Awaydays with slime

I've just returned from a few days in Wales. Cader Idris, sharp against a cloudless sky, hills tawny with bracken and woods carpeted with bright beech leaves and lit by the vivid greens of moss and ferns: all signs of the changing season. And there were smaller signs too, needing sharp eyes and a bit of luck to detect.

In the middle of a field we spotted a handsome caterpillar. It was about 3cm long, covered in a fuzz of lemon yellow bristles and sporting a tuft of stiffer red bristles at its hind end. As it undulated along, black stripes between its segments appeared and disappeared in a rippling effect, no doubt disconcerting to would-be predators. It was the caterpillar of the Pale Tussock moth, which lives and feeds in the foliage of oaks, birches or other deciduous trees. A caterpillar is a simple bag-shaped organism, designed for a life of eating and growing, so whatever was it doing clambering through the sheep-bitten turf at least 50 yards from the nearest tree?

Many moth caterpillars pupate in the soil. For some this merely involves climbing down from the canopy and forming a chrysalis in the ground below its tree. Not so the Pale Tussock. It wanders off, presumably to find just the right spot. It must be a matter of some importance to take our lemon-bristled specimen on such an arduous and, indeed, perilous journey. We left it to continue its quest in peace.

Poking about in nearby woodland, we made our next discovery. Among the leaf litter was a trail of a vivid yellow substance. It looked as if someone had dumped a spoonful of congealing egg yolk there, leaving a dribble to one side. When we returned three hours later the whole thing had moved, cohering together to form one mass around a single stalk of grass and, through a magnifying glass, we could see its texture was becoming grainy. Was it a fungus? Well, no - fungi don't move. In fact it was one of the many kinds of plasmodial slime mould to be found in the British countryside. It was once classified as a fungus, but is now in a kingdom of its own, a strange organism which is neither fully animal nor vegetable. Our specimen would have started life as a number of single-celled individuals, moving and feeding rather like amoeba. As these mature they congregate together and the amalgamated mass slithers around over the leaves and grass, feeding and growing. When fully ready, it firms up and releases spores and the cycle begins again.
Plasmodial slime moulds come in varying colours and sizes (ours was a tiny one). I've seen one which looked like lumpy milk pudding and another like lentils. So, it's worth keeping your eyes open at this time of year. You never know what might be creeping over the grass or through the woods!

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