By Joe Ballenger
Dermestid beetles are usually decomposers that feed on corpses at the very last stages of decomposition. Many are pests because they damage museum specimens which cannot be replaced. Paradoxically, they are also prized by museums and hunters because they are very efficient at cleaning bits of meat away from bone without discoloring the skeleton. Others, namely ones from the genus Trogoderma, are pests of stored products because they can withstand bad conditions.
Under optimal conditions, Trogoderma glabrum has a lifespan of eight weeks and the immatures progress just like most other insects, from larvae to pupae to adults. However, when large larvae are deprived of food, their life cycle can be extended for longer than two years.
Instead of molting to progressively larger stages, Trogoderma glabrum actually MOLTS BACKWARDS into progressively smaller stages — kind of like the character played by Brad Pitt in the movie The Curious Case of Benjamin Button. This doesn’t stave off starvation indefinitely, and the larvae can starve to death if they go without food or water for extended periods. But if the larvae recover and are then deprived of food again, they’ll begin again to molt backwards as they did before. This can be repeated over and over, and the authors of the papers describing this phenomena (references below) gave up trying to test the limits after two years. It is apparent, however, that this cycle cannot continue indefinitely. As the larvae go through more cycles of regrowth, the time it takes for them to reach their previous size increases progressively.
There are other animals which go through similar cycles, and some, like jellyfish in the genus Turritopsis, are virtually immortal. But Trogoderma glabrum is really interesting because it does not reverse the process of senescence, the biological process of deterioration which eventually leads to death. Some of this, like DNA damage from byproducts of metabolism or unmetabolizable deposits of fat-like chemicals, are not under direct control of the genome. Other effects of aging are regulated in part by genetic elements, namely the FOXO3 transcription factor, among others. These insects, under the right conditions, can turn back the clock on their development. Even though aging-related processes keep happening, they do so at a slower rate, and the beetle can multiply its lifespan by more than five times while molting backwards.
I think the fact that Trogoderma glabrum is not immortal would make it a rather interesting model to study the interaction of processes like aging, senescence, and development. In humans and other insects, when a developmental stage is reached, that stage is fixed and cannot be revisited. Trogoderma glabrum is capable of extending its life when starved, but dies after only eight weeks under optimal conditions. The question of why Trogoderma glabrum is capable of doing this is certainly worthy of further investigation.
There are some interesting things happening with Trogoderma glabrum, but the last investigation into the Benjamin Button-like properties of this insect took place more than 30 years ago.
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Joe Ballenger is an entomologist who specializes in molecular biology and has a thing for wasps. By day, he dreams about having the opportunity to work in pest management research. By night, he loves curling up with a good review paper. Follow him on Twitter at @Stylopidae and at the Biofortified Blog.