A holometabolous insect is one that undergoes complete metamorphosis, i.e. a drastic restructuring of body form and nervous system during the pupal stage. During this metamorphic phase, cascades of programmed cell death, or apoptosis, prune away many of the neurons that were present in the larva. This is particularly true of neurons in sensory systems, as sensory information that was relevant to the larva may be completely different from information that is relevant to the adult insect. In the fly species Drosophila, for example, scientists have tracked the individual survival of mushroom bodies, a type of olfactory neuron. They found that only a small subset of mushroom bodies were carried over from the larva to the adult insect.

And yet in some species, the adult insect seems to be able to remember things that it learned when it was a larva. One study showed that the species Manduca sexta, a type of moth, can remember the meaning of olfactory cues that it learned as a caterpillar. This study gave caterpillars a mild electric shock paired with a novel odor. The odor thus became a conditioned aversive stimulus, a cue that organisms learn to avoid. After several repeated trials, the caterpillars were placed in the bottom arm of a Y-shaped “maze.”

One arm contained the odor that had been paired with the shock and the other arm contained no specific odor. Overwhelmingly, caterpillars chose the arm without odor, staying away from the aversive stimulus. The researchers waited for the caterpillars to metamorphose into moths and gave them the same test. Surprisingly, the results were the same: the majority of moths chose the arm without the aversive odor. This indicates that the moths remembered that the smell had been previously paired with an electric shock, and chose to avoid the smell.

To my knowledge, no studies have yet tracked the development of the nervous system of this species of insect throughout metamorphosis. However, it seems that even throughout the radical changes that the organism undergoes during this phase, enough neurons and neural connections must survive so that some kinds of memories can persist.

Sources

Blackiston, D.J., Silva Casey, E., & Weiss, M.R. (2008). Retention of Memory through Metamorphosis: Can a Moth Remember What It Learned As a Caterpillar? PLoS ONE , 3(3): e1736.

Lee, T., Lee, A., & Luo, L.Q. (1999). Development of the Drosophila mushroom bodies: sequential generation of three distinct types of neurons from a neuroblast. Development, 126: 4065–4076.

Lowe, T., Garwood, R.J., Simonsen, T.J., Bradley, R.S., & Withers, P.J. (2013). Metamorphosis revealed: time-lapse three-dimensional imaging inside a living chrysalis. Journal of the Royal Society Interface, 10(84): 20130304.