Occurrence and predictive utility of isochronal, equiproportional, and other types of development among arthropods

In isochronal (ICD) and equiproportional development (EPD), the proportion of total immature (egg, larval, and/or juvenile) development spent in each stage (developmental proportion) does not vary among stages or temperatures, respectively. ICD and EPD have mainly been reported in copepods, and whether they occur in other arthropods is not known. If they did, then rearing studies could be simplified because the durations of later developmental stages could be predicted based on those of earlier ones. The goal of this study was to test whether different taxa have ICD, EPD, or an alternative development type in which stage-specific proportions depend on temperature, termed 'variable proportional' development (VPD), and also how well each development type allowed later-stage durations to be predicted from earlier ones. Data for 71 arthropods (arachnids, copepod and decapod crustaceans, and insects) were tested, and most (85.9%) species were concluded to have VPD, meaning that ICD and EPD do not occur generally. However, EPD predicted later-stage durations comparably well to VPD (within 19-23%), and thus may still be useful. Interestingly, some species showed a 'mixed' form of development, where some stages' developmental proportions varied with temperature while those of others did not, which should be further investigated.

Miniaturisation in Chelicerata

Arachnids and their relatives (Chelicerata) range in body length from tens of centimetres in horseshoe crabs down to little more than 80-200 μm in several groups of mites. Spiders (Araneae) show the widest range within a given Bauplan - the largest species being ca. 270 times longer than the smallest - making them excellent models to investigate scaling effects. The two mite clades (Parasitiformes and Acariformes) are the main specialists in being small. Miniaturisation, and its consequences, is reviewed for both fossil and extant chelicerates. Morphological changes potentially related to miniaturisation, or adapting to the ecological niches that small size allows, include reduction in the length and number of legs, loss of prosomal arteries (and eventually also the heart), replacement of book lungs by tracheae, or even loss of all respiratory organs. There may also be evolutionary novelties, such as the acquisition of structures by which some mites attach themselves to larger hosts. The observed character distributions suggest a fairly fundamental division between larger pulmonate (lung-bearing) arachnids and smaller, non-pulmonate, groups which could reflect a phylogenetic dichotomy. However, it is worth noting that lineages of tiny spiders were originally fully pulmonate, but have acquired some typically non-pulmonate features, while camel spiders (Soli-fugae) can be large but have a Bauplan suggestive of smaller, non-pulmonate, ancestors.

The disjunct pattern of the Neotropical harvestman Discocyrtus dilatatus (Gonyleptidae) explained by climate-driven range shifts in the Quaternary: Paleodistributional and molecular evidence

The disjunct distribution of the harvestman Discocyrtus dilatatus (Opiliones, Gonyleptidae) is used as a case study to test the hypothesis of a trans-Chaco Pleistocene paleobridge during range expansion stages. This would have temporarily connected humid regions (‘Mesopotamia’ in northeastern Argentina, and the ‘Yungas’ in the northwest, NWA) in the subtropical and temperate South American lowlands. The present study combines two independent approaches: paleodistributional reconstruction, using the Species Distribution Modeling method MaxEnt and projection onto Quaternary paleoclimates (6 kya, 21 kya, 130 kya), and phylogeographic analyses based on the cytochrome oxidase subunit I molecular marker. Models predict a maximal shrinkage during the warm Last Interglacial (130 kya), and the rise of the hypothesized paleobridge in the Last Glacial Maximum (21 kya), revealing that cold-dry stages (not warm-humid ones, as supposed) enabled the range expansion of this species. The disjunction was formed in the mid-Holocene (6 kya) and is intensified under current conditions. The median-joining network shows that NWA haplotypes are peripherally related to different Mesopotamian lineages; haplotypes from Santa Fe and Córdoba Provinces consistently occupy central positions in the network. According to the dated phylogeny, Mesopotamia-NWA expansion events would have occurred in the last glacial period, in many cases closely associated to the Last Glacial Maximum, with most divergence events occurring shortly thereafter. Only two (out of nine) NWA haplotypes are shared with Mesopotamian localities. A single, presumably relictual NWA haplotype was found to have diverged much earlier, suggesting an ancient expansion event not recoverable by the paleodistributional models. Different measures of sequence statistics, genetic diversity, population structure and history of demographic changes are provided. This research offers the first available evidence for the historical origin of NWA disjunct populations of a Mesopotamian harvestman.

