现存蜉蝣目昆虫的原始特征和独特性状

现存蜉蝣具有一系列引人注目的特征 ,它们为重建原始昆虫模式、探讨翅的起源、脉相的演化、附肢的演变等起到重要作用。该文系统总结了现生蜉蝣具有的原始特征和独特性状 (如原变态、稚虫水生、交尾行为、脉相、翅面皱褶、翅位、较多的附肢和蜕皮次数以及口器、生殖系统等等 ) ,并讨论比较了有关这些特征起源和演化的主要观点。     

Formation of the insect head involves lateral contribution of the intercalary segment, which depends on Tc-labial function

The insect head is composed of several segments. During embryonic development, the segments fuse to form a rigid head capsule where obvious segmental boundaries are lacking. Hence, the assignment of regions of the insect head to specific segments is hampered, especially with respect to dorsal (vertex) and lateral (gena) parts. We show that upon Tribolium labial (Tc-lab) knock down, the intercalary segment is deleted but not transformed. Furthermore, we find that the intercalary segment contributes to lateral parts of the head cuticle in Tribolium. Based on several additional mutant and RNAi phenotypes that interfere with gnathal segment development, we show that these segments do not contribute to the dorsal head capsule apart from the dorsal ridge. Opposing the classical view but in line with findings in the vinegar fly Drosophila melanogaster and the milkweed bug Oncopeltus fasciatus, we propose a “bend and zipper” model for insect head capsule formation.     

Morphology of the nasal capsule of Heloderma suspectum with comments on the systematic position of helodermatids (Squamata: Helodermatidae)

A serially sectioned embryonic head of Heloderma suspectum formed the basis for a three-dimensional reconstruction of the cartilaginous nasal capsule and its membrane bones. Further, the soft parts of the nasal capsule were analysed microscopically. The embryonic nasal capsule is described and compared to the morphological conditions in other lizards, especially the varanids, which many recent authors believe to be their closest relatives.
The results support the close affinities between the helodermatids and the varanids. Both groups have a well developed recessus lateralis in a cartilaginous capsule, entered by the nasal concha. Further, in both groups a fragmentation of the cartilago paraseptalis occurs in a comparable location and form. Heloderma has no double lacrimal duct, in contrast to previous accounts, but the two canaliculi at the origin of the duct fuse noteworthy far ahead (in other squamates they fuse in front of the bulbus oculi). In the varanids the two branches of the lacrimal duct remain separate and open separately.
Some unique features of Heloderma are presented as well, above all the presence of a second concha.     

Morphological and molecular data argue for the labrum being non-apical, articulated, and the appendage of the intercalary segment in the locust

Our analysis of head segmentation in the locust embryo reveals that the labrum is not apical as often interpreted but constitutes the topologically fused appendicular pair of appendages of the third head metamere. Using molecular, immunocytochemical and retrograde axonal staining methods we show that this metamere, the intercalary segment, is innervated by the third brain neuromere-the tritocerebrum. Evidence for the appendicular nature of the labrum is firstly, the presence of an engrailed stripe within its posterior epithelium as is typical of all appendages in the early embryo. Secondly, the labrum is innervated by a segmental nerve originating from the third brain neuromere (the tritocerebrum). Immunocytochemical staining with Lazarillo and horseradish peroxidase antibodies reveal that sensory neurons on the labrum contribute to the segmental (tritocerebral) nerve via the labral nerve in the same way as for the appendages immediately anterior (antenna) and posterior (mandible) on the head. All but one of the adult and embryonic motoneurons innervating the muscles of the labrum have their cell bodies and dendrites located completely within the tritocerebral neuromere and putatively derive from engrailed expressing tritocerebral neuroblasts. Molecular evidence (repo) suggests the labrum is not only appendicular but also articulated, comprising two jointed elements homologous to the coxa and trochanter of the leg.     

