Grasshopper Schistocerca Gregaria Research Articles

DNA damage in hemocytes of Schistocerca gregaria (Orthoptera: Acrididae) exposed to contaminated food with cadmium and lead

We measured in a comet assay the damage of DNA in the hemocytes of various stages of the grasshopper Schistocerca gregaria after ex-posing them to various doses of Cd and Pb in the food. The mechanisms of Cd and Pb toxicity for grasshopper are discussed. The accumula-tion of heavy metals and stage of the insect may play important roles in causing the DNA damage. S. gregaria may be considered a valuable bio-indicator for evaluation the genotoxicity of en-vironmental pollutants.

An ontogenetic analysis of locustatachykinin-like expression in the central complex of the grasshopper Schistocerca gregaria

We have investigated the ontogenetic basis of locustatachykinin-like expression in a group of cells located in the pars intercerebralis of the grasshopper midbrain. These cells project fibers to the protocerebral bridge and the central body via a characteristic set of fiber bundles called the w, x, y, z tracts. Lineage analyses associate the immunoreactive cells with one of four neuroblasts (termed W, X, Y, Z) in each protocerebral hemisphere of the early embryo. Locustatachykinin is a ubiquitous myotropic peptide among the insects and its expression in the pars intercerebralis begins at approximately 60–65% of embryogenesis. This coincides with the appearance of the columnar neuroarchitecture characteristic of the central body. The number of immunoreactive cells in a given lineage is initially small, increases significantly in later embryogenesis, and attains the adult situation (about 7% of a lineage) in the first larval instar after hatching. Although each neuroblast generates progeny displaying a spectrum of cell body sizes, there is a clear morphological gradient, which reflects birth order within the lineage. Locustatachykinin expressing cells are located stereotypically at or near the tip of their lineage, which an age profile reveals places them amongst the first born progeny of their respective neuroblasts. Although these neuroblasts begin to generate progeny at approximately 25–27% of embryogenesis, their daughter cells remain quiescent with respect to locustatachykinin expression for over 30% of embryogenesis.

Evidence that the primary brain commissure is pioneered by neurons with a peripheral-like ontogeny in the grasshopper Schistocerca gregaria

The commissures represent a major neuroarchitectural feature of the central nervous system of insects and vertebrates alike. The adult brain of the grasshopper comprises 72 such commissures, the first of which is established in the protocerebral midbrain by three sets of pioneer cells at around 30% of embryogenesis. These pioneers have been individually identified via cellular, molecular and intracellular dye injection techniques. Their ontogenies, however, remain unclear. The progenitor cells of the protocerebral midbrain are shown via Annulin immunocytochemistry to be compartmentalized, belonging either to the protocerebral hemispheres or the so-called median domain. Serial reconstructions based on bromodeoxyuridine incorporation confirm that their lineages do not intermingle. Dye injection into progenitor cells and progeny confirms this compartmentalization, and reveals that none of the pioneers are associated with a lineage of cells deriving from a protocerebral neuroblast or midline precursor. Immunocytochemical data as well as dye injection into identified pioneers over several developmental stages indicate that they differentiate directly from epithelial cells, but not from classical progenitor cells. That the commissural pioneers of the protocerebrum represent modified epithelial cells involves a different ontogeny to that described for pioneers in the ventral nerve cord, but parallels that of pioneer neurons of the peripheral nervous system.

Building the central complex of the grasshopper Schistocerca gregaria: axons pioneering the w, x, y, z tracts project onto the primary commissural fascicle of the brain

The central complex is a major neuropilar structure in the insect brain whose distinctive, modular, neuroarchitecture in the grasshopper is exemplified by a bilateral set of four fibre bundles called the w, x, y and z tracts. These columns represent the stereotypic projection of axons from the pars intercerebralis into commissures of the central complex. Each column is established separately during early embryogenesis in a clonal manner by the progeny of a subset of four identified protocerebral neuroblasts. We report here that dye injected into identified pioneers of the primary brain commissure between 31 and 37% of embryogenesis couples to cells in the pars intercerebralis which we identify as progeny of the W, X, Y, or Z neuroblasts. These progeny are the oldest within each lineage, and also putatively the first to project an axon into the protocerebral commissure. The axons of pioneers from each tract do not fasciculate with one other prior to entry into the commissure, thereby prefiguring the modular w, x, y, z columns of the adult central complex. Within the commissure, pioneer axons from columnar tracts fasciculate with the growth cones of identified pioneers of the existing primary fascicle and do not pioneer a separate fascicle. The results suggest that neurons pioneering a columnar neuroarchitecture within the embryonic central complex utilize the existing primary commissural scaffold to navigate the brain midline.

