Circulatory organs of Diplura (Hexapoda): the basic design in Hexapoda?

The circulatory systems of Campodea augens and Catajapyx aquilonaris (Hexapoda: Diplura) have been examined by means of light and electron microscopy. Hemolymph flow has also been investigated in vivo. Both species share features that deviate conspicuously from the common textbook design of the insect circulatory system: (i) antennal vessels connected to the anterior end of the dorsal vessel; (ii) presence of a circumoesophageal vessel ring in the head; (iii) a bidirectional flow within the dorsal vessel, made possible by intracardiac valves; (iv) posterior end of the dorsal vessel tube opens into a caudal chamber connected to cercal vessels (in Campodea) or to cercal channels (in Catajapyx); (v) dorsal diaphragm barely realized, ventral diaphragm absent altogether, and (vi) legs without specific organs serving hemolymph circulation. Comparative analysis has revealed that these characters in Diplura represent the most plesiomorphic condition in the circulatory organs of all extant Hexapoda. In the basic evolutionary lineages of insects, some organ components have been lost and the peripheral vessels decoupled from the dorsal vessel; as a result, autonomous accessory pulsatile organs have evolved to supply hemolymph to long body appendages and a unidirectional hemolymph flow mode prevailed within the dorsal vessel.     

Beyond aerodynamics: The critical roles of the circulatory and tracheal systems in maintaining insect wing functionality

Insect wings consist almost entirely of lifeless cuticle; yet their veins host a complex multimodal sensory apparatus and other tissues that require a continuous supply of water, nutrients and oxygen. This review provides a survey of the various living components in insect wings, as well as the specific contribution of the circulatory and tracheal systems to provide all essential substances. In most insects, hemolymph circulates through the veinal network in a loop flow caused by the contraction of accessory pulsatile organs in the thorax. In other insects, hemolymph oscillates into and out of the wings due to the complex interaction of several factors, such as heartbeat reversal, intermittent pumping of the accessory pulsatile organs in the thorax, and the elasticity of the wall of a special type of tracheae. A practically unexplored subject is the need for continuous hydration of the wing cuticle to retain its flexibility and toughness, including the associated problem of water loss due to evaporation. Also, widely neglected is the influence of the hemolymph mass and the circulating flow in the veins on the aerodynamic properties of insect wings during flight. Ventilation of the extraordinarily long wing tracheae is probably accomplished by intricate interactions with the circulatory system, and by the exchange of oxygen via cutaneous respiration.     

The insect abdomen--a heartbeat manager in insects?

Different possibilities of coordination between circulation, respiration and abdominal movements were found in pupae of Pieris brassicae, Tenebrio molitor, Galleria mellonella and Leptinotarsa decemlineata. Coordination principles depend on metabolic rate: the need to support circulation with abdominal movements appears only at higher metabolic rates. Integration between different abdominal movements and circulation depends on species, on physiological state and, supposedly, on internal morphology. At low metabolic rates, there is no need for a very intensive hemolymph flow, and the dorsal vessel is capable of initiating and/or maintaining necessary hemolymph flow. Starting from a certain metabolic level, it is possible that the abdomen is used to accelerate hemolymph flow in the case of a large amount of hemolymph. When the necessary flow speed has been reached, relatively weak pulsation of the dorsal vessel with accessory pulsatile organs and diaphragms can easily maintain the necessary flow intensity. Heart activity may sometimes be initiated by abdominal movements via cardiac reflex or mechanical excitation. Sometimes, when heart function is weakened by histolysis, the abdomen may temporarily take over the main circulatory function or occasionally contribute to acceleration of low-speed hemolymph flow. In this case the functions are simultaneous and may be triggered by some mediator(s). In active adult insects the whole body is moving, and hence hemolymph circulates and the tracheal system is effectively ventilated by a whole body ensemble consisting of the dorsal vessel, moving organs, body appendages and accessory pulsatile organs. The mechanism of autocirculation (analogous to autoventilation in gas exchange) is a probable mechanism in circulation in adult insects.     

