In terms of their nutritional composition, phytophagous insects are clearly not what they eat. Despite having evolved the morphological, physiological, and behavioral means to ensure that they obtain adequate supplies of nutrients from their hosts, phytophages are still faced with more immediate problems, in that the nutritional quality of plants (a function of the quantity, balance, and availability of nutritients) is variable both in time and in space. 78 , 198 , 227 Additionally, the nutritional needs of an insect are not constant, but vary with the requirements of growth and development, reproduction, and so on. Unless a phytophage is able to respond to the constantly changing mismatch between what it needs and what the plant can provide, it suffers consequences such as extended development, reduced fecundity, or premature death. 227 The study of compensatory mechanisms whereby an insect reduces this discrepancy is therefore central to an understanding of insect-plant relations. It also has potential consequences for the control of phytophagous insect pests 78 , 154 and is relevant to theories about the optimality of feeding behavior. 168 , 170 , 235