Ultrastructure of the larval stemmata,the stemmata nerves and the optic neuropils of the larval cotton bollworm,Heliothis armigera (Hübner) (Lepidoptera : Noctuidae)

Ultrastructure of stemmata (larval eyes), stemmatal nerves, and the optic neuropils of 5th-instar larvae of cotton bollworm, Heliothis armigera (Hübner) (Lepidoptera : Noctuidae), were examined with scanning and transmission electron microscopes. Six stemmata are on each side of the head. Each stemma consists of 7 retinula cells arranged into 2 tiers. Stemmata I and II have 4 distal retinula cells and 3 proximal cells, the other 4 stemmata (III–IV) have 3 distal cells and 4 proximal cells. Stemmata I and IV have a short proximal rhabdom and the rhabdomere of each proximal cell has its microvilli projecting in only one direction. On the other hand, each stemma (in stemmata II–V) has a long proximal rhabdom and the rhabdomere of each proximal cell has microvilli pitched in several different directions relative to the horizontal plane. An axon projects proximally from each retinula cell body. The stemmatal nerve is composed of the 42 retinular axons from all of the 6 stemmata on the same side of the head. Each stemmatal nerve projects to the ipsilateral optic neuropil. Axons from each stemma are in a fasicle (within the stemmatal nerve), which consists of 7 axons, 3–4 of them are thick and terminate synaptically in the proximal neuropil; the others are thinner and terminate in the distal neuropil. Organelles, particularly lysosomes, undergo ultrastructural transformations relative to ambient light levels. The functional significance of abovementioned structures are discussed in light of current knowledge.     

Postembryonic changes in the optic primordia and optic bud in the flesh fly Sarcophaga ruficornis fabr. (Diptera: Sarcophagidae)

Differentiation of the optic lobe anlagen begin in the brain of second instar. Each is an elongated disc of cortical cells placed on the dorsolateral border of each protocerebrum. In the late second instar the disc elongates and its two ends bend inwards which gradually separate from the central region, thus giving three imaginal discs. The protocerebral neuropile extends into these discs and medulla interna and externa are formed. The rudiments of compound eyes (cephalic complex) appear in the early laid larva. These are attached with the brain and pharyngeal wall separately. The posterior portion of cephalic complex (optic bud), after establishing a nervous association with the central optic lobe anlage (lamina ganglionaris), forms the compound eye. Ech optic bud is attached to the brain by a non-nervous stalk. The epiblast cells of the optic bud do not migrate into the brain and the lamina is formed by the proliferation of the central imaginal disc. The reorientation of the optic lobe anlagen starts in the late third instar and the medulla interna divides into two unequal lobes. In 2 day pupa the nerve fibres from the lamina travel into the optic stalk and the optic nerve is formed. The epiblast cells of the optic bud differentiate to form a peripheral epithelial layer which becomes pigmented and gets apposed to the lateral boundary of the brain. The central epiblast cells of the optic bud form several ommatidia. The optic nerve degenerates gradually and various components of the compound eye are formed by the epiblast cells. Chiasm internum is present but chiasm externum is absent.     

Eye and optic lobe metamorphosis in the sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae)

Nearly nothing is known about the transition that visual brain regions undergo during metamorphosis, except for Drosophila in which larval eyes and the underlying neural structure are strongly reduced. We have studied the larvae of the sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae), which are sophisticated visually oriented predators characterized by six elaborate stemmata on each side of the head and an associated large optic lobe. We used general neurohistological staining and 3D reconstruction to determine how the eyes and optic lobe of T. marmoratus change morphologically during metamorphosis. We find that in third (last) instar larvae, the adult neuropils are already forming de novo dorsally and slightly anteriorly to the larval neuropils, while the latter rapidly degenerate. Larval eyes are eventually reduced to distinct areas with dark pigmentation. This complete reorganization, which may be an evolutionarily conserved trait in holometabolous insects, occurs despite the considerable costs that must apply to such a visually complex animal. Our findings are consistent with the concept that stemmata are homologous to the most posterior ommatidia of hemimetabolous insects, an idea also recently supported by molecular data.     