Embryonic development of a peripheral nervous system: Nerve tract associated cells and pioneer neurons in the antenna of the grasshopper Schistocerca gregaria

The grasshopper antenna is an articulated appendage associated with the deutocerebral segment of the head. In the early embryo, the meristal annuli of the antenna represent segment borders and are also the site of differentiation of pioneer cells which found the dorsal and ventral peripheral nerve tracts to the brain. We report here on another set of cells which appear earlier than the pioneers during development and are later found arrayed along these tracts at the border of epithelium and lumen. These so-called nerve tract associated cells differ morphologically from pioneers in that they are bipolar, have shorter processes, and are not segmentally organized in the antenna. Nerve tract associated cells do not express horseradish peroxidase and so are not classical neurons. They do not express antigens such as repo and annulin which are associated with glia cells in the nervous system. Nerve tract associated cells do, however, express the mesodermal/mesectodermal cell surface marker Mes-3 and putatively derive from the antennal coelom and then migrate to the epithelium/lumen border. Intracellular recordings show that such nerve tract associated cells have resting potentials similar to those of pioneer cells and can be dye coupled to the pioneers. Similar cell types are present in the maxilla, a serially homologous appendage on the head. The nerve tract associated cells are organized into a cellular scaffold which we speculate may be relevant to the navigation of pioneer and sensory axons in the early embryonic antennal nervous system.

Building the central complex of the grasshopper Schistocerca gregaria: temporal topology organizes the neuroarchitecture of the w, x, y, z tracts

The central complex is a brain specific structure involved in multimodal information processing and in coordinating motor behaviour. It possesses a highly organized neuroarchitecture, which is remarkably conserved across insect species. A prominent feature of this neuroarchitecture is the stereotypic projection of axons from clusters of neurons in the pars intercerebralis to the central body via the so-called w, x, y and z tracts. Despite extensive analyses of this neuroarchitecture in adults, little is known about its ontogeny in any insect. In this paper we use the expression pattern of the segment polarity gene engrailed to identify those neuroblasts belonging to the protocerebrum of the early embryonic brain of the grasshopper Schistocerca gregaria. We present a new map for this brain region in which the 95 protocerebral neuroblasts in each hemisphere are organized into seven rows, as they are in the neuromeres of the ventral nerve cord. We then identify a subset of four of these neuroblasts as being the progenitor cells for four clusters of neurons, some of whose axons we show project via discrete tracts (w, x, y, z) into the central complex. These tracts begin to form prior to 39% of embryogenesis. We show further, that the cells from one of these clusters (the Z cluster) are organized according to age, and direct axons topologically according to age into the appropriate z tract. This pattern is repeated in each of the other three clusters, thus establishing a clonally based modular system of fibre tracts consistent with the model proposed for this brain region in the adult.

Embryonic development of the sensory innervation of the antenna of the grasshopper Schistocerca gregaria

The establishment of the sensory nervous system of the antenna of the grasshopper Schistocerca gregaria was examined using immunocytochemical methods and in the light of the appendicular and articulated nature of this structure. The former is demonstrated first by the expression pattern of the segment polarity gene engrailed in the head neuromere innervating the antenna, the deutocerebrum. Engrailed expression is present in identified deutocerebral neuroblasts and, as elsewhere in the body, is continuous with cells of the posterior epithelium of the associated appendage, in this case the antenna. Second, early expression of the glial homeobox gene reversed polarity ( repo) in the antenna is by a stereotypic pair of cells at the antenna base, a pattern we show is repeated metamerically for each thoracic appendage of the embryo. Subsequently, three regions of Repo expression (A1, A2, A3) are seen within the antenna, and may represent a preliminary form of articulation. Bromodeoxyuridine incorporation reveals that these regions are sites of intense cell differentiation. Neuron-specific horseradish peroxidase and Lazarillo expression confirm that the pioneers of the ventral and dorsal tracts of the antennal sensory nervous system are amongst these differentiating cells. Sets of pioneers appear simultaneously in several bands and project confluent axons towards the antennal base. We conclude that the sensory nervous system of the antenna is not pioneered from the tip of the antenna alone, but in a stepwise manner by cells from several zones. The early sensory nervous systems of antenna, maxilla and leg therefore follow a similar developmental program consistent with their serially homologous nature.