A biomechanical analysis of prognathous and orthognathous insect head capsules: evidence for a many‐to‐one mapping of form to function

Insect head shapes are remarkably variable, but the influences of these changes on biomechanical performance are unclear. Among ‘basal’ winged insects, such as dragonflies, mayflies, earwigs and stoneflies, some of the most prominent anatomical changes are the general mouthpart orientation, eye size and the connection of the endoskeleton to the head. Here, we assess these variations as well as differing ridge and sclerite configurations using modern engineering methods including multibody dynamics modelling and finite element analysis in order to quantify and compare the influence of anatomical changes on strain in particular head regions and the whole head. We show that a range of peculiar structures such as the genal/subgenal, epistomal and circumocular areas are consistently highly loaded in all species, despite drastically differing morphologies in species with forward‐projecting (prognathous) and downward‐projecting (orthognathous) mouthparts. Sensitivity analyses show that the presence of eyes has a negligible influence on head capsule strain if a circumocular ridge is present. In contrast, the connection of the dorsal endoskeletal arms to the head capsule especially affects overall head loading in species with downward‐projecting mouthparts. Analysis of the relative strains between species for each head region reveals that concerted changes in head substructures such as the subgenal area, the endoskeleton and the epistomal area lead to a consistent relative loading for the whole head capsule and vulnerable structures such as the eyes. It appears that biting‐chewing loads are managed by a system of strengthening ridges on the head capsule irrespective of the general mouthpart and head orientation. Concerted changes in ridge and endoskeleton configuration might allow for more radical anatomical changes such as the general mouthpart orientation, which could be an explanation for the variability of this trait among insects. In an evolutionary context, many‐to‐one mapping of strain patterns onto a relatively similar overall head loading indeed could have fostered the dynamic diversification processes seen in insects.     

Embryogenesis of a springtail,Tomocerus ishibashii (Collembola,tomoceridae): External morphology

The external features of the developing embryos of the springtail, Tomocerus ishibashii, are described. The clypeolabral anlage arises as a single, unpaired swelling. The entognathy is completed by the ventral growth of the tergal anlagen of mandibular, maxillary, and labial segments. These anlagen also form the posterior part of the cranium. The palpi of maxilla and labium are homologous with the telopodites, and proximal parts of these head appendages are homologous with the coxopodites. The sternal element of the labial segment does not participate in the postmentum formation. The anlagen of abdominal appendages appear in the first to the fourth abdominal segments. The first, third, and fourth appendage anlagen form the ventral tube, tenaculum, and furcula, respectively. The fused proximal parts of the first, third, and fourth appendage anlagen are homologous with the coxopodites, and the distal parts which do not fuse are homologous with the telopodites. The anlagen of the second abdominal appendages become flattened and spread over the ventral side of this segment. The ventral structures of the first four abdominal segments are appendicular in origin.     

A scanning electron microscope study of the developing embryo of Manduca sexta (L.) (Lepidoptera : Sphingidae)

Scanning electron microscopy of the developing Manduca sexta (Lepidoptera : Sphingidae) embryo reveals that the body wall of the insect undergoes considerable morphogenesis beginning at 20 hr post-oviposition. The elongated 19 hr embryo contracts in length, which gives rise to the formation of rudimentary segments. By 33 hr, many of the appendage anlagen are visible, the presumptive spiracles appear as bifurcate pits and the proctodeum begins invagination. During this same period, prior to katatrepsis, the body walls become established, and the segments and appendages develop. Between 50 and 60 hr post-oviposition, involution of the oral cavity and reorientation of the associated gnathal appendages occurs. During this same period, katatrepsis and provisional dorsal closure take place. Developmental polarity is evident as a distinctive wave of specialization proceeding posterior to anterior in the thorax/abdomen, and anterior to posterior in the head. Configuration of the oral cavity is strikingly prognathous until just prior to eclosion. Two embryonic molts are apparent, as determined by the remnants of ecdysed “embryonic cuticles”.     