Larva-adult relationships in an ancestral dipteran: A re-examination of sensillar pathways across the antenna and leg anlagen of Chaoborus crystallinus (DeGeer, 1776; Chaoboridae)

 In one of his classical studies on insect metamorphosis, Weismann compared the imaginal anlagen of the ancestral phantom midge, Chaoborus, with those of advanced brachycerans. We have expanded his findings on the relationships between larval and imaginal organs using electron microscopy and cobalt backfilling of the antenna and leg anlagen and the axonal trajectories of corresponding larval sensilla. We show that both primordia are confluent with the larval antennae and ”leg” sensilla (an ancestral Keilin organ), respectively. These fully developed larval organs represent the distal tips of the imaginal anlagen rather than separate cell clusters. The axons of the larval antenna and leg sensilla project across the corresponding anlagen to their target neuromeres within the central nervous system (CNS). Within the discs, nerves composed of these larval axons, developing afferent fibres and efferences ascending from the CNS are found. Both the structure of the primordia and the axonal trajectories thus relate the situation found in advanced brachycerans with that seen in more ancestral insects. In addition, the larval antennae, legs, wings and even the eyes possess very similar afferent pioneer trajectories supporting the idea that the described pattern is generally used in the ontogeny of sensory systems. Received: 30 June 1998 / Accepted: 27 September 1998     

Evolutionswege zum Larvalauge der Insekten -Die Stemmata der höheren Dipteren und ihre Abwandlung zum Bolwig-Organ

Evolutionary pathways to the larval eyes of insects. Higher Dipteran stemmata and the evolutionary development of Bolwig's organ 1. A cornrehensive morphological study of the photoreceptors in the so-called hemi — Institut für Biologie I (Zoologie) der Albert-Ludwigs-Universität, Albertstr. 21a, D-7800 Freiburg i. Brsg., FRGand acephalic lrvae of Brachycera was undertaken. In Brachyceran larvae the head casule originally is more or less retracted into the thorax. The larval hotoreceptor in Musca and Drosophila, here called Bolwig's organ, is situated on the outer side of the cephalopharyngeal apparatus, well below the surface. The aim of this study is to elucidate the homology of Bolwig's organ, i. e. whether Bolwig's organ originated from typical stemmata during the evolution of Cyclor-rhaphan larval organlsation or whether it represents a unique type of hotoreceptor. Larval photoreceptors and the anatomy of the head capsules of representative Erachycerans were examined by means of light and electron microscopy. In particular, the site and the structure of the photofecetors of Stratiomys (Stratiomyidae, Solva (Solvidae), Atherix ibis (Athericidae), Rhagio (Rhagionidae), Thereva (derevidae), Lonchoptera (Lonchoteridae), Epistrohe balteata, Volu-cella bombylans, Eristalis tenax (Syrphidae), Drosophila (f;rosophilidae), jannia and Musca domestica (Muscidae) were investigated. 2. Brachyceran stemmata are either situated on the ocular plates of the free remnant of the head (Stratiomyidae, Therevidae), or on the inner wall of an epidermal invaination, which includes epithelia of the head as well as of the thorax (Tabaniformia). In adfition, they can be found on the outer side of the tentorial phragmata (Cyclorrhapha). In all taxa studied, stemmata keep contact directly with the epithelium or through short processes of eithelial cells. 3. Ancestral Brachycera have fused stemmata that correspond with typical larval eyes of other holometabolan insects. 4. A cornea is found solely in Stratiomyidae and Xylophagidae, in contrast to all other taxa, where no dioptric apparatus is found. 5. In “Orthorrapha” and in “Aschiza”, rhabdomeric photoreceptors occur, forming a fused, star-shaped or Ktticed rhabdom. In Schizophoran larvae, retinular cells are distinguished by a different type of surface enlargement of the photoreceptor cell membrane. Here, the membrane forms flat ramella. The latter originate from transformed and/or rudimentary rhabdomeric microvilli. 6. In the primitive “Orthorrhapha”, pigment cups can be found, that are composed of retinular pirnent, whereas in derived Brachycera, pigment grains are absent. In Cyclorrhapha, tentorial piramata and their optic deressions operate as external and functional piment cups 7. In Therevidae and in Syrphidae, a tracheal tapetum lucidum can be founfi which la, been evolved independently in both families. The tapetum is always found together with inversely orientated retinular cells. 8. The homology of Brachceran stemmata is shown by a transformation series of stem-mata's site and fine structure. The homology is corroborated by the identical innervation in primitive and derived taxa, the identical site within Cyclorrhaha, and structural similarities of “Orthorrhaphan” and primitive Cylorrhaphan stemmata. Although transformations of the fine structure appear as a sequence or reductions, particular adaptations such as multilication of retinular celfs (Epistrophe), or deepening of the pigment cup (Musca) have evolved separately. Despite the fact that in Brachyceran larvae the head has been reorganized, the eyes still keep contact with the epithelium of the frontal sacks and/or tentorial phragmata. 9. The transformation series given shows that Bolwig's organ represents a highly modified stemma.     

