Walking Legs Research Articles

Effect of temperature change on synaptic transmission at crayfish neuromuscular junctions.

ABSTRACTEctothermic animals in areas characterised by seasonal changes are susceptible to extreme fluctuations in temperature. To survive through varied temperatures, ectotherms have developed unique strategies. This study focuses on synaptic transmission function at cold temperatures, as it is a vital component of ectothermic animals' survival. For determining how synaptic transmission is influenced by an acute change in temperature (20°C to 10°C within a minute) and chronic cold (10°C), the crayfish (Procambarus clarkii) neuromuscular junction (NMJ) was used as a model. To simulate chronic cold conditions, crayfish were acclimated to 15°C for 1 week and then to 10°C for 1 week. They were then used to examine the synaptic properties associated with the low output nerve terminals on the opener muscle in the walking legs and high output innervation on the abdominal deep extensor muscle. The excitatory postsynaptic potentials (EPSPs) of the opener NMJs increased in amplitude with acute warming (20°C) after being acclimated to cold; however, the deep extensor muscles showed varied changes in EPSP amplitude. Synaptic transmission at both NMJs was enhanced with exposure to the modulators serotonin or octopamine. The membrane resistance of the muscles decreased 33% and the resting membrane potential hyperpolarised upon warm exposure. Analysis of haemolymph indicated that octopamine increases during cold exposure. These results suggest bioamine modulation as a possible mechanism for ensuring that synaptic transmission remains functional at low temperatures.

A revised dichotomous key for the mangrove crab genus Scylla De Haan, 1833 (Brachyura, Portunidae)

Abstract Species identification in mangrove crabs of the genus Scylla De Haan, 1833 is problematic, due to the high intraspecific variation and the continuity of diagnostic features. To determine the hierarchy of characters that are most useful in differentiating Scylla species from the Philippines, 1,185 mangrove crabs were collected from seven sites across the Philippines. Morphological species identification was contrasted with morphometric and molecular techniques. The hierarchy of characters in the existing dichotomous key resulted in the misidentification of 245 samples. Errors stem from asymmetry in the features of the chelipeds and the continuity in the features of the frontal lobe spines. Considering mainly morphological characters, the most reliable traits were the geometric patterns in the swimming and walking legs. Nonetheless, 27 individuals with morphological ambiguities exhibited unique patterns in digested nuclear ITS-1 fragments, unreported in any known species of Scylla. A revised hierarchy of characters resulted in a 97.8% match with molecular species identification.

A Study on contact positions of hind legs for a simple quadruped walking powered exoskeleton

A Study on contact positions of hind legs for a simple quadruped walking powered exoskeleton

Sensory systems and feeding behaviour of the giant freshwater prawn, Macrobrachium rosenbergii, and the marine whiteleg shrimp, Litopenaeus vannamei

Information on the sensory basis of shrimp feeding provides the means for assessment of the effectiveness of food items in terms of smell, taste, size, and colour. This chapter summarizes information about the sensory basis of the feeding behaviour of the giant freshwater prawn (Macrobrachium rosenbergii) and the marine whiteleg shrimp (Litopenaeus vannamei). Existing literature on these shrimp species and other decapod crustaceans is reviewed, and unpublished experiments using the selective sensory ablation technique to determine the involvement of vision, chemoreception, and touch sense in the feeding behavior of the juveniles of M. rosenbergii and L. vannamei are also described. To determine the role of vision in feeding, the eyes of the juveniles were painted over (deprived of vision) with white manicure and their feeding response to commercial pellets was compared with those with untreated eyes. The untreated eyed juveniles detected and approached a feed pellet right away, but the specimens blinded by the coating detected a pellet only after random accidental touch with the walking legs while roaming on the aquarium bottom. Juveniles that had learned to feed on pellets showed food search and manipulation responses to a pellet-like pebble without smell and taste. The early larvae (zoeae) of M. rosenbergii already have colour vision (that likely persists through life) and colour preference for blue and white. The adults of L. vannamei discriminated a blue-colored well among seven grey wells in a palette, also showing colour vision in this shrimp. A behavioural experiment with dyed prawn flesh showed that L. vannamei has innate color preference for yellow food over black, red, green, and blue food regardless of the background colours of the aquarium bottom. To disrupt chemoreception, the juveniles of both the species were abruptly transferred to water of drastically different salinity and the osmotic ablation destroyed the chemosensitive sensilla. The osmotically ablated juveniles approached a pellet right away but failed to ingest it; they had learned the visual cue and texture of the pellets and recognized them by vision and tactile sense. To determine the role of sensory appendages in feeding of L. vannamei juveniles, the antennal flagella, antennular flagella, and the pincers of the pereiopods were ablated. The ablated juveniles roamed the bottom, touched a pellet at random, grabbed it with the maxillipeds, and ingested it. Subsequently they learned to lower the head, actively swim forward, sweep the bottom with the maxillipeds, detect a pellet, and ingest it—thus indicating a plasticity in feeding behaviour in L. vannamei.