Double-stranded RNA interference in the spider Cupiennius salei: the role of Distal-less is evolutionarily conserved in arthropod appendage formation

Chelicerates represent a basal arthropod group, which makes them an excellent system for the study of evolutionary processes in arthropods. To enable functional studies in chelicerates, we developed a double-stranded RNA-interference (RNAi) protocol for spiders while studying the function of the Distal-less gene. We isolated the Distal-less gene from the spider Cupiennius salei. Cs-Dll gene expression is first seen in cells of the prosomal segments before the outgrowth of the appendages. After the appendages have formed, Cs-Dll is expressed in the distal portion of the prosomal appendages, and in addition, in the labrum, in two pairs of opisthosmal (abdominal) limb buds, in the head region, and at the posterior-most end of the spider embryo. In embryos, in which Dll was silenced by RNAi, the distal part of the prosomal appendages was missing and the labrum was completely absent. Thus, Dll also plays a crucial role in labrum formation. However, the complete lack of labrum in RNAi embryos may point to a different nature of the labrum from the segmental appendages. Our data show that the expression of Dll in the appendages is conserved among arthropods, and furthermore that the role of Dll is evolutionarily conserved in the formation of segmental appendages in arthropods.     

A revision of brain composition in Onychophora (velvet worms) suggests that the tritocerebrum evolved in arthropods

Background  

The composition of the arthropod head is one of the most contentious issues in animal evolution. In particular, controversy surrounds the homology and innervation of segmental cephalic appendages by the brain. Onychophora (velvet worms) play a crucial role in understanding the evolution of the arthropod brain, because they are close relatives of arthropods and have apparently changed little since the Early Cambrian. However, the segmental origins of their brain neuropils and the number of cephalic appendages innervated by the brain - key issues in clarifying brain composition in the last common ancestor of Onychophora and Arthropoda - remain unclear.     

A conserved genetic mechanism specifies deutocerebral appendage identity in insects and arachnids

The segmental architecture of the arthropod head is one of the most controversial topics in the evolutionary developmental biology of arthropods. The deutocerebral (second) segment of the head is putatively homologous across Arthropoda, as inferred from the segmental distribution of the tripartite brain and the absence of Hox gene expression of this anterior-most, appendage-bearing segment. While this homology statement implies a putative common mechanism for differentiation of deutocerebral appendages across arthropods, experimental data for deutocerebral appendage fate specification are limited to winged insects. Mandibulates (hexapods, crustaceans and myriapods) bear a characteristic pair of antennae on the deutocerebral segment, whereas chelicerates (e.g. spiders, scorpions, harvestmen) bear the eponymous chelicerae. In such hexapods as the fruit fly, Drosophila melanogaster, and the cricket, Gryllus bimaculatus, cephalic appendages are differentiated from the thoracic appendages (legs) by the activity of the appendage patterning gene homothorax (hth). Here we show that embryonic RNA interference against hth in the harvestman Phalangium opilio results in homeonotic chelicera-to-leg transformations, and also in some cases pedipalp-to-leg transformations. In more strongly affected embryos, adjacent appendages undergo fusion and/or truncation, and legs display proximal defects, suggesting conservation of additional functions of hth in patterning the antero-posterior and proximo-distal appendage axes. Expression signal of anterior Hox genes labial, proboscipedia and Deformed is diminished, but not absent, in hth RNAi embryos, consistent with results previously obtained with the insect G. bimaculatus. Our results substantiate a deep homology across arthropods of the mechanism whereby cephalic appendages are differentiated from locomotory appendages.     

Early Cambrian arthropods—new insights into arthropod head and structural evolution

The Cambrian Maotianshan-Shale lagerstätten have yielded fossils of a great variety of taxa, particularly arthropods. We report further information on two of these arthropods, Fuxianhuia protensa and Chengjiangocaris longiformis and present a new species, Shankouia zhenghei. Their morphologies shed new light on the early evolution of Arthropoda, which can be resolved into three major phases. In the first phase, new features such as segmental uniramous limbs were developed. The second phase is characterized by segmental sclerotizations of body and appendages. In this phase, the head included only the segments bearing lateral eyes and antennae. A shield-like structure is recognized as the expanded tergite of the second head segment. This tergite housed a pair of sac-like structures. Formerly interpreted as appendages in Fuxianhuia, they are regarded here as gut diverticula. Behind the head, all tergites free from the shield-like one are tripartite, made of an elevated middle region and pleurotergites. All post-antennal limbs comprise a multi-annulated rod and an exopod flap. The antenna is limb-shaped and was, most likely, the only structure suitable for food gathering. Remarkably, the two head segments are those anterior segments, which are not expressed by Hox genes. The third step is the development of euarthropod characteristics.     

