Defences
Aphids are soft-bodied, and have little protection from predators and diseases. Some species of aphid interact with plant tissues forming a gall, an abnormal swelling of plant tissue. Aphids can live inside the gall, which provides protection from predators and the elements. A number of galling aphid species are known to produce specialised "soldier" forms, sterile nymphs with defensive features which defend the gall from invasion. For example, Alexander's horned aphids are a type of soldier aphid that has a hard exoskeleton and pincer-like mouthparts. Infestation of a variety of Chinese trees by Chinese sumac aphids (Melaphis chinensis) can create a "Chinese gall" which is valued as a commercial product. As "Galla Chinensis", Chinese galls are used in Chinese medicine to treat coughs, diarrhoea, night sweats, dysentery and to stop intestinal and uterine bleeding. Chinese galls are also an important source of tannins.
Some species of aphid, known as "woolly aphids" (Eriosomatinae), excrete a "fluffy wax coating" for protection.
The cabbage aphid, Brevicoryne brassicae, stores and releases chemicals that produce a violent chemical reaction and strong mustard oil smell to repel predators.
It was common at one time to suggest that the cornicles were the source of the honeydew, and this was even included in the Shorter Oxford English Dictionary and the 2008 edition of the World Book Encyclopedia. In fact, honeydew secretions are produced from the anus of the aphid, while cornicles mostly produce defensive chemicals such as waxes. There also is evidence of cornicle wax attracting aphid predators in some cases. Aphids are also known to defend themselves from attack by parasitoid wasps by kicking.
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