Tracking down the "head blob": comparative analysis of wingless expression in the developing insect procephalon reveals progressive reduction of embryonic visual system patterning in higher insects

The evolution of larval head morphology in holometabolous insects is characterized by reduction of antennal appendages and the visual system components. Little insight has been gained into molecular developmental changes underlying this morphological diversification. Here we compare the expression of the segment polarity gene wingless (wg) in the pregnathal head of fruit fly, flour beetle and grasshopper embryos. We provide evidence that wg activity contributes to segment border formation, and, subsequently, the separation of the visual system and protocerebrum anlagen in the anterior procephalon. In directly developing insects like grasshopper, seven expression domains are formed during this process. The activation of four of these, which correspond to polar expression pairs in the optic lobe anlagen and the protocerebral ectoderm, has shifted to postembryonic stages in flour beetle and Drosophila. The remaining three domains map to the protocerebral neuroectoderm, and form by disintegration of a large precursor domain in flour beetle and grasshopper. In Drosophila, the precursor domain remains intact, constituting the previously described “head blob”. These data document major changes in the expression of an early patterning gene correlated with the dramatic evolution of embryonic visual system development in the Holometabola.     

Neural development in an imaginal disc mutant of Drosophila melanogaster

The neural phenotype of an imaginal disc degenerate mutant l(1)d deg-3 was studied in histological sections. The mutant larvae showed severe abnormalities in the imaginal neural development. Gynandromorphs, which are composed of genetically mutant and nonmutant cells, were generated and analyzed as late larvae. The results of mosaic analysis were consistent with l(1)d deg-3 gene acting autonomously in the imaginal disc and imaginal neural cells. The optic lobe development patterns observed in the larval mosaics provided evidence for an eye disc-optic lobe interaction during the late third instar larval stage.     

Ultrastructure and central projections of extraocular photoreceptors in caddiesflies (Insecta: Trichoptera)

Summary Retained larval eyes (stemmata) were studied in the imagines of three species of Trichoptera: Phrygania grandis, Agrypnia varia, and Trichostegia minor. At the light-microscopic level the stemmata of all three species appeared to represent different stages of reduction with respect to size, shape and number of lenses. However, in all three species electron-microscopic studies showed units with monolayered rhabdoms, each formed by four retinula cells. By use of immunocytochemistry the presence of S-antigen was demonstrated in the retinula cells and their axons. This method also revealed the central projections of the axons of the retinula cells, which were found (i) to terminate either in the lamina accessoria or (ii) to penetrate this area to join the fibers of the outer chiasma of the optic lobes and then terminate in the medulla accessoria. The lamina accessoria and the medulla accessoria are the assumed remnants of the larval optic lobes. It is suggested that the imaginal stemmata might still be functioning photoreceptors.     