THE CIRCULATORY SYSTEM OF THE ALDER FLY SIALIS LUTARIA

This is a comprehensive account of the circulatory system of all stages of Sialis lutaria L. The circulatory organs and pathways are described. In the larva an organ for circulating blood within the terminal segment is described. In the adult, blood vessels are described in the appendages and over the surface of the brain. Associated with these vessels are two types of accessory circulatory organs, pulsatile and valvular. The rates of heart beat, of circulation and pulsation of the scutellar organs are recorded.     

Circulatory organs of abdominal appendages in primitive insects (Hexapoda: Archaeognatha, Zygentoma and Ephemeroptera)

The abdominal circulatory organs and the haemolymph supply of the terminal filament and the cerci were investigated in a total of nine species (serial semithin sections, TEM, in vivo observations). In all investigated species, the dorsal vessel features a bidirectional flow. In the 10th abdominal segment, there is an intracardiac valve preventing flow in the anterior direction. The posterior portion of the dorsal vessel differs significantly from the anterior portion in design and wall structure. Pumping actions and frequencies do not correspond with each other in the two portions. Vessels supply haemolymph to the long terminal appendages. The terminal filament vessel is connected to the dorsal vessel, the two cercal vessels originate from a transverse septum at the base of the terminal filament. All taxa show the same flow pattern through the caudal appendages. Ephemeroptera have a unique spherical body at the posterior end of the dorsal vessel functioning as a backflow valve. The results indicate that the plesiomorphic state of the circulatory organs in the abdomen of primitive insects differ distinctly from that of higher insects which serve as bases for generalized schemes common to entomological textbooks.     

The controversial origin of the abdominal appendage‐like processes in immature insects: Are they true segmental appendages or secondary outgrowths? (Arthropoda hexapoda)

In this article, I review the major characteristics of different types of appendage‐like processes that develop at the abdominal segments of many immature insects, and I discuss their controversial morphological value. The main question is whether the abdominal processes are derived from segmental appendages serially homologous to thoracic legs, or whether they are “secondary” outgrowths not homologous with true appendages. Morphological and embryological data, in particular, a comparison with the structure and development of the abdominal appendages in primitive apterygote hexapods, and data from developmental genetics, support the hypothesis of appendicular origin of many of the abdominal processes present in the juvenile stages of various pterygote orders. For example, the lateral processes, such as the tracheal gills in aquatic nymphs of exopterygote insects, are regarded as derived from lateral portions of appendage primordia, homologous with the abdominal styli of apterygotan insects; these processes correspond either to rudimentary telopodites or to coxal exites. The ventrolateral processes, such as the prolegs of different endopterygote insect larvae, appear to be derived from medial portions of the appendicular primordia; they correspond to coxal endites. These views lead to the rejection of Hinton's hypothesis (Hinton [1955] Trans R Entomol Soc Lond 106:455–545) according to which all the abdominal processes of insect larvae are secondary outgrowths not derived from true appendage anlagen. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

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.     

Evolution of Insect Eyes: Tales of Ancient Heritage, Deconstruction, Reconstruction, Remodeling, and Recycling

The visual organs of insects are known for their impressive evolutionary conservation. Compound eyes built from ommatidia with four cone cells are now accepted to date back to the last common ancestor of insects and crustaceans. In species as different as fruit flies and tadpole shrimps, the stepwise cellular patterning steps of the early compound eye exhibit detailed similarities implying 500 million years of developmental conservation. Strikingly, there is also a cryptic diversity of insect visual organs, which gives proof to evolution’s versatility in molding even the most tenacious structures into something new. We explore this fascinating aspect in regard to the structure and function of a variety of different insect eyes. This includes work on the unique compound–single-chamber combination eye of twisted-winged insects and the bizarre evolutionary trajectories of specialized larval eyes in endopterygote insects.     