Head segmentation in the embryo of the Colorado beetleLeptinotarsa decemlineata as seen with anti-en immunostaining

Segmentation in the head of the embryo of the Colorado beetleLeptinotarsa decemlineata is described on the basis of anti-engrailed (en) immunostaining of germ band stages. Six segmental units can be identified with this technique. Three segmentalen stripes can be distinguished in the gnathal region, a weak stripe interrupted medially shows the intercalary segment rudiment, a pair of oblique stripes indicate the antennal segment, and one pair of preantennalen spots are taken to indicate a sixth segment. In the broad head lobes of the beetle the spacing of the six segmental units as demarcated byen regions is similar to that in other parts of the germ band. The results are discussed with respect to old and new data concerning the number of head segments and origin of the compound eye in insects.     

Segmental organisation of the head in the embryo of Drosophila melanogaster

Summary Embryos of Drosophila melanogaster were irradiated in the presumptive head region with a UV-laser microbeam of 20 m diameter at two developmental stages, the cellular blastoderm and the extended germ band. The ensuing defects were scored in the cuticle pattern of the head of the first-instar larva, which is described in detail in this paper. The defects caused by irradiating germ band embryos when morphologically recognisable lobes appear in the head region were used to establish the segmental origin of various head structures. This information enabled us to translate the spatial distribution of blastoderm defects into a fate map of segment anlagen. The gnathal segments derive from a region of the blastoderm between 60% and 70% egg length (EL) dorsally and 60% and 80% ventrally. The area anterior to the mandibular anlage and posterior to the stomodaeum is occupied by the small anlagen of the intercalary and antennal segments ventrally and dorsally, respectively. The labrum, which originates from a paired anlage dorsally at 90% EL, is separated from the remaining head segments by an area for which we did not observe cuticle defects following blastoderm irradiation, presumably because those cells give rise to the brain. The dorsal and lateral parts of the cephalo-pharyngeal skeleton appear to be the only cuticle derivatives of the non-segmental acron. These structures derive from a dorso-lateral area just behind the putative brain anlage and may overlap the latter. In addition to the segment anlagen, the regions of the presumptive dorsal pouch, anterior lobe and post-oral epithelium, whose morphogenetic movements during head involution result in the characteristic acephalic appearance of the larva, have been projected onto the blastoderm fate map. The results suggest that initially the head of the Drosophila embryo does not differ substantially from the generalised insect head as judged by comparison of fate map and segmental organisation.     

Biology of the Hyolitha

Hyoliths are Paleozoic fossils that have a calcareous exoskeleton consisting of an elongate, usually bilaterally symmetrical cone, a close fitting operculum, and a pair of curved appendages. Their skeletal ultrastructure resembles the crossed-lamellar shell layers of some molluscs. Several specimens from the Ordovician of France and the Cambrian of Antarctica have parts of the gut preserved by infilling matrix, showing that both mouth ad anus were located near the cone aperture. Muscle scars in other hyolith shells indicate that the animal had a series of dorsoventral and longitudinal, or longitudinal and circular muscles, which operated through a hydrostatic skeleton to protract and retract the head, to open and close the operculum, and to move the appendages. Although the shell form and skeletal ultra-structure of hyoliths are of a molluscan type, the muscle insertions suggest that the hyolith cone is not homologous with the dorsal exoskeleton of primitive molluscs. Hyoliths probably constitute a small extinct branch of phylum size, related to the Mollusca and the Sipunculoidea. All three groups may have had common ancestors in the late Precambrian.     

Captopodus poschmanni gen. et sp. nov. a new stem-group arthropod from the Lower Devonian Hunsrück Slate (Germany)

A new arthropod from the Lower Devonian Hunsrück Slate is described on the basis of four specimens. The body of Captopodus poschmanni comprises a head, a trunk with an anal portion. The high number of trunk appendages (≥66 segments) is unusual. The function of one pair of cupola-like structures of the head shield is unclear. The presence of large grasping appendages in the head superficially resembles the ‘short great appendages’ of other euarthropods and grasping appendages of thylacocephalans. The phylogenetic position of the arthropod cannot be determined in detail, though several morphological aspects indicate a phylogenetic position as a stem lineage representative of the Euarthropoda, the morphology of the trunk appendages seem to indicate a more advanced phylogenetic position. This new taxon underlines the exceptional diversity of arthropods within the Hunsrück Slate in comparison to other Devonian fossil sites and highlights the significance of the Hunsrück Slate for the evolution of early arthropods.     