Ultrastructure of the single-chamber stemmata of Arge pagana (Panzer, 1798) (Hymenoptera: Argidae)

Stemmata are peculiar visual organs of most larvae in holometabolous insects. In Hymenoptera, Symphyta larvae exclusively possess a pair of stemmata, whose cellular organizations have not been thoroughly elucidated to date. In this paper, the morphology and fine structure of stemmata were investigated in the large rose sawfly Arge pagana (Panzer, 1798) using light and electron microscopy. The larvae possess a pair of stemmata, which belong to the “unicorneal composite eye” or single-chamber stemmata. Each stemma is composed of a biconvex cornea lens, a layer of corneagenous cells, numerous pigment cells, and hundreds of retinula cells. According to the number of retinula cells forming a rhabdom, the stemma can be divided into two regions, the larger Region I and the smaller Region II. The former occupies the largest area of the stemma and contains the majority of rhabdoms, each of which is formed by the rhabdomeres of eight retinula cells. The latter occupies a narrow posterior margin, where each rhabdom consists of nine retinula cells. Based on the different cellular organizations of rhabdoms, the stemma of Argidae is likely developed by the fusion of two types of ommatidial units.     

Structure of the visual system of the larva of the tiger beetle (Cicindela chinensis)

The visual system of the larval tiger beetle (Cicindela chinensis) consists of six (two large, two mediumsized, and two small) stemmata on either side of the head, and an underlying neuropil mass. Each stemma exhibits a corneal lens and an underlying rhabdom layer. Retinular cells extend single proximal axons into the neuropil mass. The neuropil mass has a flattened heart-shape, and consists of two juxtaposed identical structures, each being a neuropil complex of each of the two large stemmata. The complex consists of lamina and medulla neuropils. Most retinular axons terminate in the lamina neuropil. Axons of two types of lamina monopolar neurons descend parallel to each other into the lamina neuropil. Moreover, each lamina neuropil contains a single giant monopolar neuron. Possible centrifugal processes and tangential neurons also occur. Lamina monopolar axons descend straight into the medulla neuropil. Medulla neurons spread fan-shaped dendrites distally in the medulla neuropil and send single axons toward the protocerebrum. These data are discussed with respecct to the unique visual behavior of this larva and in comparison with other insect visual systems.     

Structure of the visual system of the two-spotted spider-mite, Tetranychus urticae

The pair of eyes on each side of Tetranychus urticae consists of fifteen retinular cells, six pigment cells, six corneal cells, and one vitreous cell. Five rhabdomeres lie beneath the anterior lens, twelve beneath the posterior lens. The pigment and corneal cells appear to determine the positions at which rhabdomeres occur. The volumes of all rhabdomeres have been measured. A pair of fibrous masses are associated with tendons at the dorsal apodemes; the nerves of these ‘apodemal organs’ join the optic nerves en route to the brain. There are a pair of optic neuropiles, each surmounted by a large secretory cell.     

Post-larval development of compound eyes and stemmata of Chaoborus crystallinus (De Geer, 1776) (Diptera : Chaoboridae): Stage-specific reconstructions within individual organs of vision