Molecular Evolution and Structural Features of IRAK Family Members

The interleukin-1 receptor-associated kinase (IRAK) family comprises critical signaling mediators of the TLR/IL-1R signaling pathways. IRAKs are Ser/Thr kinases. There are 4 members in the vertebrate genome (IRAK1, IRAK2, IRAKM, and IRAK4) and an IRAK homolog, Pelle, in insects. IRAK family members are highly conserved in vertebrates, but the evolutionary relationship between IRAKs in vertebrates and insects is not clear. To investigate the evolutionary history and functional divergence of IRAK members, we performed extensive bioinformatics analysis. The phylogenetic relationship between IRAK sequences suggests that gene duplication events occurred in the evolutionary lineage, leading to early vertebrates. A comparative phylogenetic analysis with insect homologs of IRAKs suggests that the Tube protein is a homolog of IRAK4, unlike the anticipated protein, Pelle. Furthermore, the analysis supports that an IRAK4-like kinase is an ancestral protein in the metazoan lineage of the IRAK family. Through functional analysis, several potentially diverged sites were identified in the common death domain and kinase domain. These sites have been constrained during evolution by strong purifying selection, suggesting their functional importance within IRAKs. In summary, our study highlighted the molecular evolution of the IRAK family, predicted the amino acids that contributed to functional divergence, and identified structural variations among the IRAK paralogs that may provide a starting point for further experimental investigations.     

Parthenogenesis in Hexapoda: Entognatha and non‐holometabolous insects

In hexapods, unlike the majority of animals, development without fertilization is a common phenomenon. They evolved a striking diversity of unisexual reproductive types that include a variety of modes starting from spontaneous parthenogenesis in females to the production of impaternate males with different variants in between. Many reports about parthenogenetic species have accumulated over time. Here, we present a review of various parthenogenetic hexapod groups with a particular focus on their chromosome systems and ploidy level. We show that conclusions about the reproductive mode often lack solid evidence and sometimes inefficiently demonstrate how parthenogenesis is maintained in corresponding groups. In this review, basal hexapods (Protura, Collembola, Diplura), primarily wingless insect groups (‘Apterygota’) and non‐holometabolous insects are listed with references to a variety of their unisexual reproductive modes.     

Association of tracheal placodes with leg primordia in Drosophila and implications for the origin of insect tracheal systems

Adaptation to diverse habitats has prompted the development of distinct organs in different animals to better exploit their living conditions. This is the case for the respiratory organs of arthropods, ranging from tracheae in terrestrial insects to gills in aquatic crustaceans. Although Drosophila tracheal development has been studied extensively, the origin of the tracheal system has been a long-standing mystery. Here, we show that tracheal placodes and leg primordia arise from a common pool of cells in Drosophila, with differences in their fate controlled by the activation state of the wingless signalling pathway. We have also been able to elucidate early events that trigger leg specification and to show that cryptic appendage primordia are associated with the tracheal placodes even in abdominal segments. The association between tracheal and appendage primordia in Drosophila is reminiscent of the association between gills and appendages in crustaceans. This similarity is strengthened by the finding that homologues of tracheal inducer genes are specifically expressed in the gills of crustaceans. We conclude that crustacean gills and insect tracheae share a number of features that raise the possibility of an evolutionary relationship between these structures. We propose an evolutionary scenario that accommodates the available data.     

Insect appendages and comparative ontogenetics

It is arguable that the evolutionary and ecological success of insects is due in large part to the versatility of their articulated appendages. Recent advances in our understanding of appendage development in Drosophila melanogaster, as well as functional and expression studies in other insect species have begun to frame the general themes of appendage development in the insects. Here, we review current studies that provide for a comparison of limb developmental mechanisms acting at five levels: (1) the specification of ventral appendage primordia; (2) specification of the limb axes; (3) regulation and interactions of genes expressed in specific domains of the proximal-distal axis, such as Distal-less; (4) the specification of appendage identity; and (5) genetic regulation of appendage allometry.     

