Plant sap
A large number of animals, all of the single insect order Hemiptera, feed directly on phloem sap, and make it the primary component of their diet. Phloem sap is "nutrient-rich compared with many other plant products and generally lacking in toxins and feeding deterrents, [but] it is consumed as the sole diet by a very restricted range of animals". Phloem sap is physiologically extreme in terms of animal digestion: it is thought that few animals take direct advantage of this because they lack adaptations to this diet.