The ultimate legs of Chilopoda (Myriapoda): a review on their morphological disparity and functional variability.

The arthropodium is the key innovation of arthropods. Its various modifications are the outcome of multiple evolutionary transformations, and the foundation of nearly endless functional possibilities. In contrast to hexapods, crustaceans, and even chelicerates, the spectrum of evolutionary transformations of myriapod arthropodia is insufficiently documented and rarely scrutinized. Among Myriapoda, Chilopoda (centipedes) are characterized by their venomous forcipules—evolutionarily transformed walking legs of the first trunk segment. In addition, the posterior end of the centipedes’ body, in particular the ultimate legs, exhibits a remarkable morphological heterogeneity. Not participating in locomotion, they hold a vast functional diversity. In many centipede species, elongation and annulation in combination with an augmentation of sensory structures indicates a functional shift towards a sensory appendage. In other species, thickening, widening and reinforcement with a multitude of cuticular protuberances and glandular systems suggests a role in both attack and defense. Moreover, sexual dimorphic characteristics indicate that centipede ultimate legs play a pivotal role in intraspecific communication, mate finding and courtship behavior. We address ambiguous identifications and designations of podomeres in order to point out controversial aspects of homology and homonymy. We provide a broad summary of descriptions, illustrations, ideas and observations published in past 160 years, and propose that studying centipede ultimate legs is not only essential in itself for filling gaps of knowledge in descriptive morphology, but also provides an opportunity to explore diverse pathways of leg transformations within Myriapoda.

Exceptional appendage and soft-tissue preservation in a Middle Triassic horseshoe crab from SW China

Horseshoe crabs are classic “living fossils”, supposedly slowly evolving, conservative taxa, with a long fossil record back to the Ordovician. The evolution of their exoskeleton is well documented by fossils, but appendage and soft-tissue preservation is extremely rare. Here we analyse details of appendage and soft-tissue preservation in Yunnanolimulus luopingensis, a Middle Triassic (ca. 244 million years old) horseshoe crab from Yunnan Province, SW China. The remarkable preservation of anatomical details including the chelicerae, five pairs of walking appendages, opisthosomal appendages with book gills, muscles, and fine setae permits comparison with extant horseshoe crabs. The close anatomical similarity between the Middle Triassic horseshoe crabs and their recent analogues documents anatomical conservatism for over 240 million years, suggesting persistence of lifestyle. The occurrence of Carcinoscorpius-type claspers on the first and second walking legs in male individuals of Y. luopingensis indicates that simple chelate claspers in males are plesiomorphic for horseshoe crabs, and the bulbous claspers in Tachypleus and Limulus are derived.

