Discovery

Early in the 20th century, scientists began to demonstrate roles for circulating hormones in the development and maturation of insects. Stefan Kopec, working in Poland, observed that removal of the brain in gypsy moths (Lymantria dispar), caused developmental arrest. Amazingly, reimplantation of the brain allowed development to resume. In other experiments, Kopec used an experimental tool called a ligature to demonstrate the timing of hormone release from the brain. A ligature is applied with a loop of string that is pulled tight, like a tourniquet, interrupting blood flow between front and back blood compartments. If a ligature is tied around a last instar before the "critical period" of 5 to 7 days, the front part of the animal will develop pupal cuticle, whereas development in the back part will be arrested (Fig. 1). However if the ligature is applied after the critical period, both sides will develop. These observations implied that something was released from the front end of the animal during the interval between days 5 and 7 of development. Consistent with Kopec's earlier work, this signal was later shown to come from the brain. These ingenious experiments illustrated the surprising fact that the insect brain, in addition to its well-known function in electrical signaling, is a secretory organ, controlling developmental processes through release of a hormone into the bloodstream.

The experiments of Kopec were published in two classic papers in the early 1920s, but little happened for about a

FIGURE 1 Ligature tied prior to the critical period (arrow) leads to arrested development in the posterior part of the animal, where green larval cuticle is retained. The thorax synthesizes a new, dark pupal cuticle in response to ecdysteroid release by the prothoracic glands. [Modified from Farb, P. (1962). "The Insects." Time, Incorporated, New York.]

FIGURE 1 Ligature tied prior to the critical period (arrow) leads to arrested development in the posterior part of the animal, where green larval cuticle is retained. The thorax synthesizes a new, dark pupal cuticle in response to ecdysteroid release by the prothoracic glands. [Modified from Farb, P. (1962). "The Insects." Time, Incorporated, New York.]

decade. The notion that hormones program insect development was not readily accepted because thinking was dominated by mid-19th century studies of reproductive hormones in birds. These experiments showed that implantation of the avian male testes causes masculinizing effects, whereas removal of the testes produced loss of male characteristics. Such experiments performed in insects had no apparent effect, and as a consequence it became accepted that insects did not engage in hormonal signaling.

Nevertheless, Vincent Wigglesworth, S. Fukuda, and later Carroll Williams extended Kopec's work, by providing evidence for a second signal located in the thorax of the insect that is released in response to the brain hormone. It became evident that implantation of the brain, as Kopec had done, worked only if the brain was placed in the thoracic area. When the brain was implanted into an isolated insect abdomen, developmental arrest persisted. The source of this second factor was the prothoracic gland. Williams showed that, if both the brain and the prothoracic glands were implanted into an isolated abdomen, development resumed. The prothoracic glands alone could accomplish this, provided they had prior exposure to the brain. It therefore became evident that the brain provides a hormonal signal that induces release from the prothoracic glands of a "molting hormone" that is critical to promoting growth.

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