Several groups of ants have independently evolved the habit of keeping domestic ‘dairy’ animals in the form of aphids. Unlike other symbiotic insects that live inside ants’ nests and don’t benefit the ants, the aphids are pastured out in the open, sucking sap from plants as they normally do. As with mammalian cattle, aphids have a high throughput of food, taking only a small amount of nutriment from each morsel. The residue that emerges from the rear end of an aphid is sugar-water—‘honeydew’—only slightly less nutritious than the plant sap that goes in at the front.
