The “True Bugs” evolved mouthparts that are modified into a feeding tube (proboscis) for sucking liquids. The advantages of feeding on liquids are many. Many plants are defended against herbivores by toxic chemicals or structural barriers such as tough waxy leaves or by hairs and thorns. Many insects are defended against predation by sharp spines that physically deter predators and may contain toxic or irritating chemicals. A sucking proboscis that can be inserted into the tissue of a plant or insect can bypass some of the defense.
The sucking proboscis is derived from mouthparts that were originally adapted to chewing food. In the True Bugs, the shapes of all the mouthparts are elongated and interlock to form canals for liquid to travel. The third set of mouthparts (Labium) forms a protective sheath around the stylets. The stylets are formed by the first two sets of mouthparts, the mandibles and maxillae. The labium protects and braces the stylets. As the stylets penetrate the food, the labium folds out of the way. The stylets have separate channels for saliva and digested food. The stink bug pumps saliva containing digestive enzymes into its food. The enzymes digest the cells, releasing the cell content. The nutrients are sucked up and imbibed in the food canal.
The mode of feeding used by True Bugs has been quite successful. There are close to 100,000 species of True Bugs and their relatives.