Itsy bitsy neurons

Encarsia formosa is a wasp that has been well researched as a biocontrol agent. But there’s something else about it you can’t miss, which is how easy it would be to miss:

It is one tiny little insect. For those of you who are practising biologists, maybe this will provide an even better sense of how small this wasp is:

The whole beast pretty much fits inside the eye of a fruit fly.

Extremes never fail to be fascinating in biology, because they often challenge your notions of what is possible. What do you have to sacrifice to be able to make an animal that small? In a new paper, Hustert argues that this wasp hasn’t given up too much behaviour: it is still able to do the things that other wasps do, like fly, respond to stimuli, groom themselves, walk, and jump.

Has this wasp sacrificed neurons at the cellular level? Are the neurons just smaller, or are there fewer of them? The shape of the nervous system is different than larger insects: everything is clumped together into fewer masses of cells. And there are probably a smaller number of sensory hairs than in other species.

The neurons themselves are also tiny. The cell bodies about 2-3 µm, though they seem to have a fairly normal structure otherwise. Most axons have diameters 0.2 µm across or less. No, that is not a typo. One micrometer is one millionth of a metre, so these are one fifth of one millionth of a metre. The smallest axons are about one twentieth of a millionth of a metre (~0.05 µm).

These tiny neurons raise interesting physiological questions that are asked, but not answered, by this paper. Small neurons conduct action potentials more slowly, so how does the animal cope with having slow communication withing the nervous system?

For that matter, do these neurons conduct action potentials at all? An axon can only get so small before it cannot transmit action potentials: you just can’t fit enough voltage gated sodium channels (needed to start an action potential) on the cell.

Hustert quotes earlier theoretical work predicting that you could not make an axon smaller than 0.1 µm in diameter, because enough sodium channels would be opening spontaneously that you would get random action potentials, and noise is not good in a signalling system. Hustert suggests it’s possible this wasp might be using non-spiking neurons, but this seems unlikely, because in other invertebrates, non-spiking neurons tend to be fat. Alternately, maybe the wasp neurons’ physiology is different in some other way. Hustert says that maybe the neurons fire in bursts, which could increase the reliability of the signalling.

One of the things I like about this paper is not just that it documents the tiny, but asks the reverse question: what has this animal kept relatively large? After all, space is at a premium here, so if you’re going to have an axon with a diameter of over a whole micrometer, where most are 0.2 µm or less, that’s got to be a pretty special cell. Hustert does find a few of these relatively large cells, and suggests that they are responsible for detecting puffs of air, and could trigger escape.

A decent number of neurobiologists record the electrical activity from fruit flies. I have always referred to these people as “masochists.” But we may need a new breed of masoch— er, physiologists willing to try to get into these even smaller insects, because there are some great questions that will never be answered by looking at their anatomy: we’ll need to see the neurons in action.

Reference

Hustert R. 2012. Giant and dwarf axons in a miniature insect, Encarsia formosa (Hymenoptera, Calcididae). Arthropod Structure and Development: in press. DOI: 10.1016/j.asd.2012.08.002

Photo from here.

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