOR, head emphasizing bristles around mouthparts

Something interesting about that picture is the appearance of so many tiny, brown, slender bristles on the inner, cutting-edge side of the pincers. A little Internet sleuthing came up with suggestions as to what purpose the hairs probably serve. Technical works accessed through Google Books describe members of the Caterpillar Hunter genus Calosoma as "strict fluid-feeding entomophagous" species. In other words, the beetle clamps down on a caterpillar, the caterpillar's juices spill out, and the beetle feeds on the juices. Therefore, those bristles surely enable the beetle to assay by smelling/tasting a caterpillar's chemical composition before and during the flowing of the caterpillar's juices into the beetle's intestinal tract. We know that some caterpillars, such as those of Monarch butterflies, are filled with toxins that beetles might not want to deal with.