Species of the week: The money spider

Money spiders (family Linyphiidae) are pretty common, there’s thousands of described species, over 270 in Britain, with a distribution that is pretty much worldwide (they have even been found walking in snow at –7 degrees). While they might not be that rare or glamorous, they do have a particularly amazing way of getting themselves around, namely ‘ballooning’ - travelling by shooting a strand of silk into the air, and then using it to ride the wind.

Picture taken by Olaf Leillinger on 2005-08-07

To get airborne a spider climbs to the top of a platform such as a blade of grass, and then faces the wind. They stand on ‘tiptoes’ point their abdomen to the sky, and then release a stream of silk from its ‘spinneret’. Once the stream catches the wind the spider will be lifted into the air, sometimes traveling over 100s of miles - perhaps one of the reasons for the wide geographic distribution of the Linyphiidae family. David has some nice footage here.

While ballooning is most often observed in young spiders, smaller adults and "immatures" of larger species can also position themselves to travel with the wind using a number of techniques. Adult females have been observed ballooning using rising thermals on hot days, and some spiders use tens or hundreds of silk strands, formed into a triangular sheet about 1m in length, to catch the wind.

One interesting aspect of this behaviour is that it seems to be influenced by the presence in some spiders of endosymbiotic bacteria such as Rickettsia and Wolbachia. These bacteria exist as endosymbionts, and have co-evolved with the spiders resulting in a relationship that benefits both species; infected female spiders have been shown to be more successful in reproduction, for example. Infected females are less likely to display ballooning behaviour, and as communities of spiders don’t become as geographically dispersed as a result, bacteria are able to spread through populations with less difficulty.

Ballooning spiders aren’t the only insects taking the aerial route to travel; recently radar technology has been used to look at the migratory patterns of insects, and found that in any given month in Britain as many as three billion insects pass overhead at a range of altitudes (termites are the highest known ‘flyers’, having been found at an altitude of 19,000 feet!) The insects are even thought to maximise the migratory distances travelled by selecting favourable winds at high-altitudes.