Shelter availability mediates decorating in the majoid crab, Camposcia retusa

Although numerous aquatic and terrestrial species adorn their surface with items secured from their surroundings, termed decorating, the physical and environmental factors that drive this behavior are often unclear. One of the best-known examples of this phenomenon are the decorator crabs (Majoidea), with almost 75% of species known to decorate. Here, we examined patterns of decorating in a coral reef-associated majoid, Camposcia retusa, to identify what factors determine patterns of, and investment in, decorating. Observations of natural decoration patterns indicate this species primarily decorate with sponges, fleshy and filamentous algae, and detritus. Decorations were primarily distributed on the carapace and hind walking legs which may reflect exoskeleton morphology. However, decoration cover did not decline with size, as is observed in some other majoid species, suggesting the factors driving decoration investment remain consistent throughout growth stages. From behavioral experiments, we determined that nonuniform decorating is a result of active selection, with crabs preferentially decorating their hind legs and carapace, with only small items placed on the carapace. Decorating in C. retusa appears to function primarily as an antipredator response, with crabs decorating at higher rates when access to shelter is limited. This study is the first to quantify the decorating habits of C. retusa and suggests that behaviorally-mediated decorating has a primary antipredator function. This study also highlights the value of manipulative behavioral experiments as a tool for assessing the behavioral mechanisms that drive decorating in animals.

Elevated expression of neuropeptide signaling genes in the eyestalk ganglia and Y-organ of Gecarcinus lateralis individuals that are refractory to molt induction

Molting is induced in decapod crustaceans via multiple leg autotomy (MLA) or eyestalk ablation (ESA). MLA removes five or more walking legs, which are regenerated and become functional appendages at ecdysis. ESA eliminates the primary source of molt-inhibiting hormone (MIH) and crustacean hyperglycemic hormone (CHH), which suppress the production of molting hormones (ecdysteroids) from the molting gland or Y-organ (YO). Both MLA and ESA are effective methods for molt induction in Gecarcinus lateralis. However, some G. lateralis individuals are refractory to MLA, as they fail to complete ecdysis by 12weeks post-MLA; these animals are in the “blocked” condition. Quantitative polymerase chain reaction was used to quantify mRNA levels of neuropeptide and mechanistic target of rapamycin (mTOR) signaling genes in YO, eyestalk ganglia (ESG), thoracic ganglion (TG), and brain of intact and blocked animals. Six of the seven neuropeptide signaling genes, three of four mTOR signaling genes, and Gl-elongation factor 2 (EF2) mRNA levels were significantly higher in the ESG of blocked animals. Gl-MIH and Gl-CHH mRNA levels were higher in the TG and brain of blocked animals and levels increased in both control and blocked animals in response to ESA. By contrast, mRNA levels of Gl-EF2 and five of the 10 MIH signaling pathway genes in the YO were two to four orders of magnitude higher in blocked animals compared to controls. These data suggest that increased MIH and CHH synthesis in the ESG contributes to the prevention of molt induction by MLA in blocked animals. The up-regulation of MIH signaling genes in the YO of blocked animals suggests that the YO is more sensitive to MIH produced in the ESG, as well as MIH produced in brain and TG of ESA animals. Both the up-regulation of MIH signaling genes in the YO and of Gl-MIH and Gl-CHH in the ESG, TG, and brain appear to contribute to some G. lateralis individuals being refractory to MLA and ESA.

EMG synchrony to assess impaired corticomotor control of locomotion after stroke

Adapting one’s gait pattern requires a contribution from cortical motor commands. Evidence suggests that frequency-based analysis of electromyography (EMG) can be used to detect this cortical contribution. Specifically, increased EMG synchrony between synergistic muscles in the Piper frequency band has been linked to heightened corticomotor contribution to EMG. Stroke-related damage to cerebral motor pathways would be expected to diminish EMG Piper synchrony. The objective of this study is therefore to test the hypothesis that EMG Piper synchrony is diminished in the paretic leg relative to nonparetic and control legs, particularly during a long-step task of walking adaptability. Twenty adults with post-stroke hemiparesis and seventeen healthy controls participated in this study. EMG Piper synchrony increased more for the control legs compare to the paretic legs when taking a non-paretic long step (5.02±3.22% versus 0.86±2.62%), p<0.01) and when taking a paretic long step (2.04±1.98% versus 0.70±2.34%, p<0.05). A similar but non-significant trend was evident when comparing non-paretic and paretic legs. No statistically significant differences in EMG Piper synchrony were found between legs for typical walking. EMG Piper synchrony was positively associated with walking speed and step length within the stroke group. These findings support the assertion that EMG Piper synchrony indicates corticomotor contribution to walking.