During metamorphosis, the dioptric apparatus of the larval compound eye of Chaoborus crystallinus (Diptera : Nematocera) is radically reconstructed. The thin larval cornea of the ommatidia is replaced by strongly curved corneal lenses, and the eucone larval cone is replaced by an imaginal cone of the acone type. Curvature of the future lens is already apparent in very young pupae, in which the cornea consists only of a thin epicuticle with corneal nipples. Fibrillary cuticle is secreted by cone and primary pigment cells throughout pupal development. Lens formation is accompanied by movement of the nuclei of the accessory pigment cells. The larval cone disintegrates unexpectedly late in young, images. During late pupal development, 7 cone cell projections emerge. In contrast to the dioptric apparatus, the retinula cells and rhabdom remain almost unchanged during metamorphosis. The main refractive element of the larval ommatidium appears to be the cone, while that of the imaginal ommatidium is the corneal lens. In addition to the compound eyes, the pairs of stemmata are retained during the whole post-larval development. Pupal stemmata show no structural differences from the larval stemmata. The stemmata are still present in 2-day-old images (“retained stemmata”), but the primary stemma loses its dioptric apparatus and is proximally relocated to the basal region of the compound eye. The reconstructions in the visual system of Chaoborus, which occur during ontogeny, are probably connected with the change from aquatic living larvae to aerial adults, and appear to fulfill stage-specific needs of vision.     

昆虫单眼的结构和功能

大多数昆虫的视觉器官除了复眼外还有一些简单的小眼,称为单眼。昆虫成虫和半变态类若虫的单眼称为背单眼,位于头顶两复眼之间。背单眼在数目和结构上都有较大变化,但基本结构包括角膜晶体、一层角膜生成细胞(覆盖在角膜晶体上)、视网膜(由大约1000个感光细胞构成,视类群而不同)。背单眼对弱光比较敏感,但在图像感知方面的作用并不显著;它是一种“激发器官”,可以增加复眼的感知能力。全变态昆虫的幼虫既没有复眼也没有背单眼,但在其头部两侧有些类似复眼小眼的侧单眼。侧单眼的结构也与小眼相似,包括角膜,晶体和由一些视网膜细胞组成的视杆。侧单眼是完全变态类昆虫幼虫仅有的感光器官,与复眼一样,它们可以感知颜色、形状、距离等等。     

The fine structure of the lateral ocellus of the dobsonfly larva

The lateral ocelli of the dobsonfly (Protohermes grandis, Neuroptera) larva have been examined with light and electron microscopy. The larva has six ocelli on both sides of the head, each containing a single corneal lens. A conical crystalline body, of some 10–20 cells is situated immediately posterior to the lens. From 100 to 300 elongated retinular cells are arranged perpendicular to the crystalline body except at the innermost surface of the lens, where they are absent. The distal process of each retinular cell is enclosed by a tube-like rhabdom formed by the close association of microvilli from the same and adjacent distal processes. The distal process contains many mitochondria, multivesicular bodies, microtubles and pigment granules. In the dark-adapted ocellus the pigment granules are concentrated near the nucleus which lies under the rhabdomic layer. The granules diffuse toward the rhabdomic microvilli during light adaptation. Each retinular cell has a single axon, which extends from the ocellus as an ocellar nerve fiber into the optic lobe, where it frequently synapses upon second order neurons. In addition to these afferent synapses, there are two other synaptic combinations: (1) a feedback synapse from a second order neuron to a retinular axon, and (2) a synapse between second order neurons. These results suggest that photic signals reach the more proximal part of the brain via second order neurons after some degree of integration in the optic lobe.     

Postembryonic development of serotonin-immunoreactive neurons in the central nervous system of the blowfly,Calliphora erythrocephala

