Interestingly, it has now been shown that the saliva of the aphids has an analogue to the anti-coagulant properties of blood suckers, subverting the wound repair mechanism of the plant.

ants tending mealybugs” width=”218″ height=”218″>Plant sap is an important food source, being rich in sugars. It suffers, however, the disadvantage of being very poor in nitrogen, especially in the form of amino acids. It is exploited by various animals, such as the sap-suckers (respectively in the wood-peckers and mammals), but the extraction of sap (and sometimes the much more dilute solution from the xylem) is taken to a high art by the insects, especially the hemipterans where it has evolved multiple times. The key structure is the stylet, and this has occasionally been turned to other uses, notably for blood extraction in the bed-bugs.

Hemipteran sap-suckers

A variety of plants are attacked, and their deprivation is best known from such hemipterans as the aphids, planthoppers (including the cicadas) and mealybugs. All are serious pests to agriculture and the gardener. The stylet can follow a convoluted path before it finds a suitable “vein”, the sieve-tubes of the plant that carry the sap. Interestingly, it has now been shown that the saliva of the aphids has an analogue to the anti-coagulant properties of blood suckers and in what has been appropriately called “molecular sabotage” subverts the wound repair mechanism of the plant. In addition, the saliva of the aphid is remarkable because it comes in two forms: one is the usual watery variety but the other is jelling, and quickly hardens to form a sheath that presumably helps to prevent leakage.

The almost pure sugar diet, as noted, provides nutritional challenges, and significantly the hemipterans have repeatedly recruited endosymbiotic bacteria e.g. Buchnera to provide missing nutrients. An important convergence in these bacteria is genome reduction, and the sugar diet used by hemipterans recalls the challenges facing the hummingbirdoid moths.

Honey-dew and mutualisms

The hemipterans produce prodigious quantities of honey-dew, and this is an important food source, especially for the ants. This mutualism has evolved many times independently, and in some cases is obligate and can be effectively described as nomadic shepherding because the ants move the hemipterans from one “pasture” to another. Other remarkable examples of this mutualism include the storage and care (including the application of anti-fungicides) of aphid eggs by the ants in their nest. Not all is sweetness and light, however, because there is also evidence that some species of ant will not only exploit the honey-dew but the aphids themselves, perhaps to achieve what might be called a “balanced diet”.

Remarkably the exploitation of honeydew has also evolved independently in the geckos, a group of lizards. The example comes from Madagascar and involves the gecko approaching the plant-hopper with great caution, bouncing its head against the bark which in sending vibrational signals induces the plant-hopper to raise its abdomen and place a drop of honey-dew on its snout, which is then licked off.