Geometrid Moth (Larva)
Sitting motionless on a dead twig, Camp Creek Maricopa Co., Arizona. 14 May 2017.
Geometridae -- Inch-worm Moth Family
When I first looked I saw nothing among the tangle of twigs. Suddenly a rapid, arching twitch made by the subject insect larva caught my attention. Normally the highly cryptic immature stages of moths remain perfectly motionless during daylight so as not to be noticed by a hungry bird. But some predators of caterpillars use chemicle sensing (smell) and the rapid twitch I saw was a defense against just such a threat. It was a small parasitic wasp (Ichneumonidae) and was about to alight upon the caterpillar and insert a deadly egg of its own into the body. In a lightning fast move the forelegs and mandibles nearly caught the wasp before it could do harm. The parasite flew off and did not attempt another attack lest the tables be turned upon the parasite!