Summary The postembryonic development of serotonin-immunoreactive (5-HTi) neurons was studied in the optic lobe of the blowfly. In the adult fly there are 24 5-HTi neurons invading each optic lobe. The perikarya of two of these neurons are situated in the dorso-caudal part of the protocerebrum (LBO-5HT neurons; large bilateral optic lobe 5-HTi neurons). The cell bodies of the remaining 22 neurons are located anteriorly at the medial base of the medulla (2 innervating the lobula, LO-5HT neurons; and 20 neurons innervating the medulla, ME-5HT neurons). The two central neurons (LBO-5HT neurons) are derived from metamorphosing larval neurons, while the ME- and LO-5HT neurons are imaginai optic lobe neurons differentiating during pupal development.The 5-HTi neurons of the optic lobe seem to have different ancestors. The LBO-5HT neurons are probably derived from segmental protocerebral neuroblasts, whereas the ME-and LO-5HT neurons are most likely derived from the inner optic anlage. The first 5-HTi fibers to reach the imaginal optic lobes are seen in the late third instar larva and are derived from the LBO-5HT neurons. The first ME- and LO-5HT neurons become immunoreactive at 24 h (10%) pupal development. At about 96 h (40%) of pupal development all the 5-HTi neurons of the optic lobes have differentiated and attained their basic adult morphology. The further development mainly entails increase in volume of arborizations and number of finer processes. The differentiation and outgrowth of 5-HTi processes follows that of, e.g., columnar neurons in the optic lobe neuropils. Hence, 5-HTi processes invade neuropil relatively late in the differentiation of the optic lobe.     

Receptive field of the stemmata in the swallowtail butterflyPapilio

Summary Receptive fields of individual retinular cells in the stemmata ofPapilio xuthus L. were examined electrophysiologically, and the receptive field of the complete stemmatal system was reconstructed (Fig. 8).In stemmata I-IV, proximal retinular cells have narrow receptive fields (acceptance angles of = 1.7–5 °, Fig. 5) and small inclinations of the visual axes (inclinations of = 0.7–1.5 °, Fig. 2) with respect to the axis of the stemma, while distal ones have wide fields ( =7–13 °, Fig. 5) and large inclinations of the visual axes ( = 5–10 °, Fig. 3). In stemmata V and VI, both proximal and distal retinular cells have wide receptive fields ( = 7–26 °, Fig. 6) and have large inclinations of their visual axes ( = 9–19 °) with respect to the axis of the stemma except for one proximal cell ( = 0 °) (Fig. 4).The spatial properties of distal and proximal retinular cells, combined with the finding that distal cells are homogeneous in the spectral sensitivity while proximal ones are heterogeneous (Ichikawa and Tateda 1980), suggest that the distal cells may be concerned largely with the detection of objects and proximal cells are involved with the discrimination of the color and shape of the detected objects.     

The embryonic development of the Drosophila visual system

We have used electron-microscopic studies, bromodeoxyuridine (BrdU) incorporation and antibody labeling to characterize the development of the Drosophila larval photoreceptor (or Bolwig's) organ and the optic lobe, and have investigated the role of Notch in the development of both. The optic lobe and Bolwig's organ develop by invagination from the posterior procephalic region. After cells in this region undergo four postblastoderm divisions, a total of approximately 85 cells invaginate. The optic lobe invagination loses contact with the outer surface of the embryo and forms an epithelial vesicle attached to the brain. Bolwig's organ arises from the ventralmost portion of the optic lobe invagination, but does not become incorporated in the optic lobe; instead, its 12 cells remain in the head epidermis until late in embryogenesis when they move in conjunction with head involution to reach their final position alongside the pharynx. Early, before head involution, the cells of Bolwig's organ form a superficial group of 7 cells arranged in a rosette pattern and a deep group of 5 cells. Later, all neurons move out of the surface epithelium. Unlike adult photoreceptors, they do not form rhabdomeres; instead, they produce multiple, branched processes, which presumably carry the photopigment. Notch is essential for two aspects of the early development of the visual system. First, it delimits the number of cells incorporated into Bolwig's organ. Second, it is required for the maintenance of the epithelial character of the optic lobe cells during and after its invagination.     

