The gut is modified so that the excess water and sugar can quickly pass from foregut to hindgut then rectum, bypassing the midgut (see right which shows the sap feeding hemipteran digestive tract). The midgut is where the nitrogen and the amino acids are absorbed. This means that the excreted liquid is very sweet, and it is sometimes called honeydew. It tastes quite nice, but soon goes mouldy as you find out if you park your car under an aphid-infested tree.
Some aphids can excrete as many as seven droplets of this sugar-rich liquid an hour - that can be as much as 133% of the insect's weight! And some hemipterans consume more than 100 times their body weight per day.
Sometimes the honeydew is in quantities large enough to be used by man. In the Old Testament the manna given to the Israelites was probably anal excretions of Trabutina mannipara, which feeds on the tamarisk. The Arabs still collect it today, and call it "man". The Australian aborigines also collect honeydew.
Cornicles. The cornicles or siphunculi (see the drawing below) release an alarm pheromone, and secrete a waxy liquid that deters predators. They do not secrete honeydew as stated in some older books. The alarm pheromone allows the aphids to communicate to others in their group.
Ants farm aphids
The photograph on the left shows ants guarding aphids while the aphids suck sap from a rose bush. The aphids get too much sugar from the sap and excrete it (this liquid is called honeydew) and the ants lap it up, drinking some themselves, and taking the rest home for their nestmates and grubs.
The tip of every leaf was guarded by at least one ant, and other ants patrolled over the aphids, and up and down the stems. Any other insect, or even a camera strap would be nipped and squirted with formic acid. So the aphids are free to suck in peace and the ant gets a sugary reward. And the rose bush? Well it was quite a big bush and a short time later it had many fragrant blooms.
When there are no ants around to collect the honeydew the aphids flick or squirt the droplets away.
On the right is a drawing showing the position of the stylets as it enters the plant cell wall to suck sap.
Lizards, tits, sparrows, swallows and swifts.
Solitary wasps, sawfly larva (each one eats 200 - 1000 in its lifetime), lacewing larva, Anthocoridae (hemiptera), snake flies, ladybirds (adults eat about 20 a day, larva will eat around 200 - 500 during larval period), parasitic wasps, scorpion flies, social wasps.