Ontogeny of identified cells from the median domain in the embryonic brain of the grasshopper Schistocerca gregaria

In this paper, we propose an ontogeny for previously identified cells from the median domain in the midline of the embryonic brain of the grasshopper Schistocerca gregaria. The so-called lateral cells (LCs) are characteristically located laterally within the median domain at its border with the protocerebral hemispheres. The LC occurs singly and can be identified in the early embryo on the basis of their expression of the cell surface lipocalin Lazarillo. Using immunocytochemical, dye injection, electron microscopical and histological methods, we show that these LC are neurons and derive as postmitotic cells directly from the epithelium of the median domain. Further, they and the other identified cells of the median domain such as the protocerebral commissure pioneers (PCP), co-express the Mes-3 antigen, consistent with a derivation from the mesectodermal germ layer of the embryo. Subsequent to axogenesis, electron microscopy reveals that these Mes-3-expressing LC fasciculate with the co-expressing PCPs within the developing protocerebral commissure. We present a model for the origin of all these cells based on histological data and bromodeoxyuridine incorporation. The model suggests a delamination of cells from the mesectoderm followed by a migration to their ultimate sites within the median domain.

Germ Line Development in the Grasshopper Schistocerca gregaria: vasa As a Marker

Vasa is a widely conserved germline marker, both in vertebrates and invertebrates. We identify a vasa orthologue, Sgvasa, and use it to study germline development in the grasshopper Schistocerca gregaria, a species in which no germ plasm has been identified. In adults, Sgvasa is specifically expressed in the ovary and testis. It is expressed at high levels during early oogenesis, but no detectable vasa RNA and little Vasa protein are present in mature unlaid eggs. None appears to be localized to any defined region of the egg cortex, suggesting that germline specification may not depend on maternal germ plasm expressing vasa. Vasa protein is expressed in most cleavage energids as they reach the egg surface and persists at high levels in most cells aggregating to form the embryonic primordium. However, after gastrulation, Vasa protein persists only in extraembryonic membranes and in cells at the outer margin of the late heart-stage embryo. In the embryo, it then become restricted to cells at the dorsal margin of the forming abdomen. In older embryos, these Vasa-positive cells move toward the midline; Vasa protein accumulates asymmetrically in their cytoplasm, a pattern closely resembling that of germ cells in late embryonic gonads. Thus, we suggest that the Vasa-stained cells in the abdominal margin are germ cells, as proposed by Nelson (1934), and not cardioblasts, as has been proposed by others.

A single cell analysis of engrailed expression in the early embryonic brain of the grasshopper Schistocerca gregaria: ontogeny and identity of the secondary headspot cells

The expression pattern of the segment polarity gene engrailed was studied at the single cell level in the protocerebrum of the early embryonic brain of the grasshopper Schistocerca gregaria, the neuromere containing the secondary headspot cells. The engrailed protein is first expressed in the protocerebrum at about 22% of embryogenesis by a group of identified neuroblasts bordering the antennal lobe. The number of immunoreactive neuroblasts increases up to 26% of embryogenesis and then rapidly declines so that by 30% only the three most posterior remain immunoreactive. These three neuroblasts become incorporated into the developing antennal lobe of the deutocerebrum. Subsequently, there is a progressive re-expression of the engrailed protein in the protocerebrum by the so-called six secondary headspot cells. These are the first born sibling progeny of three identified protocerebral neuroblasts which themselves expressed the engrailed protein prior to generating their lineages, and so represents a reacquisition of engrailed expression within identified clones. The secondary headspot cells are neurons which direct axonal processes into the developing optic tract and so contribute to the primary axon scaffold of the brain. From our analysis of their ontogeny, we conclude that the secondary headspot cells do not represent a segmental border in the brain.

