Behavior And Ecology Feeding and Digestion

Adult and nymphal Auchenorrhyncha feed by inserting the two pairs of feeding stylets (modified mandibles and maxillae) into the host plant tissue, injecting saliva, and ingesting fluid. Unlike Sternorrhyncha, in which the stylets pass between the cells of the host tissue (intercellular feeding), Auchenorrhyncha stylets usually pierce the cells (intracellular feeding). After selecting an appropriate feeding site based on visual and chemical cues, the insect presses the tip of the labium onto the plant surface and inserts the feeding stylets. Just prior to, and during probing of the plant tissue with the stylets, the insect secretes sheath saliva that hardens on contact with air or fluid to form an impervious salivary sheath surrounding the stylets. The sheath forms an airtight seal that prevents leakage of air or fluid during feeding. Stylet probing continues until a suitable tissue is found (xylem, phloem, or mesophyll, depending on the species), after which feeding can commence. During feeding, watery saliva is injected into the plant to aid digestion and to prevent clogging of the stylet opening. This is also the mechanism by which the insect may infect the plant with pathogens (see later). Feeding may last from a few seconds to many hours at a time, depending on the auchenorrhynchan species and the quality of the plant tissue. During feeding, droplets of liquid excretion are ejected from the anus, several droplets per second in some xylem feeders.

Plant sap is a nutritionally imbalanced food source; phloem is high in sugar and xylem is, in general, nutrient poor and extremely dilute. Auchenorrhyncha have acquired various adaptations that enable them to convert the contents of plant sap into usable nutrients. Most Cicadomorpha have part of the midgut modified into a filter chamber that facilitates rapid removal of excess water. Fulgoroidea lack a distinct filter chamber but have the midgut tightly coiled and partially or completely enclosed in a sheath of specialized cells that apparently absorb solutes from the gut contents. A broad array of transovarially transmitted (i.e., from the mother through her eggs to her offspring) prokaryotic endosymbionts have also been identified in various Auchenorrhyncha species. The roles of these endosymbionts have not been fully elucidated, but presumably they function in the conversion of the nutritionally poor plant sap on which the insects feed into essential vitamins, amino acids, and sterols. The sym-bionts are housed either intracellularly in specialized fat body cells called mycetocytes, intracellularly in the fat body, or in the gut epithelium. Several distinct mycetomes, consisting of groups of mycetocytes, are often present. In Cicadomorpha, each mycetome may house up to six different kinds of endo-symbiont. In Fulgoroidea, only a single kind of endosymbiont is housed in each mycetome.

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