Expression of engrailed-family genes in the jumping bristletail and discussion on the primitive pattern of insect segmentation

It has been shown that segmentation in the short-germ insects proceeds by a two-step mechanism. The anterior region is simultaneously segmented in a manner similar to that in Drosophila, which is apparently unique to insects, and the rest of the posterior region is segmented sequentially by a mechanism involving a segmentation clock, which is derived from the common ancestor of arthropods. In order to propose the evolutionary scenario of insect segmentation, we examined segmentation in the jumping bristletail, the basalmost extant insect. Using probes for engrailed-family genes for in situ hybridization, we found no sign of simultaneous segmentation in the anterior region of the jumping bristletail embryos. All segments except the anteriormost segment are formed sequentially. This condition shown in the jumping bristletail embryos may represent the primitive pattern of insect segmentation. The intercalating formation of the intercalary segment is assumed to be a synapomorphic trait shared among all insects after the branching of the jumping bristletail.     

Functional morphology and evolutionary aspects of unusual antennal circulatory organs in Labidura riparia Pallas (Labiduridae), Forficula auricularia L. and Chelidurella acanthopygia géné (Forficulidae) (Insecta : Dermaptera)

The antennal circulatory organs in the earwigs Labidura riparia Pallas (Labiduridae), Forficula auricularia L. and Chelidurella acanthopygia Géné (Forficulidae) (Dermaptera) represent a functional type that has not been found in other insects. An independent organ exists for each antenna, consisting of a pulsatile ampulla connected to an antennal blood vessel. The ampulla is attached to the frontal cuticle medial to the antenna base and forms a thin-walled sac with a valved ostium on its ventral side. The ampulla wall epithelium is not muscular, but consists of elastic connective tissue. The pumping movements are affected by a precerebral frontopharyngeal muscle, which causes systolic compression of the ampulla upon contraction. The elasticity of both the ampulla and a band of connective tissue, which suspends it in the head capsule, passively effect diastole. The antennal vessel is quite voluminous in the head capsule, but narrows remarkably upon entrance into the antenna. It extends with a constant diameter to the apex, where it opens with a terminal pore. At the base of the vessel, near the ampulla, is a very delicate valve flap which prevents hemolymph backflow during diastole. A comparison of the antennal heart types in insects revealed fundamental differences in the attachments and functions of the associated muscles. In the Dermaptera, the involvement of a precerebral frontopharyngeal muscle suggests an ancestral condition.     

Comparative analysis of the intrinsic leg musculature of the American cockroach,Periplaneta americana (L.)

The American cockroach has a total of 368 muscles inserting on the post-coxal segments of its legs. By using a narrow morphological definition for delimiting individual muscles, it is shown (i) that the protrochanteral musculatures (23 muscles/leg) differ from the essentially identical meso- and metatrochanteral musculatures (24 and 26 muscles/leg) in number and disposition of extensors and in having a completely different flexor composition, and (ii) that the musculatures of the more distal segments of the legs are completely serially homologous, there being 2 muscles for moving each femur, 23 for each tibia, 7 for each first tarsomere, and 5 for each of the paired pretarsal claws. In all six legs, the trochanteral and tibial musculatures each contain single slender muscles that may be acting proprioceptively to measure the angular displacements between, respectively, the coxas and trochanters, and the femurs and tibias. Neurological and phylogenetic considerations are used to demonstrate why a narrow morphological definition should be employed, and why the widely used functional definition of Snodgrass ('35) is not only fallacious on evolutionary grounds, but also leads to making erroneous conclusions regarding the manner in which insect musculature is controlled by the insect central nervous system. Finally, it is hypothesized that the physiological limitations imposed by having an open circulatory system and the problems inherent in the neural control of large muscles may have been major evolutionary factors in forcing insects to use many slender muscles to control their body movements.     