Unmyelinated Peripheral Nerves Can Be Stimulated in Vitro Using Pulsed Ultrasound

Appreciation for the medical and research potential of ultrasound neuromodulation is growing rapidly, with potential applications in non-invasive treatment of neurodegenerative disease and functional brain mapping spurring recent progress. However, little progress has been made in our understanding of the ultrasound–tissue interaction. The current study tackles this issue by measuring compound action potentials (CAPs) from an ex vivo crab walking leg nerve bundle and analysing the acoustic nature of successful stimuli using a passive cavitation detector (PCD). An unimpeded ultrasound path, new acoustic analysis techniques and simple biological targets are used to detect different modes of cavitation and narrow down the candidate biological effectors with high sensitivity. In the present case, the constituents of unmyelinated axonal tissue alone are found to be sufficient to generate de novo action potentials under ultrasound, the stimulation of which is significantly correlated to the presence of inertial cavitation and is never observed in its absence.

Elemental characterization of the exoskeleton in the whipscorpions Mastigoproctus giganteus and Typopeltis dalyi (Arachnida: Thelyphonida)

AbstractThe arthropod cuticle is a structurally diverse secretion that is largely composed of lipids, proteins, and α‐chitin that function together in protection, prey capture, and as a skeletal framework for efficient and diverse means of locomotion. Aquatic, aerial, and terrestrial arthropods are well known to enrich their cuticles with trace elements that are proposed to strengthen or otherwise enhance specific regions of the exoskeleton. In this study, we provide evidence that whipscorpions (vinegaroons) enrich their exoskeleton with up to 14 trace elements. We use energy‐dispersive x‐ray spectroscopy (EDS) to provide the first comparisons of trace element composition in two species of Thelyphonida: Mastigoproctus giganteus (female) and Typopeltis dalyi (female and male). We analyzed the chemical composition of eight regions that have sensory, locomotory, taxonomic, protective, and/or predatory significance: the flagellum, prosoma, tarsal claws of the walking legs, tarsi of the antenniform legs, chelicerae, and the terminal three segments of the pedipalps. Our results reveal the presence of 14 trace elements across both species (12 elements in T. dalyi, 10 elements in M. giganteus), and only four elements are present at significant levels (≥1% weight): Si, Cl, Ca, and Zn. The flagellum, antenniform leg tarsi, and prosoma keel lack these elements in both species, while the chelicerae of all species are enriched with Ca, Zn, and Cl, and the tarsal claws are enriched with Zn. Significantly, we note the presence of Si in the prosomal carapace (but not the keel) of males and females of Typopeltis only, which appears to be the first evidence of this transition metal in the arthropod exoskeleton. We discuss the significance of these chemical enrichments in whipscorpions and provide hypotheses about their functional significance.

Correlation between investment in sexual traits and valve sexual dimorphism in Cyprideis species (Ostracoda).

