Within the Hemiptera, the Sternorrhyncha form the largest and most complex group. Within the Sternorrhyncha, the same applies for the Aphidomorpha, the aphids and relatives. Aphidomorpha have an exceedingly long proboscis (that may be retracted within the when not in use).
Many Aphidomorpha species demonstrate hostplant alternation. A fertilised female then deposits her eggs on plant species A; the progeny develops there but, once full grown migrates to plant species B, where a number of parthenogenetic generations follow. Against the end of the season along with females also males are formed, and the cycle restarts. Plant species A, the primary hostplant, generally is a tree or shrub, B, the secondary one, a herb.
Also in the Adelgidae, a small aphid family, host alternation takes place, but both primary and secondary host plant are conifers (Pinaceae). In the parthenogenetic phase no living young are born, like in the true aphids, but unfertilised eggs. Galls are produced on the primary host plant, which invariably is a Picea species. Contrary to true aphids, Aphididae, Adelgidae do not possess siphunculi. but they do have an ovipositor.
Blackman & Eastop (2014), Börner & Heinze (1957a), Carter (1971a), Havill & Foottit (2007a), Richards & Davies (1977a).