Segmentation and tagmosis in Chelicerata

Patterns of segmentation and tagmosis are reviewed for Chelicerata. Depending on the outgroup, chelicerate origins are either among taxa with an anterior tagma of six somites, or taxa in which the appendages of somite I became increasingly raptorial. All Chelicerata have appendage I as a chelate or clasp-knife chelicera. The basic trend has obviously been to consolidate food-gathering and walking limbs as a prosoma and respiratory appendages on the opisthosoma. However, the boundary of the prosoma is debatable in that some taxa have functionally incorporated somite VII and/or its appendages into the prosoma. Euchelicerata can be defined on having plate-like opisthosomal appendages, further modified within Arachnida. Total somite counts for Chelicerata range from a maximum of nineteen in groups like Scorpiones and the extinct Eurypterida down to seven in modern Pycnogonida. Mites may also show reduced somite counts, but reconstructing segmentation in these animals remains challenging. Several innovations relating to tagmosis or the appendages borne on particular somites are summarised here as putative apomorphies of individual higher taxa. We also present our observations within the concept of pseudotagma, whereby the true tagmata - the prosoma and opisthosoma - can be defined on a fundamental change in the limb series while pseudotagmata, such as the cephalosoma/proterosoma, are expressed as divisions in sclerites covering the body without an accompanying change in the appendages.

A multilocus phylogeny of Podoctidae (Arachnida, Opiliones, Laniatores) and parametric shape analysis reveal the disutility of subfamilial nomenclature in armored harvestman systematics

The taxonomy and systematics of the armored harvestmen (suborder Laniatores) are based on various sets of morphological characters pertaining to shape, armature, pedipalpal setation, and the number of articles of the walking leg tarsi. Few studies have tested the validity of these historical character systems in a comprehensive way, with reference to an independent data class, i.e., molecular sequence data. We examined as a test case the systematics of Podoctidae, a family distributed throughout the Indo-Pacific. We tested the validity of the three subfamilies of Podoctidae using a five-locus phylogeny, and examined the evolution of dorsal shape as a proxy for taxonomic utility, using parametric shape analysis. Here we show that two of the three subfamilies, Ibaloniinae and Podoctinae, are non-monophyletic, with the third subfamily, Erecananinae, recovered as non-monophyletic in a subset of analyses. Various genera were also recovered as non-monophyletic. As first steps toward revision of Podoctidae, the subfamilies Erecananinae Roewer, 1912 and Ibaloniinae Roewer, 1912 are synonymized with Podoctinae Roewer, 1912 new synonymies, thereby abolishing unsubstantiated subfamilial divisions within Podoctidae. We once again synonymize the genus Paralomanius Goodnight & Goodnight, 1948 with Lomanius Roewer, 1923 revalidated. We additionally show that eggs carried on the legs of male Podoctidae are not conspecific to the males, falsifying the hypothesis of paternal care in this group.

How to stay on mummy's back: Morphological and functional changes of the pretarsus in arachnid postembryonic stages

A specific type of maternal care occurs in several groups of Arachnida: mothers carry their offspring on their back (pulli-carrying behaviour). In scorpions, whip scorpions and whip spiders it is the prenymphal stage that settles on the mother. The prenymph is not yet fully developed for a free life and very limited in its mobility, but its feet are equipped with special adhesive organs (arolia) that become lost at the nymphal stage. Here we study the morphology, ultrastructure and mechanical function of the arolia. In scorpions (Scorpiones) the contact area between arolia and substrate and thus adhesion of the pad is controlled by the antagonistic work of hydrostatic pressure and muscular retraction. Arolia of whip scorpions (Thelyphonida) do not require muscular action for strong attachment. Arrays of long, branching fibres in the mesocuticle lead to high compliancy of the pad. In whip spiders (Amblypygi) the prenymphal pretarsus is already equipped with sclerites and claws. Its arolium is retained in nymphs and adults in some taxa, but acquires a more complex structure. These results contribute to our knowledge on the postembryonic development of arachnids and to the understanding of attachment pad evolution among arthropods. Some of the described developmental, structural, and mechanical phenomena are not known from other animals and might be of potential interest for further biomimetic developments.