Hox genes and the phylogeny of the arthropods

The arthropods are the most speciose, and among the most morphologically diverse, of the animal phyla. Their evolution has been the subject of intense research for well over a century, yet the relationships among the four extant arthropod subphyla - chelicerates, crustaceans, hexapods, and myriapods - are still not fully resolved. Morphological taxonomies have often placed hexapods and myriapods together (the Atelocerata) [1, 2], but recent molecular studies have generally supported a hexapod/crustacean clade [2-9]. A cluster of regulatory genes, the Hox genes, control segment identity in arthropods, and comparisons of the sequences and functions of Hox genes can reveal evolutionary relationships [10]. We used Hox gene sequences from a range of arthropod taxa, including new data from a basal hexapod and a myriapod, to estimate a phylogeny of the arthropods. Our data support the hypothesis that insects and crustaceans form a single clade within the arthropods to the exclusion of myriapods. They also suggest that myriapods are more closely allied to the chelicerates than to this insect/crustacean clade.     

Phylogeography and evolution of a fungal–insect association on the Tibetan Plateau

Parasitoidism refers to a major form of interspecies interactions where parasitoids sterilize and/or kill their hosts typically before hosts reach reproductive age. However, relatively little is known about the evolutionary dynamics of parasitoidism. Here, we investigate the spatial patterns of genetic variation of Chinese cordyceps, including both the parasitoidal fungus Ophiocordyceps sinensis and its host insects. We sampled broadly from alpine regions on the Tibetan Plateau and obtained sequences on seven fungal and three insect DNA fragments from each of the 125 samples. Seven and five divergent lineages/cryptic species were identified within the fungus and host insects, respectively. Our analyses suggested that O. sinensis and host insects originated at similar geographic regions in southern Tibet/Yunnan, followed by range expansion to their current distributions. Cophylogenetic analyses revealed a complex evolutionary relationship between O. sinensis and its host insects. Significant congruence was found between host and parasite phylogenies and the time estimates of divergence were similar, raising the possibility of the occurrence of cospeciation events, but the incongruences suggested that host shifts were also prevalent. Interestingly, one fungal genotype was broadly distributed, consistent with recent gene flow. In contrast, the high‐frequency insect genotypes showed limited geographic distributions. The dominant genotypes from both the fungus and the insect hosts may represent ideal materials from which to develop artificial cultivation of this important Chinese traditional medicine. Our results demonstrate that both historical and contemporary events have played important roles in the phylogeography and evolution of the O. sinensis–ghost moth parasitoidism on the Tibetan Plateau.     

Expression and characterization of an epoxide hydrolase from Anopheles gambiae with high activity on epoxy fatty acids

In insects, epoxide hydrolases (EHs) play critical roles in the metabolism of xenobiotic epoxides from the food resources and in the regulation of endogenous chemical mediators, such as juvenile hormones. Using the baculovirus expression system, we expressed and characterized an epoxide hydrolase from Anopheles gambiae (AgEH) that is distinct in evolutionary history from insect juvenile hormone epoxide hydrolases (JHEHs). We partially purified the enzyme by ion exchange chromatography and isoelectric focusing. The experimentally determined molecular weight and pI were estimated to be 35 kD and 6.3 respectively, different than the theoretical ones. The AgEH had the greatest activity on long chain epoxy fatty acids such as 14,15-epoxyeicosatrienoic acids (14,15-EET) and 9,10-epoxy-12Z-octadecenoic acids (9,10-EpOME or leukotoxin) among the substrates evaluated. Juvenile hormone III, a terpenoid insect growth regulator, was the next best substrate tested. The AgEH showed kinetics comparable to the mammalian soluble epoxide hydrolases, and the activity could be inhibited by AUDA [12-(3-adamantan-1-yl-ureido) dodecanoic acid], a urea-based inhibitor designed to inhibit the mammalian soluble epoxide hydrolases. The rabbit serum generated against the soluble epoxide hydrolase of Mus musculus can both cross-react with natural and denatured forms of the AgEH, suggesting immunologically they are similar. The study suggests there are mammalian sEH homologs in insects, and epoxy fatty acids may be important chemical mediators in insects.