Saturday, 6 August 2011

Water skaters

Larval Water Strider, Usa River, Aug 2011 They can move like lightening!
Here's a little guy that doesn't get much attention. Just about every pond or stream has them, but have you ever looked closely at what they're doing? These ones were on a stream and just keeping themselves in the sunniest spot, each time a little leaf or twig floated down steam (often provided by my daughter!) they'd wizz off like lightening to check if it was some small invertebrate to eat. On ponds they're not easily fooled - their feet pick up all the vibrations on the surface of the water from struggling insects and immediately know the difference between a still leave and a moving item of prey, but that's obviously not so easy on a flowing stream.

Water Strider, Usa River, Aug 2011

So, let's try the three things I ask about when interpreting wildlife - what is is? That's easy to a certain level, it's a pond/water skater of some description. These insects (check the body structure and 6 legs - the front two are short and used to capture prey, the others are long and used to propel the animal across the surface of the water) are actually true bugs - order Hemiptera. Now, I think these one are probably family Gerridae (maybe even genus Gerris), the typical pond skater (though they usually have a long thin body), and not family Veliidae, but I'm not sure - unless you're an expert the only certain way to ID them seems to involve dissection of the genitalia. Nice (what is it with entomologists and dissecting genitalia?). Still, pond skater or water strider is probably enough for most people anyway, and knowing it's a true bug might be a little bit interesting too. Look at the picture below, however, and you'll see something more - there are no wings, yet true bugs (usually) have wings? In fact, this tells us we're looking at one of the larval forms - just as juvenile grasshoppers don't have functional wings, nor do juvenile Gerrids, which suggests we know something about the age of these little guys too. But even more interestingly, this is a group where in some species developing animals in low population densities never develop wings, but when growing up in higher density populations, the adults do grow wings - a phenomenon known as phenotypic plasticity, when the same genes under different envorinmental conditions (high or low population density) result in different body plans.
Larval water strider, Usa River, Aug 2011

What's it doing? Well, obviously, it's looking for food, but rater remarkably it's doing that by walking on water! How? Well, you probably remember being told that water has a high surface tension - water-molecules rather like to cling to one another. If you're small, that means that it's as though the water has a skin, and by having very long legs that spread it's weight out neatly over the water,  the bug can exploit that skin to stand on the surface aided by one further adaptation - tiny hairs on the legs that are hydrophobic - that repel water. These three things allow the critters to take up their life on the water surface.

And what impact does it have on the ecology of the world it lives in? Well, here I'm not to sure, whilst there's quite a lot fo research into the group, not much has focussed on it's ecology or role inthe ecosystem. Certainly they get eaten by lots of things and eat lots of others (they're also very successful as a group, with over 1700 species), but no-one seems to have speculated aout how the world - or even the pond - would be different without these creatures. There's a project for someone...

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