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SUMMARY “Hormone manipulation” is being used increasingly in evo-devo studies as the sole or primary technique to investigate the regulation of insect polymorphism by hormones, most notably juvenile hormone (JH). This manuscript critically evaluates the limitations and strengths of this indirect method for inferring aspects of endocrine regulation, and conclusions derived from recent endocrine studies of evolution and development in which data have been obtained primarily or exclusively by this method. The main conclusions of this critique are as follows: first, when used alone, or as the primary empirical technique, hormone manipulation is a superficial method that is fraught with problems with respect to identifying a hormone that regulates developmental–morphological variation, let alone identifying its mode of action. Second, conclusions reported in studies using this technique as the exclusive, or nearly exclusive experimental approach, most notably recent studies of JH regulation of horn polymorphism in dung beetles, and some studies of wing polymorphism should be considered, at best, weakly supported until substantiated by well-validated, direct methods. Finally, there are many reliable and well-validated techniques that can be used to directly and accurately quantify JH levels, and activities of JH regulators, in many insects, even in small, nonmodel species. Some of the most important of these assays will be briefly described and their strengths and weaknesses will be discussed.