[John Chapman (OP)]

A monster's moved itself into my house. And I've not even shagged 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 foot 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 it’s 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 it would probably mean instant death even if it is native.

Any entomologists out there? Has anyone got any idea what it is? What can I do with it?

Monster, Naked Sci 1.JPG (49.19 kB . 403x293 - viewed 12835 times)
 Monster posing 2 weeks ago.


Monster, Naked Sci 3.JPG (43.31 kB . 392x294 - viewed 11249 times)
 Monster posing tonight.


Monster, Naked Sci 2.JPG (49.31 kB . 392x294 - viewed 11458 times)
 Monster's home.
 

[RD]

I can't recall seeing aquatic beasties posted on "what's that bug" but may be worth a try ... http://www.whatsthatbug.com/

A fishtankologist would be a better bet ... http://forums.tfhmagazine.com/

[Geezer]

Could it be a dragonfly larvae? I think they come in a lot of different shapes and sizes.

[RD]

Monster, Naked Sci 3.JPG (43.31 kB . 392x294 - viewed 11249 times)

I see you've got one of those newfangled 3D TVs   []

[Geezer]

Here you go.

http://www.google.com/images?hl=en&q=dragonfly+larvae+pictures&wrapid=tlif12909218456741&um=1&ie=UTF-8&source=univ&ei=jufxTNPkDJH6sAP_o9DqCw&sa=X&oi=image_result_group&ct=title&resnum=1&ved=0CCAQsAQwAA&biw=1491&bih=736

The front end looks a lot a lot like that of a dragonfly.

[RD]

bug in fishtank.jpg (54.8 kB . 597x304 - viewed 9423 times)

These look like a match ...


lowc.jpg (45.15 kB . 800x542 - viewed 9338 times)
http://integraonline.com/~pondhawk//odonata/ips_odonata.html

but the caption suggests other species look similar, i.e. your "monster" is not necessarily dragonfly. 

[Bored chemist]

Glad the mystery is probably solved. I just thought I'd mention to RD that John's 3d tv looks more interesting than most programs I have seen advertised.

[John Chapman (OP)]

Thanks RD and Geezer

I reacon you've cracked it. Some of those pictures look almost identical. I showed Monster a picture of a dragonfly and it got all excited and shouted "Mummy! Mummy!". So there you go.

The problem now is what to do with it. Dragonfly larvae are voracious. I think I'd better get some extra fish!

I've heard that dragonflys spend the vast majority of their lives in their larval stage and only a month or two as adults. I've also heard that British dragonfly larvae can over-winter for one or two years depending on species. I wonder if I can keep it until next spring and then release it. It seems to be growing very fast in my heated aquarium.
 

[John Chapman (OP)]

I have also sent pictures to the British Amateur Entomologist Society, the British Museum and the Invertibrate Section of Chester Zoo. I wonder if anybody can pin down the exact species without dissecting it. I'll keep you posted about any of their replies.
 

[Don_1]

Its either a dragonfly or damselfly larvae. It may become more obvious as it grows. The dragonfly larvae will get much bigger than that of the damselfly and can take up to two years to reach maturity, while the damselfly will mature in around a year.

Both are voracious predators, the dragonfly larvae especially so, and will eat just about anything not bigger than themselves. They will shed their skins a number of times during their growth.

The question is, how did it get into your fish tank? The two adults will usually couple on the stem of a plant protruding from the water. The eggs are laid and fertilised, just below the water line, there and then. They will fly, still coupled, to several such sites to lay and fertilise the eggs. Do you put pond plants in your aquarium? Alternately, it might be possible that a larvae was in with some blood worms you may have used for fish food. These are also prey to the larvae.