Cicadas and aphids are bugs. The bugs were for years classified in the order Hemiptera, which means half-wings, and from there into two suborders: Homoptera and Heteroptera. In recent years, some taxonomists have elevated the two suborders to the status of full orders. Whether considered to be a suborder or an order, the Homoptera are an important group of insects, both for their numbers, estimated to be over 32,000 species worldwide and for their economic effects on human crops, both by eating them and by spreading plant diseases..
Homoptera means same or uniform wings and winged homopterans are characterised by front wings that are uniformly membranous, with no hardening at the front. This is opposed to the other bugs, (sub)order Heteroptera, where the foreparts of the wings are hard and quite different from the membranous hind parts (examples are assassin bugs and stink bugs). Other characteristics that homopterans and heteropterans share are sucking mouthparts and an incomplete life cycle (egg, nymph, adult with no pupation between). The wings of homopterans are usually folded in a roof-like shape over their backs.
As well as the cicadas and aphids, leafhoppers, spittlebugs, psyllids, soft-scale insects and whiteflies are all homopterans. Gardeners will recognise many of these as common garden pests. This is because all homopterans feed by sucking the sap of plants. In many homopterans, the gut is specialised to remove excess water from the sap and this is excreted as ‘honeydew’, a product that many ants find irresistible. For this reason some ants actually farm aphids like little cows, in order to collect the honeydew.
The largest and most harmless of this order are the cicadas. As nymphs they live underground where they feed on plant materials. Eventually they mature and come out to form part of the summer chorus of insect songs. They may live underground for years but their adult lives are confined to a single summer.
Two bugs produce useful products. The lac insect, which produces natural shellac, is a scale insect. The cochineal insect is a scale insect that live on the prickly pear cactus and used to be the main source of red dye. Both have been replaced by artificial shellacs and dyes, but they were important economically for centuries before modern chemistry.
Aphids and scale insects are major pest species, both by sap sucking and by spreading disease. Millions are spent each year on insecticides to control these pests, so they also contribute to pesticide pollution of the environment. Oils and soaps are used by some gardeners as natural pest control methods. Fungi have been released that attack aphids. In some cases, other plants can be grown around vulnerable crops in order to keep the aphids out. There is still much research to be done in this field.
For more information: http://www.cals.ncsu.edu/course/ent425/compendium/homopt~1.html