Last spring I picked up a bag of compost from the side of the shed and found a couple of New Zealand flatworms underneath. Yikes! These horrors are invasive species, and they eat earthworms so are not very welcome. They are about 5-10 cm long when stretched out, although they are normally found curled up like a Cumberland sausage as in the first picture. In the second image they are uncurled (5p for scale) and moved into the light for a better picture. You can see the white lump where the largest worm would have been about to lay an egg had it not met an untimely end!
I started reading up on the subject and found out they were first spotted in the UK in the 1960s in Belfast and Glasgow and have been steadily marching south ever since. University College London OPAL project has a map to show where flatworms have been found, but this basically overlays a population map of the UK – their invasion seems to be fairly comprehensive.
Since then I have had occasional sightings (and squashings) of flatworms in the garden, including the three pictured above. Oddly, I only ever find them in one particular place down the side of the shed. This is a storage area of gravel and clinker rather than soil. I have sometime put bought plants there to rest in the shade when I haven’t had time to immediately plant them out so the visitors maybe hitchhiked into the garden that way. I never see flatworms anywhere else in the garden, even when I have placed traps by leaving black plastic bag weighted down on the soil and checking underneath.
The shed is surrounded by paving, which may confine the flatworms to that small area. In any case, the earthworm population in the garden appears healthy and the number of flatworms isn’t increasing so I shall keep up a policy of search and squash and hope for the best!