An ant-acacia Acacia collinsii like the one on which the vegetarian jumping spiders were found. Photo taken by Thomas Hesselberg.

The spider belongs to the group of jumping spiders, which are usually active hunters without a web. They have a good vision and use it stalk prey at a distance before they are within jumping distance. However, this particular jumping spider does not hunt any prey, but spend its life on a plant. Not just some plant though, but an ant-acacia, which is a plant that lives in a close mutualism with an ant. The acacia offers the ants housing in its thorns and food in the form of special fat and protein rich Beltian bodies and in return the ants protects the plant against herbivores and other encroaching plants.

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The small male approaching the much larger female of the golden orb spider, Nephila clavipes. Photo: T. Hesselberg.

A new study explains the observed size difference between females and males (sexual dimorphism) in spiders by referring to gravity. Gravity gives rise to an optimal size above where climbing speed is reduced. Since male spiders are often required to climb to access females located in their webs in the vegetation, large spiders should thus have larger sexual dimorphism. The scientists were able to show that this is true for a wide range of different spiders.

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A colony of aphids feeding on a fennel stalk. Wikimedia Commons.

Many animals, especially in aquatic systems, have been shown to react to dead members of their own species by using their presence to assess the predation risk. The more dead individuals they encounter the higher the perceived predation risk. In aphids such a system would also work well since they live in high density colonies on plants and tree, where they feed on the sugar rich liquid. Aphid colonies are often farmed and protected by ants, but nonetheless suffer a very high predation rate from predators such as ladybird beetles, lace wings and parasitic wasps. A team of French scientists from the University of Rennes investigated how aphids respond to death colony members predated by parasitic wasps, which are especially suitable for such studies since the wasps leave the empty corpses intact on the plant.

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Social interaction among hyenas. (Courtesy: Oliver Hoener)

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