During metamorphosis in the hawkmoth Manduca sexfa, muscles of the abdominal body wall undergo a reorganization. Many die at the end of larval life and are replaced in the adult by newly generated muscles. We have identified several of the motoneurons innervating these muscles and followed them through metamorphosis.The morphology of larval motoneurons is correlated with their target location. Those with medial targets have bilateral dendritic fields, whereas those with lateral targets have dendrites restricted to one side of the segmental ganglion. Some motoneurons innervate the same muscle in all stages of life, but the majority lose their larval targets following entry into the pupal stage. Although some of the latter group also die at this time, most survive to innervate a new adult target. These "respecified" motoneurons undergo a period of dramatic dendritic growth during metamorphosis.The results demonstrate that these identified neurons are capable, under the appropriate conditions of existing in more than one stable morphology.Typically the development of the nervous system does not end with the completion of embryogenesis. A postembryonic phase of development is often required to allow fine tuning of sensory (Hubel et al., 1977) and motor (Brown et al., 1976) systems. Even functioning systems may be modified as animals acquire new behavior as they mature (Nottebohm, 1981). The discrete larval, pupal, and adult stages of the holometabolous insects each have their own characteristic morphology and behavior. Their nervous systems must undergo an extensive reorganization in order to direct the behavior of these radically different stages as the animal progresses through its life history.In some instances this reorganization during metamorphosis involves the generation of new neurons by retained embryonic neuroblasts (Edwards, 1969). This is particularly true with regard to the major sensory processing areas of the insect brain (Nordlander and Edwards, 1969a, b, 1970; Meinertzhagen, 1973; White and Kankel, 1978; Matsumoto and Hildebrand, 1981), which receive inputs from a large number of imaginal disc-derived sensory neurons. However, the generation of new neurons is not the only strategy employed by

Programmed cell death occurs in the nervous and muscular system of newly emerged adult Drosophila melanogaster. Many of the abdominal muscles that were used for eclosion and wing-spreading behavior degenerate by 12 hr after eclosion. Related neurons in the ventral ganglion also die within the first 24 hr. Ligation experiments showed that the muscle breakdown is triggered by a signal from the anterior region, presumably the head, that occurs about 1 hr before adult emergence. The timing of this signal suggests that eclosion hormone may be involved. Although muscle death is triggered prior to ecdysis, it can be delayed, at least temporarily, by forcing the emerging flies to show a prolonged ecdysis behavior. In contrast to the muscles, the death of the neurons is triggered after emergence. The signal for neuronal degeneration is closely correlated with the initiation of wing inflation behavior. Ligation and digging experiments and behavioral manipulations that either blocked or delayed wing expansion behavior had a parallel effect in suppressing or delaying neuronal death.

The metamorphosis of insects is controlled by the blood titers of a small number of developmental hormones including a class of steroids, the ecdysteroids. We have studied the developmental fates of several muscles and their motoneurons during the larval-pupal transformation of the tobacco hornworm, Manduca sexta. The endocrine events which trigger pupal development are first, a fall in the blood titer of juvenile hormone, followed by two subsequent elevations of blood ecdysteroids. The small "commitment pulse" of ecdysteroids commits tissues to pupal development, whereas the sustained "prepupal peak" causes the new pupa to be formed (Riddiford, L. M. (1980) In Progress in Ecdysone Research, J.A. Hoffmann, ed., pp. 409-430, Elsevier/North-Holland Biomedical Press, Amsterdam). In the present experiments we were able to correlate specific aspects of the changing blood steroid titers with the degeneration of larval muscles, and with the dendritic regression and death of their motoneurons. The abdominal prolegs, which are the principal locomotory appendages of the caterpillar, are lost during the larval-pupal transformation. We have followed the fates of a proleg retractor muscle, PPRM, and its single motoneuron, PPR. Two other differently fated abdominal muscles not associated with the proleg were also studied. Surgical and endocrinological manipulations showed that PPRM degenerates in response to the rising phase of the prepupal ecdysteroid peak and that interactions with its motoneuron are not involved in the muscle's death. Motoneuron PPR responds to the rising prepupal peak by first reducing its dendritic arbor by 40% and then dying. Other proleg motoneurons regress but do not die, indicating that dendritic regression is programmed separately from neuronal death. Neither the dendritic reduction nor the death of PPR involves interactions with its target muscle. These results indicate that ecdysteroids have independent and parallel effects in the periphery, where they cause muscle degeneration, and in the central nervous system, where they cause dendritic regression and death of motoneurons.

In insects, the neuropeptide eclosion hormone (EH) acts on the CNS to evoke the stereotyped behaviors that cause ecdysis, the shedding of the cuticle at the end of each molt. Concomitantly, EH induces an increase in cyclic GMP (cGMP). Using antibodies against this second messenger, we show that this increase is confined to a network of 50 peptidergic neurons distributed throughout the CNS. Increases appeared 30 min after EH treatment, spread rapidly throughout these neurons, and were extremely long lived. We show that this response is synaptically driven, and does not involve the soluble, nitric oxide (NO)-activated, guanylate cyclase. Stereotyped variations in the duration of the cGMP response among neurons suggest a role in coordinating responses having different latencies and durations.

Manduca sexta larvae accumulate large amounts of iron during their larval feeding period. When 59Fe was fed to 5th instar larvae, it was evenly distributed among the hemolymph, gut and carcass until the cessation of feeding. By pupation 95% of the labelled iron was found in the fat body. In the adult a significant portion of this iron was found in flight muscle. Studies of the hemolymph disclosed two iron-containing proteins. The first was composed of a single polypeptide chain of 80 kD, containing one atom of iron. This protein bound ionic iron in vitro and was able to transfer this iron to ferritin when incubated with fat body in vitro. Therefore, it appeared to serve a transport function. The second protein had a molecular weight of 490 kD with subunits of 24 and 26 kD and contained 220 micrograms of iron/mg protein. Its chemical and ultrastructural characteristics were those of ferritin. These studies demonstrate the presence of both a transport protein and a unique circulating ferritin in Manduca sexta, the latter serving a storage function during development and possibly also a transport function.