Evolutionary biology of harvestmen (Arachnida, Opiliones)

Opiliones are one of the largest arachnid orders, with more than 6,500 species in 50 families. Many of these families have been erected or reorganized in the last few years since the publication of The Biology of Opiliones. Recent years have also seen an explosion in phylogenetic work on Opiliones, as well as in studies using Opiliones as test cases to address biogeographic and evolutionary questions more broadly. Accelerated activity in the study of Opiliones evolution has been facilitated by the discovery of several key fossils, including the oldest known Opiliones fossil, which represents a new, extinct suborder. Study of the group's biology has also benefited from rapid accrual of genomic resources, particularly with respect to transcriptomes and functional genetic tools. The rapid emergence and utility of Phalangium opilio as a model for evolutionary developmental biology of arthropods serve as demonstrative evidence of a new area of study in Opiliones biology, made possible through transcriptomic data.

Comparative analyses of reproductive structures in harvestmen (opiliones) reveal multiple transitions from courtship to precopulatory antagonism

Explaining the rapid, species-specific diversification of reproductive structures and behaviors is a long-standing goal of evolutionary biology, with recent research tending to attribute reproductive phenotypes to the evolutionary mechanisms of female mate choice or intersexual conflict. Progress in understanding these and other possible mechanisms depends, in part, on reconstructing the direction, frequency and relative timing of phenotypic evolution of male and female structures in species-rich clades. Here we examine evolution of reproductive structures in the leiobunine harvestmen or “daddy long-legs” of eastern North America, a monophyletic group that includes species in which males court females using nuptial gifts and other species that are equipped for apparent precopulatory antagonism (i.e., males with long, hardened penes and females with sclerotized pregenital barriers). We used parsimony- and Bayesian likelihood-based analyses to reconstruct character evolution in categorical reproductive traits and found that losses of ancestral gift-bearing penile sacs are strongly associated with gains of female pregenital barriers. In most cases, both events occur on the same internal branch of the phylogeny. These coevolutionary changes occurred at least four times, resulting in clade-specific designs in the penis and pregenital barrier. The discovery of convergent origins and/or enhancements of apparent precopulatory antagonism among closely related species offers an unusual opportunity to investigate how major changes in reproductive morphology have occurred. We propose new hypotheses that attribute these enhancements to changes in ecology or life history that reduce the duration of breeding seasons, an association that is consistent with female choice, sexual conflict, and/or an alternative evolutionary mechanism.

Harvestman (Opiliones) fauna associated with Maine lowbush blueberry fields in the major production areas of Washington and Hancock counties

Over a period of 19 yr, the harvestman (Opiliones) community associated with the lowbush blueberry agro-ecosystem in Maine was studied. Eight species representing five genera, four subfamilies, and two families of harvestmen belonging to the suborder Eupnoi were collected. The harvestman community was dominated by two introduced, synanthropic species: Phalangium opilio in all but 1 yr (that year dominated by Rilaena triangularis). Rilaena was recorded for the first time from eastern North America. Relative abundance of harvestman adults increases throughout the season and the temporal pattern of trap capture does not refute speculated life cycles of the harvestmen being univoltine with overwintering eggs. Some blueberry management practices were found to affect trap capture. We did find that on average (with opposite results 1 yr) trap captures are greater in pruned fields than in fruit-bearing fields. Organic fields were found to have higher relative abundance of harvestmen than conventionally managed fields. Conventionally managed fields with reduced-risk insecticides showed no difference in harvestmen relative abundance compared with those conventionally managed fields using the older more persistent organophosphate insecticides. Insecticide trials with common insecticides used in blueberry insect pest management showed that the organophosphate insecticide, phosmet, and the pyrethroid insecticide, esfenvalerate, were detrimental to P. opilio adults when exposed to leaf residues, whereas the reduced-risk insecticide, spinosad, showed no negative effects compared with nonsprayed foliage.

Soil macroinvertebrates as indicators of pollution by heavy metals

A broad range of soil pollutants were found to decrease with distance from a zinc smelter from 35,000 to 77, 8270 to 40 and from 190 to less than 1 ppm for zinc, lead and cadmium, respectively. Along this gradient, observed species richness of soil macro-organisms seemed to be more affected by the land-use type than by soil pollution--minimum in crops (21), maximum in woody sites (126). IndVal index allowed isolation of 21 indicator species from the 339 morphospecies identified. Most of these indicator species were characteristic of the unpolluted sites: only two diplopods and one gastropod from polluted poplar plantations, and none from the most polluted site. Since soil invertebrates respond to different environmental factors, including direct effect of heavy metals, we suggest there may be some confounding factors generating spurious relationships between the values of species as bioindicators and the pollution status they are supposed to indicate.

