Thrips are tiny insects, typically just a millimetre in length. Some are barely half that size. If that’s how big the adults are, imagine how small a thrips’ egg must be. Now, consider that there are insects that lay their eggs inside the egg of a thrips.
That’s one of them in the image above – the wasp, Megaphragma mymaripenne. It’s pictured next to a Paramecium and an amoeba at the same scale. Even though both these creatures are made up of a single cell, the wasp – complete with eyes, brain, wings, muscles, guts and genitals – is actually smaller. At just 200 micrometres (a fifth of a millimetre), this wasp is the third smallest insect alive* and a miracle of miniaturisation.
The wasp has several adaptations for life at such a small scale. But the most impressive one of all has just been discovered by Alexey Polilov from Lomonosov Moscow State University, who has spent many years studying the world’s tiniest insects.
Polilov found that M.mymaripenne has one of the smallest nervous systems of any insect, consisting of just 7,400 neurons. For comparison, the common housefly has 340,000 and the honeybee has 850,000. And yet, with a hundred times fewer neurons, the wasp can fly, search for food, and find the right places to lay its eggs.
On top of that Polilov found that over 95 per cent of the wasps’s neurons don’t have a nucleus. The nucleus is the command centre of a cell, the structure that sits in the middle and hoards a precious cache of DNA. Without it, the neurons shouldn’t be able to replenish their vital supply of proteins. They shouldn’t work. Until now, intact neurons without a nucleus have never been described in the wild.
And yet, M.mymaripenne has thousands of them. As it changes from a larva into an adult, it destroys the majority or its neural nuclei until just a few hundred are left. The rest burst apart, saving space inside the adult’s crowded head. But the wasp doesn’t seem to suffer for this loss. As an adult, it lives for around five days, which is actually longer than many other bigger wasps. As Zen Faulkes writes, “It’s possible that the adult life span is short enough that the nucleus can make all the proteins the neuron needs to function for five days during the pupal stage.”
As they get smaller, insects can do away with many of their organs. The feather-winged beetles – twice as big as the M.mymaripenne, but still impressively tiny – have drastically reduced the size of their genitals, guts and breathing tubes. They have totally lost their hearts: at their size, diffusion is enough to carry liquids around their body without the need for a pump. Their wings, like those of thrips and fairy wasps, are little more than wispy strands, rather than the flat oars of most other insects. That’s all they need to paddle through thick air currents.
But the nervous system is trickier to shrink. There’s a lower limit to how tiny neurons can be, and many of them have to be clustered in a chunky brain. This is one of the main things that prevent insects from becoming even smaller. Many insects have solved this problem by partitioning the brain into chest or abdomen, but wasps can’t do that. They only have a very thin connection between their heads and the rest of their bodies. No brain-shifting for them; they have to rely on more extreme adaptations, like paring down their number of neurons, and getting rid of their nuclei.
* The world’s second smallest insect is a close relative of M.mymaripenne called Megaphragma caribea, slightly smaller at 170 micrometres. The record holder is yet another wasp – Dicopomorpha echmepterygis. The males, blind and wingless, are just 130 micrometres long. The females are slightly bigger than M.caribea.
Update: It's been pointed out to be that "fairy wasps" are members of the family Mymaridae, whereas Megaphragma belongs to the separate family Trichogrammatidae, members of which do not have a common name in English. Curses.
Reference: Polilov. 2011. The smallest insects evolve anucleate neurons. Arthropod Structure and Development http://dx.doi.org/10.1016/j.asd.2011.09.001
Hat tip to David Gregory, for pointing me to the paper