Assessing the long-term macroevolutionary consequences of sexual selection has been hampered by the difficulty of studying this process in the fossil record. Cytheroid ostracodes offer an excellent system to explore sexual selection in the fossil record because their readily fossilized carapaces are sexually dimorphic. Specifically, males are relatively more elongate than females in this superfamily. This sexual shape difference is thought to arise so that males carapaces can accommodate their very large copulatory apparatus, which can account for up to one-third of body volume. Here we test this widely held explanation for sexual dimorphism in cytheroid ostracodes by correlating investment in male genitalia, a trait in which sexual selection is seen as the main evolutionary driver, with sexual dimorphism of carapace in the genus Cyprideis. We analyzed specimens collected in the field (C. salebrosa, USA; C. torosa, UK) and from collections of the National Museum of Natural History, Washington, DC (C. mexicana). We digitized valve outlines in lateral view to obtain measures of size (valve area) and shape (elongation, measured as length to height ratio), and obtained several dimensions from two components of the hemipenis: the muscular basal capsule, which functions as a sperm pump, and the section that includes the intromittent organ (terminal extension). In addition to the assessment of this primary sexual trait, we also quantified two dimensions of the male secondary sexual trait—where the transformed right walking leg functions as a clasping organ during mating. We also measured linear dimensions from four limbs as indicators of overall (soft-part) body size, and assessed allometry of the soft anatomy. We observed significant correlations in males between valve size, but not elongation, and distinct structural parts of the hemipenis, even after accounting for their shared correlation with overall body size. We also found weak but significant positive correlation between valve elongation and the degree of sexual dimorphism of the walking leg, but only in C. torosa. The correlation between the hemipenis parts, especially basal capsule size and male valve size dimorphism suggests that sexual selection on sperm size, quantity, and/or efficiency of transfer may drive sexual size dimorphism in these species, although we cannot exclude other aspects of sexual and natural selection.

Unexpected UBX expression in the maxilliped of the mystacocarid crustacean Derocheilocharis remanei-evidence for a different way of making a maxilliped?

In terms of morphology, crustacean maxillipeds are hybrid appendages. They arise in anterior thoracic segments and display characteristics of both locomotory (thoracic) and feeding (gnathal) appendages. Maxillipeds are functionally integrated with the anterior gnathal appendages. Hox gene expression patterns and immunolabeling with the FP6.87 antibody, which detects conserved epitopes of UBX and ABD-A proteins, reveal that maxillipeds are consistently associated with a shift in the expression of the homeotic gene Ubx. Ubx transcription products or proteins only appear in thoracic segments with a typical locomotory thoracopod and are consistently absent in the maxilliped. This pattern is found in various crustaceans: the copepod Mesocyclops, the mysid Mysidium, the decapods Homarus and Periclimenes, the isopod Porcellio, and the amphipod Parhyale. In Parhyale, which possesses maxillipeds on the first thoracic segment, gene manipulation experiments have shown that a leg-like thoracic appendage can be recovered by mis-expressing Ubx in that segment and walking legs can be transformed into maxillipeds by Ubx-knockdown. This survey focuses on the expression of UBX/ABD-A proteins, studied using the FP6.87 antibody, in the larval stages of the mystacocarid crustacean Derocheilocaris remanei. Mystacocarids inhabit the intertidal meiofauna zone of sandy beaches and possess one pair of maxillipeds on the first thoracic segment. Strong UBX/ABD-A expression in the developing maxilliped makes Derocheilocaris unique among crustaceans. Our data might also show that the transformation from locomotory thoracopod to maxilliped cannot be accounted for by the mere presence or absence of UBX, because in mystacocarids, UBX is present in both kinds of limbs. The role of the other Hox gene known to be involved in this transformation, Sex combs reduced (Scr), is unclear. The results presented here may document a new example of a shift in Hox gene function in arthropods. The difference in UBX/ABD-A expression between D. remanei and the copepod maxillipeds is of particular interest because correspondences between the feeding apparatus-including the maxilliped-in mystacocarids and copepods have been suggested as being of phylogenetic significance.

