Hemiptera plant. Phloem sap, which has a higher concentration of sugars and nitrogen, is under positive pressure unlike the more dilute xylem sap. Most of the Sternorrhyncha and a number of Auchenorrhynchan groups feed on phloem. Phloem feeding is common in the Fulgoromorpha, most Cicadellidae and in the Heteroptera. The Typhlocybine Cicadellids specialize in feeding on non-vascular mesophyll tissue of leaves, which is more nutritious than the leaf epidermis. Most Heteroptera also feed on mesophyll tissue where they are more likely to encounter defensive secondary plant metabolites which often leads to the evolution of host specificity. Obligate xylem feeding is a
Tonlé Sap the Tonle Sap River and surrounding wetlands, waters of the lake's basin drains via the Tonle Sap River into the Mekong. As a result the Tonle Sap River (length around 115 km/71 mi) flows six months a year southeast (Mekong) to northwest (lake) and six months a year in the opposite direction. The mean annual reverse flow volume in the Tonle Sap is , or about half of the maximum lake volume. A further 10 percent is estimated to enter the system by overland flow from the Mekong. The Mekong branches off into several arms near Phnom Penh and reaches