Aphids
Aphids are insects that feed off of plants making them a hazard to farmers and forests. Aphids use stylets – sucking mouthparts – to eat with. They have long and thin legs, are two jointed, a two-clawed tarsi, and usually a pair of cornicles, abdominal tubes through which a waxy secretion is exuded.
Aphids have two compound eyes and two ocular tubercles made up of three lenses, each of which is located behind and above the compound eyes. When host plant quality becomes poor or is crowded, female aphids will produce winged offspring that can disperse to other food sources.
Aphids feed on sap of phloem vessels in plants. This sap being kept under high pressure, once a phloem vessel is punctured, it is forced into the food canal. As they feed, aphids often transmit plant viruses to their food plants. These viruses can sometimes kill the plants.
Some species of ants “farm” aphids, protecting them on the plant they eat, and eating the honeydew that the aphids secrete; this is a mutualistic relationship. Aphid honeydew is rich in carbohydrates, of which the aphids ingest an excess, being phloem-feeders. Many aphids are host to endosymbiont bacteria, Buchnera, which live in specialized cells called bacteriocytes inside the aphid. These bacteria synthesize some essential amino acids that are absent in the phloem that the aphids eat.