Evolution of Ftz protein function in insects

The Drosophila gene fushi tarazu (ftz) encodes a homeodomain-containing transcriptional regulator (Ftz) required at several stages during development. Drosophila melanogaster ftz (Dm-ftz) is first expressed in seven stripes defining alternate parasegments of the embryo—a “pair-rule” segmentation function [1, 2]. It is then expressed in specific neural precursor cells in the central nervous system and finally in the developing hindgut [3]. An Orthopteran ortholog of ftz (Sg-ftz, formally Dax) has been isolated from the grasshopper Schistocerca gregaria[4]. The pattern of Sg-ftz expression in Schistocerca embryos suggests that some developmental roles of the ftz gene are likely to be conserved between these two species (e.g., CNS functions) while others may have diverged (e.g., segmentation functions). To test whether the function of the Ftz protein itself differs between these two species, here we compare the functions of Sg-Ftz and Dm-Ftz proteins by expressing both in Drosophila embryos. Sg-ftz mimics only poorly several segmentation roles of Dm-ftz (engrailed activation, wingless repression, and embryonic cuticle transformation). However, the two proteins are similarly active in the rescue of a CNS-specific ftz mutant. These findings argue that this ftz CNS function is mediated by conserved parts of the protein, while efficient pair-rule function requires sequences present specifically in the Drosophila protein.

Maternal expression and early zygotic regulation of the Hox3/zen gene in the grasshopper Schistocerca gregaria.

In insects, a key step in the early patterning of the egg is to distinguish the primordium of the embryo proper from those regions that will form extra-embryonic membranes. In Drosophila, where these processes are well understood, the structure of the extra-embryonic membranes is highly derived. The distinct amnion and serosa typical of lower insects is replaced by a single, fused, and much reduced membrane, the amnioserosa, which never secretes an embryonic cuticle. We have used the Zen gene as a marker to study the formation of the extra-embryonic membranes, and other aspects of early embryonic patterning, in the grasshopper Schistocerca gregaria (African Plague Locust). Zen genes are derived from Hox genes, but in Drosophila they appear to have lost any role in patterning the A/P axis of the embryo; instead, they are involved in D/V patterning and the specification of the extra-embryonic membranes. We show that the Schistocerca zen gene is expressed during embryogenesis in three distinct phases. The first of these is during cleavage, when Sgzen is transiently expressed in all energids that reach the cell surface. The second phase of expression initiates in a ring of "necklace cells" that surround the forming embryo, and demarcate the boundary between the amnion and serosa. This leads to expression throughout the serosa. The final phase of expression is in the amnion, after this has separated from the serosa. This complex pattern implies that the role of Sgzen in Schistocerca is not limited solely to the specification of cell identity in the extra-embryonic membranes. We also report that the Schistocerca zen gene is expressed maternally, unlike its Drosophila and Tribolium counterparts. A distinct maternal transcript, and maternal Zen protein, accumulate in the developing oocyte from early post-meiotic stages. They remain uniformly distributed in the oocyte cytoplasm until late vitellogenic stages, when the protein and RNA become somewhat concentrated at the egg cortex and in the posterior polar cap of the oocyte, probably by passive exclusion from the yolk. The cytoplasmic localization of Sgzen protein in the oocyte, and at some stages during embryogenesis, implies that nuclear exclusion of this transcription factor is specifically controlled.

Building the antennal lobe: engrailed expression reveals a contribution from protocerebral neuroblasts in the grasshopper Schistocerca gregaria

The expression pattern of the engrailed protein was studied in neuroblasts which delaminate at the border of the protocerebrum and antennal lobe of the deutocerebrum in the early embryonic brain of the grasshopper. The antennal lobe is a complex structure comprising both glomerular and non-glomerular components, a cellular organization which distinguishes it from the striate-like neuropil comprising the remainder of the deutocerebrum. Early in embryogenesis engrailed expression in the protocerebrum is restricted to a compact block of neuroblasts located at its interface with the antennal lobe. Subsequently engrailed expression in these cells disappears in a stepwise manner from anterior to posterior so that by 37% of embryogenesis only a single row of three engrailed positive neuroblasts and their progeny remains. Contemporaneously engrailed expression reappears in a group of more anterior progeny deriving from neuroblasts which are no longer immunoreactive. The three remaining engrailed positive neuroblasts then become separated from their non-immunoreactive neighbours by an invagination of the perineurium called the lateral cleft and come to lie completely within the developing antennal lobe. These cells then direct columns of immunoreactive progeny centrifugally towards the centre of the lobe. Such a protocerebral contribution to the antennal lobe suggests that the evolution and ontogeny of this brain region need to be reconsidered.

Lazarillo expression reveals a subset of neurons contributing to the primary axon scaffold of the embryonic brain of the grasshopper Schistocerca gregaria.

