Stylet: Any sharp, piercing, needle-like organ such as those that form the mouth parts of bugs, fleas, and mosquitoes.*
Mealy bugs, scale insects, and aphids are examples of pests which use their stylets to suck juices out of plants. In the picture above, a mealy bug is starting in on an orchid leaf.
*O’Toole, C. (ed.). (2002). Firefly encyclopedia of insects and spiders. Toronto: Firefly Books.