JOHN CHAPMAN: Monster in my Aquarium (From the Bug Club email list)

I have a monster living in my house. And I've not even dated it. I have a small indoor heated aquarium, which fits within a picture frame and hangs on the wall of my living room. The tank is about 2.5 feet long by about 18 inches high by about 2 or 3 inches deep, and is fully enclosed in a wooden frame. It contains guppies and a lot of weed and snails. A couple of weeks ago I saw what I thought was a spider drowning, thrashing/swimming in the tank.

I tried to fish it out but it buried itself in the gravel and I lost it. A few hours later I was amazed to see it sitting in the weed. Upon closer examination I could see it has six legs, clearly has a head, abdomen and thorax, has a laterally flattened body and its abdomen was pulsating, presumably to facilitate breathing. It was then about 10mm long but is now about 15. No fish seem to be disappearing and I have a number of baby guppies, which are smaller than it is.

But I suspect it is eating snails. I assume it is an insect larva and was probably introduced on the weed, possibly as an egg. But since it is a tropical tank I have no idea whether the weed is of native or foreign origin. Obviously it would be irresponsible to release a non-native insect species into the wild – although at this time of year (end of Nov) it would probably mean instant death even if it is native. I live in North Wales, UK. Has anyone got any idea what it is? What can I do with it?