Ultrastructure of the larval eye of the scorpionfly Panorpa dubia (mecoptera: Panorpidae) with implications for the evolutionary origin of holometabolous larvae

The evolutionary origin of holometabolous larvae is a long‐standing and controversial issue. The Mecoptera are unique in Holometabola for their larvae possessing a pair of compound eyes instead of stemmata. The ultrastructure of the larval eyes of the scorpionfly Panorpa dubia Chou and Wang, 1981 was investigated using transmission electron microscopy. Each ommatidium possesses a cornea, a tetrapartite eucone crystalline cone, eight retinula cells, two primary pigment cells, and an undetermined number of secondary pigment cells. The rhabdomeres of the eight retinula cells form a centrally‐fused, tiered rhabdom of four distal and four proximal retinula cells. The rhabdomeres of the four distal retinula cells extend distally into a funnel shape around the basal surface of the crystalline cone. Based on the similarity of the larval eyes of Panorpidae to the eyes of the hemimetabolous insects and the difference from the stemmata of the holometabolous larvae, the evolutionary origin of the holometabolous larvae is briefly discussed. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Optic lobes of the larval and imaginal scorpionfly Panorpa vulgaris (Mecoptera,Panorpidae): A neuroanatomical study of neuropil organization,retinula axons,and lamina monopolar cells

Panorpa larvae possess stemmata (lateral ocelli), which have the structure of compound eyes, and stemma lamina and stemma medulla neuropils. A distinct lobula neuropil is lacking. The stemma neuropils have a columnar organization. They contain lamina monopolar cells, and both short and long visual fibers. All the identified larval monopolar neurons have radially arranged dendrites along the entire depth of the lamina neuropil and a single terminal arborization within the medulla (L1/L2-type). The terminals of visual fibers have short spiny lateral projections. Long fibers possess en passant synapses within the lamina. The same principles of organization of first and second order visual neuropils are found in Panorpa imagines. In contrast to the larvae, a lobula neuropil is present. Adults have monopolar cells of the L1-type that are similar to the L1-neurons found in Diptera. The columnar organization, the presence of short and long visual fibers, and lamina monopolar neurons are thus features common to both visual systems, viz., the larval (stemmata) and the imaginal (compound eyes).     

Distribution of FMRFamide-like immunoreactivity in the brain and suboesophageal ganglion of the sphinx mothManduca sexta and colocalization with SCPB-, BPP-, and GABA-like immunoreactivity

Summary Using an antiserum against the tetrapeptide FMRFamide, we have studied the distribution of FMRFamide-like substances in the brain and suboesophageal ganglion of the sphinx mothManduca sexta. More than 2000 neurons per hemisphere exhibit FMRFamide-like immunoreactivity. Most of these cells reside within the optic lobe. Particular types of FMRFamide-immunoreactive neurons can be identified. Among these are neurosecretory cells, putatively centrifugal neurons of the optic lobe, local interneurons of the antennal lobe, mushroom-body Kenyon cells, and small-field neurons of the central complex. In the suboesophageal ganglion, groups of ventral midline neurons exhibit FMRFamide-like immunoreactivity. Some of these cells have axons in the maxillary nerves and apparently give rise to FMRFamide-immunoreactive terminals in the sheath of the suboesophageal ganglion and the maxillary nerves. In local interneurons of the antennal lobe and a particular group of protocerebral neurons, FMRFamide-like immunoreactivity is colocalized with GABA-like immunoreactivity. This suggests that FMRFamide-like peptides may be cotransmitters of these putatively GABAergic interneurons. All FMRFamide-immunoreactive neurons are, furthermore, immunoreactive with an antiserum against bovine pancreatic polypeptide, and the vast majority is also immunoreactive with an antibody against the molluscan small cardioactive peptide SCP_B. Therefore, it is possible that more than one peptide is localized within many FMRFamide-immunoreactive neurons. The results suggest that FMRFamide-related peptides are widespread within the nervous system ofM. sexta and might function as neurohormones and neurotransmitters in a variety of neuronal cell types.Abbreviations AL antennal lobe - BPPLI bovine pancreatic polypeptide-like immunoreactivity - FLI FMRFamide-like immunoreactivity - GLI GABA-like immunoreactivity - NSC neurosecretory cell - SCP _B LI small cardioactive peptide_B-like immunoreactivity - SLI serotonin-like immunoreactivity - SOG suboesophageal ganglion