garlick’s notebook / By Don Garlick

Pupal Memories

(July 10, 2008)  I remember watching a chick emerge from an egg. It was miraculous. I have also seen a mosquito dragging itself from its pupa, standing on water for a moment to inflate its wings, and then flying away in search of blood. It was equally awesome. I mention the mosquito to quell the enthusiasm of those who think that miracles created all life.

I herein explore insect metamorphosis in more detail, inspired by experiments regarding learning and memory. Insects mature though stages, called instars, separated by molts. Some insects undergo a spectacular transformation from larva to adult imago, while encased in a pupa. Such holometabolic insects include beetles, bees, butterflies, moths, flies, fleas and mosquitoes. Examples of insects that mature more gradually are grasshoppers, dragonflies, cicadas and aphids.

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To quote the New World Encyclopedia: “In the pupal stage, the insect excretes digestive juices to destroy much of the larva’s body, leaving a few cells intact, while groups of cells, called imaginal disks, develop into tissues of the adult, using the nutrients from the broken down larva.” I was astonished, therefore, to learn of the following experiments by Douglas Blackiston, Elena Casey and Martha Weiss (Dev. Bio. July 2006 and Plos.org March 2008):

“During metamorphosis, insects undergo a drastic re-organization of the body; including widespread cell death and the proliferation of new tissue. The present study examined the ability of memory to survive metamorphosis, from the larval to adult stage, in Tobacco Hornworm Manduca sexta. Fourth instar larvae were electro-shocked in the presence of ethyl acetate to generate an odor avoidance behavior. Larvae were assayed for learning in a Y-choice chamber, and were additionally assayed as adults to assess if memory persisted through metamorphosis. Larvae demonstrated strong aversive learning, which was retained in subsequent adults. These are the first studies in Lepidoptera demonstrating memory through metamorphosis.”

So, we gain an appreciation for the learning ability of caterpillars and are astonished by the persistence of memory through a drastic redesign and restructuring of their bodies and lifestyles. The brain must surely retain some integrity during the process. Go find a Tomato Hornworm in your garden, teach it a trick like bright-light avoidance, guard it through the three weeks of metamorphosis, watch in amazement as it bursts from its pupa, and then test whether your moth resists its inherent tendency to spiral into bright lights.

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