Safranek Louis, Williams Carroll M
Biol Bull. 1984 Dec;167(3):555-567. doi: 10.2307/1541409.
Fifth-instar larvae of the tobacco hornworm underwent a supernumerary larval molt under several feeding regimens. Supernumerary molting occurred only when the molt was initiated at a live weight less than about 3 g; at higher weights pupation took place. The supernumerary sixth instars were normal in appearance and behavior and went on to form normal pupae only when the fifth instar had begun at weights less than 1 g. Otherwise the supernumerary sixth instars exhibited precocious metamorphosis of their crochets and imaginal discs and failed to undergo normal pupation. Thus, the commitment to pupal development appears to occur in stages. A first stage, initiated at a weight of about 1 g, commits the crochets and imaginal discs to metamorphosis at the next molt. A second stage, entered at a weight of about 3 g, permits the full and complete pupation of the remaining tissues; this transition probably reflects the elimination of juvenile hormone (JH). Starvation apparently alters the normal patterns of ecdysone production, breakdown, or sensitivity so as to elicit ecdysone-dependent development at lower than normal weights. When starvation results in fifth-instar larvae initiating a developmental response at weights below 3 g, a supernumerary larval molt occurs. But in this case the critical effect of starvation is on the timing of the molt rather than on the JH titer, since the latter even in normally fed early fifth instars is high at weights below 3 g.
烟草天蛾的五龄幼虫在几种饲养方案下经历了额外的幼虫蜕皮。只有当蜕皮开始时活重小于约3克时才会发生额外蜕皮；体重更高时则会化蛹。额外的六龄幼虫在外观和行为上是正常的，并且只有当五龄幼虫开始时体重小于1克时才会继续形成正常的蛹。否则，额外的六龄幼虫会出现其趾钩和成虫盘的早熟变态，并且无法正常化蛹。因此，向蛹发育的转变似乎分阶段发生。第一阶段在约1克的体重时开始，使趾钩和成虫盘在下一次蜕皮时发生变态。第二阶段在约3克的体重时进入，允许其余组织完全化蛹；这种转变可能反映了保幼激素（JH）的消除。饥饿显然改变了蜕皮激素产生、分解或敏感性的正常模式，从而在低于正常体重时引发依赖蜕皮激素的发育。当饥饿导致五龄幼虫在体重低于3克时开始发育反应时，就会发生额外的幼虫蜕皮。但在这种情况下，饥饿的关键作用在于蜕皮的时间而非保幼激素滴度，因为即使在正常饲养的早期五龄幼虫中，体重低于3克时保幼激素滴度也是高的。