The authors studied the contribution of seven clusters of Lazarillo-expressing cells to the primary axon scaffold of the brain in the grasshopper Schistocerca gregaria from 26% to 43% of embryogenesis. Each cluster, which was numbered according to when Lazarillo expression first appeared, was uniquely identifiable on the basis of its stereotypic position in the brain and the number of Lazarillo-expressing cells it contained. At no time during embryogenesis was Lazarillo expression found in brain neuroblasts: It was found only in progeny. For ease of analysis, axogenesis was followed in a cell cluster that contained only a single Lazarillo-expressing cell (the lateral cell) in the dorsal median domain of the brain midline. Bromodeoxyuridine incorporation revealed the presence of only a single midline precursor cell in this region during embryogenesis. Intracellular injection of Lucifer yellow into the lateral cell at various ages showed that there was no dye coupling to the midline precursor or to the nearby term-1-expressing primary commissure pioneers. The lateral cell is not related lineally to these cells and most likely differentiates directly from the neuroectoderm of the brain midline. Lazarillo expression appears at the onset of axogenesis as the lateral cell projects an axon laterally toward the next Lazarillo-expressing cell cluster. The cells of this target cluster direct axons into separate brain regions, thereby establishing an orthogonally organized scaffold that the lateral cell axon follows as it navigates away from the brain midline. The primary axon scaffold of the brain results from a stepwise interlinking of discrete brain regions, as exemplified by axons from neighboring Lazarillo-expressing cell clusters.

Synaptic input from serial chordotonal organs onto segmentally homologous interneurons in the grasshopper Schistocerca gregaria

We investigated the synaptic inputs from the serially homologous pleural, tympanal and wing-hinge chordotonal organs onto a set of identified homologous interneurons (714, 539, 529) in the ventral nerve cord of the grasshopper Schistocerca gregaria. Cobalt backfills show that afferents from all chordotonal organs project into stereotypic tracts in the central nervous system in which intracellular staining reveals the interneurons to have dendritic arborizations. Neuron 714 was found to receive excitatory bilateral synaptic input from all the serial chordotonal organs tested, from the second thoracic segment down to the seventh abdominal segment. Neuron 531, by contrast, only receives input from the chordotonal afferents on the first abdominal segment; those on the axon side are excitatory, while those on the soma side are inhibitory. The pattern of chordotonal input onto neuron 529 is similar to that seen for neuron 714, with the exception that neuron 529 receives no input from the forewing chordotonal organs. The pattern of afferent connectivities onto neurons 714, 531 and 529 differs with respect to those afferents which synapse directly or indirectly with the respective neuron. The synaptic inputs demonstrate a segmental specialization in the chordotonal system and thereby offer an insight into information processing in a modular sensory system.

Neurogenesis in the median domain of the embryonic brain of the grasshopperSchistocerca gregaria

Embryonic development in the median domain of the brain of the grasshopper Schistocerca gregaria was investigated with immunohistochemical, histological, and intracellular dye injection techniques. The early head midline is divisible into a dorsal median domain and a ventral median domain based on the orientation of cell somata in each region. At 25% of embryogenesis, a single large midline precursor differentiates in the dorsal median domain and produces a lineage of six neuronal progeny before degenerating. No further precursors arise. In addition, the primary commissure pioneers and a pair of lateral neurons differentiate directly from the ectoderm in this region. Lucifer yellow dye injected into the midline precursor stains only this cell and its progeny. Similarly, there is no dye coupling from the primary commissure pioneers to the midline lineage or to neuroblasts of the brain hemispheres. Neurogenesis in the dorsal median domain therefore proceeds separately within each subset of cells, and is not related to development in the brain hemispheres. Beginning at 42% of embryogenesis, the primary commissure pioneers undergo a morphological transformation and concomittantly express the Term-1 antigen. Expression continues throughout embryogenesis and into the adult, where the midline primary commissure pioneer cells are the only ones labeled by Term-1 in the entire brain. The cellular organization of the dorsal median domain therefore remains remarkably conserved throughout embryogenesis, even as the brain undergoes extensive morphological transformation.

