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Spring 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

geometrid moth caterpillar, or inchworm, hanging by a thread.

April 24. 2012

Inchworm on a thread

Suspended by a barely visible silk filament, a small green caterpillar dangles underneath a box elder tree.

It's a geometer moth larva, otherwise known as an inchworm, named after its distinctive looping and lunging gait. The caterpillar appears to measure its path in units of its own body length.

Geometrid moth larva, or inchworm.

But a climb back up a silk line happens at a much slower, steadier pace, as the inchworm uses only its front three pairs of legs to hoist itself back up to the leaf from where it leapt.

Inchworms don't bungee jump for kicks; they reserve that option for escaping predatory insects. Inchworms have lousy eyesight but are quite adept at detecting vibrations, especially those made by the wasps and predatory stink bugs that hunt them.

Testing inchworm sensitivity, entomologists Ignacio Castellanos and Pedro Barbosa used a machine to imitate various oscillations caused by wind, rain, birds, herbivores, wasps and stink bugs. Bird vibrations caused the caterpillars to remain motionless and try to blend in with the plant. But predatory insect vibrations inspired inchworms to anchor a silk line and bail, using long lines to avoid wasps, shorter lines to escape stink bugs.

Some parasitic wasps, however, are aware of the maneuver. They can locate the silk line and either reel up a desperate caterpillar or slide down to inject eggs inside it; the wasp larvae then slowly consume their host.

Sources: Pedro Barbosa, Department of Entomology, University of Maryland; Animal Behavior; Annals of the Entomological Society of America; Invertebrate Survival Journal; Ecological Entomology