Such formations are fairly common. Gerrids (pond skaters) have the sucking mouthparts typical of true bugs. They generally feed on insects drifting on the water surface; usually their prey are injured or non-swimmers. The pond skaters in the photograph are feeding on a partly submerged insect, perhaps a fly.

Gerrids pierce their victim, inject digestive juices and suck it dry. They can read surface waves with exquisite sensitivity: a fly buzzing helplessly in the water acts as a magnet for every nearby opportunist. In nature, the lightly built gerrids have to move fast because other creatures - whirligig beetles, for example - prey on similar victims and are faster and more robust.

When food is in short supply, any tempting prize attracts every gerrid within call. Often there is a scrum, and the prey soon has more perforations than a pincushion. When more than one attacks the same food item, the diners generally form a ring because gerrids are not averse to cannibalism, so it does not do to let one's fellows climb on top.

Jon Richfield, Somerset West, South Africa

Pond skaters or waterstriders are predators and scavengers. They eat mainly terrestrial insects and spiders dropped and trapped at the water surface. Pond skaters cling to their prey with their short front legs and inject digestive fluids into it via their proboscis before sucking out the resulting soup of body fluids.

Typically a feeding bout lasts about half an hour. If the prey is large enough, it can attract several pond skaters to eat at the same time. I suspect that in the centre of the star-like formation of pond skaters is a big fat fly.

Matti Nummelin, Department of Biological and Environmental Sciences, University of Helsinki, Finland

Thanks to Tim Gossling and Dave Challender for sending in photos of similar events. You can see Dave's photo, complete with a wasp in the middle, on Flickr via bit.ly/1sOGpw - Ed

To discover what an animal is doing, we must first look at what they are designed for by Darwin, I mean evolution.

Pond Skaters, as their name suggests, skate over ponds, lakes and other still bodies of fresh water. They are true bugs, as opposed to say, fruit flies, which means they can suck up food through a long straw-like mouthpiece called a probosicis. They race over the water, capturing little insects or other arthropods, and then eat them. To stay on the surface of the water, they utilise surface tension. Surface tension is the force that is made by the thin "film" of water on the top of a lake or pond, or even a glass. Basically, the pond skater is so light, it can rest on the unbelieveably thin surface of the water caused by surface tension, thus creating a "floor" that they can walk over. The pond skaters are most likely huddling together to create more stability, prehaps due to a rough water suface, and the star shape prevents them breaking the surface tension.

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