Seasonal- and age-dependent changes of the structure and chemical composition of the spherites in the midgut gland of the harvestmen Gyas annulatus (Opiliones)

Spherites--round laminated membrane bound structures--are located in the cytoplasm of all cell types of the midgut gland in the harvestmen Gyas annulatus. The spherites consist of an organic matrix composed of glycoproteins and proteoglycans whose sugar portion are carboxylated glycosaminoglycans. Different elements are embedded in this matrix, and their presence varies during the life cycle. All spherites in juveniles are composed of alternating concentrically arranged electron dense and electron lucent layers of organic matrix material before overwintering (December). At that time, spherites contain calcium, phosphorus and silicon. Calcium and phosphorus are located in their electron dense layers, and silicon spotwise, mainly peripheral. Material seems to be lost during overwintering of Gyas as electron lucent "empty" layers appear in spherites in March. The "lost" material could be used as energy supply and/or to provide molecules for synthesis processes during non-nourishing overwintering. Spherites do not contain calcium and phosphorus in July and October. These elements seem to have important biological relevance in the formation and hardening of the cuticle during the moultings in spring. In contrast to calcium and phosphorus, silicon is still stored in spherites in July and October but in decreasing concentration, therefore it could be involved in metabolic processes in adult Gyas. In the period from July to the end of their adult life in late autumn, an increasing number of spherites disintegrate and their remnant organic matrix material progressively aggregates with other cellular waste material in one or more huge vacuoles of glandular cells.

Resistance of cultivated tomato to cell content-feeding herbivores is regulated by the octadecanoid-signaling pathway

The octadecanoid signaling pathway has been shown to play an important role in plant defense against various chewing insects and some pathogenic fungi. Here, we examined the interaction of a cell-content feeding arachnid herbivore, the two-spotted spider mite (Tetranychus urticae Koch), with cultivated tomato (Lycopersicon esculentum) and an isogenic mutant line (defenseless-1 [def-1]) that is deficient in the biosynthesis of the octadecanoid pathway-derived signal, jasmonic acid (JA). Spider mite feeding and fecundity on def-1 plants was significantly greater than on wild-type plants. Decreased resistance of def-1 plants was correlated with reduced JA accumulation and expression of defensive proteinase inhibitor (PI) genes, which were induced in mite-damaged wild-type leaves. Treatment of def-1 plants with methyl-JA restored resistance to spider mite feeding and reduced the fecundity of female mites. Plants expressing a 35S::prosystemin transgene that constitutively activates the octadecanoid pathway in a Def-1-dependent manner were highly resistant to attack by spider mites and western flower thrips (Frankliniella occidentalis), another cell-content feeder of economic importance. These findings indicate that activation of the octadecanoid signaling pathway promotes resistance of tomato to a broad spectrum of herbivores. The techniques of amplified fragment length polymorphism (AFLP) and bulk segregant analysis were used to map the Def-1 gene to a region on the long arm of chromosome 3 that is genetically separable from the map position of known JA biosynthetic genes. Tight linkage of Def-1 to a T-DNA insertion harboring the maize (Zea mays) Dissociation transposable element suggests a strategy for directed transposon tagging of the gene.

Some ultrastructural observations on the integument of a pentastomid

The cuticle of the cephalobaenid pentastomid Reighardia sternae is described at various stages of the moult-intermoult cycle. The intermoult cuticle comprises four layers: an outer epicuticle; an underlying dense layer, the protein epicuticle; a fibrillar endocuticle; and a denser subcuticle. The overall similarity between the structure and composition of these layers and those of insects is discussed. However, the orientation of the chitin-protein fibres in the endocuticle does not show the rotating structure characteristic of many arthropod species, but this does appear in the sclerotized hooks. It is suggested that this comparatively loose, poorly oriented endocuticular structure produces a highly extensible cuticle which is precisely adapted to the specialize, endoparasitic habit of this species. Events at ecdysis, particularly the secretion of moulting fluid and the deposition of cuticulin, follow the insect pattern precisely. The phyletic significance of these observations is discussed.