The Performance of Mangrove (Rhizophora apiculata) Leaf Extract to Treat Vibriosis (Vibrio harveyi) in Mud Crab (Scylla serrata)

Mud crabs (Scylla serrata) is one of the most economic crabs in Central Java. However, mortality was reached 80% during fattening due to bacterial infection. Meantime, mangrove leaf contain bacteriostatic and bactericidal such as saponin, flavonoid, tanin, alkaloid, steroid, terpenoid and phenol that might effective to control bacterial infection. This study was carried out to observe the performance of mangrove leaf extract to treat vibriosis in mud crabs at various concentration in vitro and in vivo. In vitro study demonstrated that mangrove leaf extract at concentration of 300 mg/l to 900 mg/l were able to prevent Vibrio harveyi growth and formed clear ring between 11.37–13.67 mm. Forty eight mud crabs at body weight of 4011±253 g were injected with a bacterial V. harveyi at density of 106 CFU/ml in between the leg join. Then immersed into mangrove leaf extract at concentration of 0 mg/l, 300 mg/l, 600 mg/l, 900 mg/l for 30 minutes. Post injection of bacterial Vibrio crabs showed clinical signs such as wide apart of swimming and walking legs, blackened carapace, white spots, and red spots. Immersion of mangrove leaf extract post injection could improved survival rate, healing and histopathological features of experimental crabs. Therefore, it can be concluded that the use of mangrove leaf extract at 900 mg/l demonstrated best survival rate, histopathological feature and recovery after 14 days post infection.

The effects of potassium and muscle homogenate on proprioceptive responses in crayfish and crab.

Proprioception of limbs and joints is a basic sensory function throughout most of the animal kingdom. It is important to understand how proprioceptive organs and the associated sensory neurons function with altered environments such as increased potassium ion concentrations ([K+]) from diseased states, ionic imbalances, and damaged tissues. These factors can drastically alter neuronal activity. To assess this matter, we used the chordotonal organ in a walking leg of a blue crab (Callinectes sapidus) and the muscle receptor organ of the crayfish (Procambarus clarkii). These organs serve as tractable models for the analysis of proprioception. The preparations can help serve as translational models for these effects, which may be observed in other invertebrate species as well as mammalian species (including humans). When extracellular potassium concentration ([K+]o) is increased to 20mM in both preparations, mixed results are observed with activity increasing in some preparations and decreasing in others after mechanical displacement. However, when [K+]o is increased to 40mM, activity drastically decreases in all preparations. Additionally, proprioceptor sensory activity declines upon exposure to a diluted muscle homogenate, which contains a host of intracellular constituents. The robust effects of altered [K+] on proprioception in these models illuminate the potential detriments on neuronal function in cases of severe tissue damage as well as altered [K+]o.

Rational Engineering of a Dynamic, Entropy‐Driven DNA Nanomachine for Intracellular MicroRNA Imaging

AbstractWe rationally engineered an elegant entropy‐driven DNA nanomachine with three‐dimensional track and applied it for intracellular miRNAs imaging. The proposed nanomachine is activated by target miRNA binding to drive a walking leg tethered to gold nanoparticle with a high density of DNA substrates. The autonomous and progressive walk on the DNA track via the entropy‐driven catalytic reaction of intramolecular toehold‐mediated strand migration leads to continuous disassembly of DNA substrates, accompanied by the recovery of fluorescence signal due to the specific release of a dye‐labeled substrate from DNA track. Our nanomachine outperforms the conventional intermolecular reaction‐based gold nanoparticle design in the context of an improved sensitivity and kinetics, attributed to the enhanced local effective concentrations of working DNA components from the proximity‐induced intramolecular reaction. Moreover, the nanomachine was applied for miRNA imaging inside living cells.

Rational Engineering of a Dynamic, Entropy-Driven DNA Nanomachine for Intracellular MicroRNA Imaging.

We rationally engineered an elegant entropy-driven DNA nanomachine with three-dimensional track and applied it for intracellular miRNAs imaging. The proposed nanomachine is activated by target miRNA binding to drive a walking leg tethered to gold nanoparticle with a high density of DNA substrates. The autonomous and progressive walk on the DNA track via the entropy-driven catalytic reaction of intramolecular toehold-mediated strand migration leads to continuous disassembly of DNA substrates, accompanied by the recovery of fluorescence signal due to the specific release of a dye-labeled substrate from DNA track. Our nanomachine outperforms the conventional intermolecular reaction-based gold nanoparticle design in the context of an improved sensitivity and kinetics, attributed to the enhanced local effective concentrations of working DNA components from the proximity-induced intramolecular reaction. Moreover, the nanomachine was applied for miRNA imaging inside living cells.