Segmental differentiation processes in embryonic muscle development of the grasshopper

To analyse segmental differentiation processes in muscle development, we studied the embryogenesis of the ventral body wall muscles in thoracic and abdominal segments of the grasshopper Schistocerca gregaria at the identified cell level. We visualized differentiating muscle pioneer and muscle precursor cells by staining with a muscle-specific monoclonal antibody and with rhodamine-coupled phalloidin. Our results show that a similar pattern of serially reiterated early muscle pioneers is initially established in all segments. Subsequently, two major segmental differentiation processes occur. First, segment-specific sets of additional, later differentiating muscle pioneers are generated de novo. Second, segment-specific sets of existing early muscle precursors are eliminated through atrophy and eventual loss. These events have consequences for matching homonomy of muscles and their innervating motoneurons. Taken together, these processes in the embryo, in concert with postembryonic differentiation events, play critical roles in shaping the highly specialized muscular structures of the mature animal.

Axogenesis in the embryonic brain of the grasshopper Schistocerca gregaria: an identified cell analysis of early brain development.

Axogenesis in the embryonic brain was studied at the single cell level in the grasshopper Schistocerca gregaria. A small set of individually identifiable pioneer neurons establishes a primary axon scaffold during early embryogenesis. At the beginning of scaffold formation, pioneering axons navigate along and between glial borders that surround clusters of proliferating neuroblasts. In each brain hemisphere, an axonal outgrowth cascade involving a series of pioneer neurons establishes a pathway from the optic ganglia to the brain midline. At the midline the primary preoral commissural interconnection in the embryonic brain is pioneered by a pair of midline-derived pioneer neurons. A second preoral commissural connection is pioneered by two pairs of pars intercerebralis pioneer neurons. Descending tracts are pioneered by the progeny of identified neuroblasts in the pars intercerebralis, deutocerebrum and tritocerebrum; the postoral tritocerebral commissure is pioneered by a pair of tritocerebral neurons. All of the pioneering brain neurons express the cell adhesion molecule fasciclin I during initial axon outgrowth and fasciculation. Once established, the primary axon scaffold of the brain is used for fasciculation by subsequently differentiating neurons and, by the 40% stage of embryogenesis, axonal projections that characterize the mature brain become evident. The single cell analysis of grasshopper brain development presented here sets the stage for manipulative cell biological experiments and provides the basis for comparative molecular genetic studies of embryonic brain development in Drosophila.

Dax, a locust Hox gene related to fushi-tarazu but showing no pair-rule expression.

We describe an unusual Antennapedia class homeobox gene from the grasshopper Schistocerca gregaria (Orthoptera, African Plague Locust). Its sequence is not sufficiently similar to that of any other insect Hom-Hox gene to identify it unambiguously, but short conserved elements suggest a relationship to the segmentation gene fushi-tarazu, (ftz). We term it Sg Dax (divergent Antennapedia class homeobox gene). Antibodies raised against the protein encoded by this gene reveal two phases of expression during embryogenesis. In the early embryo, it is a marker for the posterior part of the forming embryonic primordium, and subsequently for the posterior part of the growing germ band. In older embryos, it labels a subset of neural precursor cells in each trunk segment, very similar to that defined by the expression of fushi tarazu (ftz) in Drosophila. We suggest that Schistocerca Dax and Drosophila ftz are homologous members of a gene family whose members are diverging relatively rapidly, both in terms of sequence and role in early development.

Homologous patterns in the embryonic development of the peripheral nervous system in the grasshopper Schistocerca gregaria and the fly Drosophila melanogaster

To determine the generality of developmental mechanisms involved in the construction of the insect nervous system, the embryonic development of the peripheral nervous system in the grasshopper Schistocerca gregaria was characterized at the level of identified neurons and nerve branches and then compared to that previously described from the fly Drosophila melanogaster. For this, immunocytochemistry using a neuron-specific antibody was carried out on staged grasshopper embryos. Our results show that initially a simple peripheral nerve scaffolding is established in each segment of the animal. This scaffolding consists of a pair of intersegmental nerves that are formed by identified afferent and efferent pioneer neurons and a pair of segmental nerves that are formed by afferent pioneers situated in limb buds. Subsequently, identified sets of sensory neurons differentiate in a stereotyped spatiotemporal pattern in dorsal, lateral and ventral clusters in each segment and project their axons onto these nerves. Although segment-specific differences exist, serial homologs of the developing nerves and sensory neurons can be identified. A comparison of these results with those obtained from Drosophila shows that virtually the same pattern of peripheral nerves and sensory structures is formed in both species. This indicates that the construction of the peripheral nervous system in extremely divergent modern insects relies on conserved developmental mechanisms that evolved in ancestral insects over 300 million years ago.