Homeotic evidence for the appendicular origin of the labrum in Tribolium castaneum

The ontogeny of the insect labrum, or upper lip, has been debated for nearly a century. Recent molecular data suggest a segmental appendage origin of this structure. Here we report the first arthropod mutation associated with a homeotic transformation of the labrum. Antennagalea-5 (Ag(5)) transforms both antennal and labral structures to resemble those of gnathal appendages in Tribolium castaneum. This labral transformation suggests that the labrum is a fused structure composed of two pairs of appendage endites, and is serially homologous to the gnathal appendages.     

The origin and evolution of appendages

Current awareness of gene expression patterns and developmental mechanisms involved in the outgrowth and patterning of animal appendages contributes to our understanding of the origin and evolution of these body parts. Nevertheless, this vision needs to be complemented by a new adequate comparative framework, in the context of a factorial notion of homology. It may even be profitable to categorize as appendages also gut diverticula, body ingrowths and 'virtual appendages' such as the eye spots on butterfly wings. Another unwarranted framework is the Cartesian co-ordinate system onto which the appendages are currently described and where it is supposed that one patterning system exists for each separate Cartesian axis. It may be justified, instead, to look for correspondences between the appendages and the main body axis of the same animal, as the latter might be the source of the growth and patterning mechanisms which gave rise to the former. This hypothesis of axis paramorphisms is contrasted with the current hypothesis of gene co-option. Recapitulationism is a common fault in current Evo-Devo perspectives concerning the origin of the appendages, in that the evolutionary origin of appendages is often expected to be the same as one of the key mechanisms involved in the ontogenetic inception of appendage formation. This unwarranted perspective is also evident in the current debate on the nature of the default arthropod appendage. Most likely, a default arthropod appendage never did exist, as the first appendages probably developed along the trunk of an animal already patterned extensively along the antero-posterior body axis.     

The origin,patterning and evolution of insect appendages

The appendages of the adult fruit fly and other insects and Arthropods develop from secondary embryonic fields that form after the primary anterior/posterior and dorsal/ventral axes of the embryo have been determined. In Drosophila, the position and fate of the different fields formed within each segment are determined by genes acting along both embryonic axes, within individual segments, and within specific fields. Since the major architectural differences between most Arthropod classes and orders involve variations in the number, type and morphology of body appendages, the elucidation of the embryology and molecular genetics of the origin and patterning of insect limb fields may help to facilitate an understanding of both the mechanism of appendage formation and some of the major steps in the morphological evolution of the Arthropods. In this review, we will discuss recent studies that have advanced our understanding of both the origin and patterning of Drosophila leg and wing secondary fields. These results provide fresh insights into potentially general mechanisms of how body parts develop and evolve.     

Sclerotomal origin of the ribs

The somites of vertebrate embryos give rise to sclerotomes and dermomyotomes. The sclerotomes form the axial skeleton, whereas the dermomyotomes give rise to all trunk muscles and the dermis of the back. The ribs were thought to be ventral processes of the axial skeleton and therefore to be derived from the sclerotomes; however, recently a dermomyotomal origin of the distal rib (the costal shaft) was suggested, with only the proximal parts (head and neck of the rib) being of sclerotomal origin. We have re-investigated the development of the ribs in quail-chick chimeras and carried out three experimental series. (1) Single dermomyotomes and (2) single sclerotomes were grafted homotopically, and (3) the ectoderm overlying the unsegmented paraxial mesoderm was removed in the prospective thoracic region. We found that the cells of the dermomyotome gave rise to epaxial and hypaxial trunk muscles, dermis of the back and endothelial cells, but not to ribs. Cells of the sclerotome formed the axial skeleton and all parts of the ribs. Ablation of the ectoderm, which affects dermomyotome development, results in severe malformations of the ribs, probably due to disturbed interactions between dermomyotome and sclerotome. Our results strongly confirm the traditional view of the sclerotomal origin of the ribs.