Serial and special: Comparison of podomeres and muscles in tactile vs walking legs of whip scorpions (Arachnida, Uropygi)

Uropygi are one of a small number of taxa within the Arachnida that do not walk on four pairs of post-pedipalpal legs. The second, third and fourth pair are used for locomotion, while the first pair has evolved into a pair of elongated and antenniform sensory appendages extensively covered with sensilla. The podomeres of this first pair of legs display about six additional tarsomeres, and the terminal claw-like apotele is reduced. A patella as found in the posterior three pairs of legs, and in all four pairs of legs of the closely related Amblypygi and Araneae, appears to be absent. This has caused some authors to assume that the patella has fused with the tibia to form a so-called “patellotibia”. However, this hypothesis is controversial, and various authors have used a labelling of the podomeres of the antenniform pair of legs including a “patella”. To shed more light on the matter, we analyse here the morphology of the four pairs of post-pedipalpal legs in Mastigoproctus giganteus. Our study uses micro-computed tomography (μCT)-based 3D reconstructions of the intrinsic muscles and cuticle components of the podomeres to describe and compare these morphological structures. The evolutionary changes undergone by the podomeres and the muscular system in the antenniform legs of Thelyphonidae are discussed and the homologous identity between podomeres is discussed on the basis of the hypothesis of serial homology. Our findings imply a refutal of the patellotibia hypothesis.

Fiber-type distribution in insect leg muscles parallels similarities and differences in the functional role of insect walking legs.

Previous studies have demonstrated that myofibrillar ATPase (mATPase) enzyme activity in muscle fibers determines their contraction properties. We analyzed mATPase activities in muscles of the front, middle and hind legs of the orthopteran stick insect (Carausius morosus) to test the hypothesis that differences in muscle fiber types and distributions reflected differences in their behavioral functions. Our data show that all muscles are composed of at least three fiber types, fast, intermediate and slow, and demonstrate that: (1) in the femoral muscles (extensor and flexor tibiae) of all legs, the number of fast fibers decreases from proximal to distal, with a concomitant increase in the number of slow fibers. (2) The swing phase muscles protractor coxae and levator trochanteris, have smaller percentages of slow fibers compared to the antagonist stance muscles retractor coxae and depressor trochanteris. (3) The percentage of slow fibers in the retractor coxae and depressor trochanteris increases significantly from front to hind legs. These results suggest that fiber-type distribution in leg muscles of insects is not identical across leg muscles but tuned towards the specific function of a given muscle in the locomotor system.

Oligomeric larvae of the pycnogonids revisited.

Organization and ultrastructure of the protonymphon larva were never adequately described, despite it being the common larval type of the enigmatic sea spiders and the only example of oligosegmented life stage among recent chelicerates. We have made a comprehensive examination of the newly hatched free-living protonymphons of Nymphon brevirostre using SEM, TEM, light, and confocal microscopy. Although fairly typical in their broad characters, protonymphon larvae have a number of unique and unexpected traits. Body cavity, already present at this stage, is lined with extracellular matrix and thus is conclusively identified as primary body cavity. Central nervous system includes four postocular neuromeres arranged in three ganglia: supraesophageal, subesophageal, and the first ganglion of the ventral nerve cord. Examination of the sensory organs revealed unusually organized eyes, mechanoreceptors, and chemoreceptors. We have uncovered a mixed sensory-secretory nature of chelar glands and proposed possible modalities of its receptory part. We gave first descriptions of the complex ultrastructure of three secretory organs (spinning glands, slit-like organs, proboscis glands) and hypothesized on their mode of functioning. Comparisons with another oligomeric larva, for example, nauplius, revealed discrepancies in the segmentation of these animals. Although both larvae are externally unsegmented and bear three pairs of homologous appendages, the protonymphon body includes a fourth segment of the prospective walking legs which is absent in nauplius.