Being in a Cocoon is Like Getting Drunk, Kinda

Warning: this study includes insect torture. Weird insect torture.

Retention of Memory through Metamorphosis: Can a Moth Remember What It Learned As a Caterpillar.

Author:	Blackiston, Douglas J.; Silva Casey, Elena; Weiss, Martha R.
Publisher:	Public Library of Science

Shockingly adorable. 

If you wanted to figure out whether certain creatures remembered experiences and these creatures did not talk (or really do very much at all), what would you do? Researchers in the Department of Biology at Georgetown decided the best route was to traumatize the hell out of them.

In a study titled “Retention of Memory through Metamorphosis: Can a Moth Remember What It Learned As a Caterpillar?,” they tested the effects of “conditioned odor aversion” by using “classical conditioning” (2) techniques, i.e. buzzing them. The “[l]arvae were subjected to 10 seconds of the odor alone, followed by 10 seconds of odor plus a continuous electrical shock,” (5-6). Then, they waited for them to turn into beautiful, delicate butterflies to see if they were still fucked up from that crazy day when the caterpillar gods frowned upon them.

Turns out, they were still fucked up.

Only those larva, though, in the last instar (read stage of larval development). M. sexta have five instars. If you get zapped earlier on, apparently your brain gets so muched up in the cocoon you forget about it. The cocoon is apparently the inverse of a night of heavy drinking: you forget everything except for what happened just before it.

(Un)surprisingly, this study is not unique. There are tons of studies on the memory of instincts employing sometimes even more bizarre methods. My (least) favorite is  one called “Food-aversion learning in two polyphagous caterpillars.” (Dethier VG (1980) Food-aversion learning in two polyphagous caterpillars, Diacrisia virginica and Estigmene congrua. Physiological Entomology 5: 321–325.) Instead of giving them shocks, they give them toxic food. Even the authors of this study are surprised: stimulus. Interestingly, larval food aversion learning has not been observed in Manduca sexta, nor in several other lepidopteran taxa, despite the extreme negative consequences of ingestion of the noxious or toxic food. Speaking for caterpillars everywhere, you can say that again.

Memorable Quotations: To investigate learning in M. sexta larvae, we used classical conditioning to train caterpillars to avoid the odor of ethyl acetate (EA) by pairing it with a mild electric shock. (2)

We saw neither attraction nor aversion to EA in moths that as larvae were not exposed to EA, were exposed to EA alone, or were shocked in the absence of EA. (2)

[C]ontrol pupae were washed and demonstrated no change in behavior as adults compared to unwashed larvae. (4)

Individual caterpillars received a 16–18v (AC) shock depending on their size and position. (6)

Strange Findings: If olfactory memories are retained across metamorphosis, they are likely to be located in the mushroom bodies (MB), paired structures in the larval and adult insect brain that receive input from the antennal lobes [13–15]. The fate of the MB cells during the transition from larva to adult is poorly understood. (2)

If synaptic connections do indeed persist through metamorphosis, the carryover of larval memory into adulthood might depend on the timeframe of larval experience. In Drosophila, those MB neurons that are pruned prior to pupation form early in larval development, whereas those that persist through metamorphosis are formed later. Thus memory of later larval experience may persist into adulthood, while memory of early experience may not. (2)

For example, washing the pupal cases of Drosophila that had been reared as larvae on menthol-scented diet eliminated an adult attraction to the odor, whereas application of menthol to the pupal cases of larvae naive to that odor resulted in an increased attraction to menthol in the emergent adults. (1)