Aphids feed by inserting their rostrum-borne stylets into a plant and ratcheting them between plant cells, seldom penetrating any until they enter the phloem sieve tubes and extract sap. Stylet advancement is lubricated by saliva containing a pectinase that loosens the bonding between plant cells. The saliva forms a stylet sheath that is left in the plant when the stylets are withdrawn. To cope with a sap diet, aphid guts have specialized groups of cells, mycetomes, containing rickettsia-like symbiotic bacteria, mycetocytes, which aid in synthesis of nutrients. These bacteria, which are passed from mother to embryonic daughter, have coevolved with aphid lineages, differing among them.
Whereas aphids largely rely on a high reproductive rate and great dispersive ability to maximize survival in a hostile environment, morph-specific behaviors exist to promote genetic survival of the individual or its clone. Behavior of alatae optimizes dispersion and finding a successful host. When alatae initially take to flight, they are attracted to the short wavelengths of light that predominate in a clear or cloudy sky, and fly up toward them. After flying a while, however, they come to prefer the longer light wavelengths reflected from plants, and they descend, moving to them. In some species, alatae have shown a preference for colors characteristic of their host plant's leaves. Generally alatae are attracted to yellow, a predominant hue in growing or senescent plants, which are better nitrogen sources. Upon alighting on a plant, they briefly probe below the epidermis with the rostrum to locate specialized secondary plant compounds that are of no nutritional value but are specific to the aphids' given host. If these feeding triggers are not found, the alatae move on.
In contrast, apterae usually move only when necessary to procure a better feeding site or if a predator or parasite molests them. Ants tend apterae in many aphid groups in a form of facultative mutualism; in some relationships, ants actively "farm" their aphid "cattle" by moving them among locations. Generally, however, aphid groups with elongate siphunculi are less likely to be tended by ants. In exchange for the aphid's sugary honeydew waste, the ants protect them from predators, such as coccinellid, lacewing, and syrphid fly larvae, or specialized aphid parasites, such as chalcidoid and braconid wasps. When stroked by the ant's antennae, the aphid will raise the tip of its abdomen, extruding a honeydew drop, which may be retracted if not accepted by the ant. If an ant does not accept honeydew after a while, the aphid will revert to its normal behavior of flicking the honeydew drop away with its hind leg or cauda, to prevent an accumulation of honeydew from fouling the aphid colony.
Aphids communicate by chemicals and sound. Parasites and predators are often foiled by the use of an aphid alarm pheromone, such as trans-P-farnesene. When molested, aphids exude microdroplets of alarm pheromone from their siphuncular pores, and in response adjacent aphids quickly withdraw their stylets from their host and drop to the ground. Shortly thereafter, the fallen aphids visually orient to vertical lines or structures and move toward them in an attempt to climb the plant stem. Aphid sexual pheromones are also used as male attractants by oviparae, being released from specialized pores on their hind tibiae. Sound communication is used by Toxoptera spp., which have a stridulatory mechanism consisting of a row of short pegs on the legs, which are rubbed against filelike ridges on the lower epidermis of the abdomen, just below the siphunculi. When disturbed, colonies of T. aurantii emit an audible high piercing stridulatory sound, to which their apterae respond.
Fundatrices of gall-forming species use species-specific patterns of feeding or probing behavior to induce characteristically shaped galls on their specialized hosts, in which their progeny can safely develop. The fundatrix of P. bursarius climbs the developing leaf petiole on Populus nigra about halfway and probes its rostrum around the petiole to create an array of punctures oriented perpendicularly to the petiole shaft. This induces a swollen globular gall with a slit oriented perpendicularly to the petiole shaft. In contrast, on the same host, a Pemphigus spyrothecae fundatrix probes the petiole shaft in an upward spiral array of punctures, yielding a corkscrew-shaped petiole gall. Not only do plant galls provide a protective encasement for aphid development, but aphids of even nongalling species do better on galled tissue, probably because of a local increase in plant nutrients in that tissue.
Many aphid species have some lower degree of sociality, especially among apterae, which is expressed as a gregarious-ness within colonies and probably confers better protection or response to attacks by natural enemies. Alatae of Drepanosiphum platanoides, sycamore aphid, are more likely to be distributed in a clustered manner among sycamore leaves, in groups in which the tips of their antennae and legs touch among the aphids. Some aphids have evolved a higher degree of sociality, however. The tribe Cerataphidini of the Hormaphidinae has genera in which species produce a soldier morph with enlarged forelegs, which defend their relative clones differentially. Soldiers discriminate between soldiers and nonsoldiers but do not attack soldiers of their own species. The investment in soldier production by the colony is related to areas needing defense, such as a gall's surface.
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