Histological and ultrastructural analysis of the respiratory tracheae of Galeodes granti (Chelicerata: Solifugae)

Outsourcing a visual neuropil - The central visual system of the median eyes of Galeodes granti Pocock, 1903 (Arachnida: Solifugae)

Only a few studies have examined the central visual system of Solifugae until now. To get new insights suitable for phylogenetic analysis we studied the R-cell (or retinula cell) projections and visual neuropils of Galeodes granti using various methods. G. granti possesses large median eyes and rudimentary lateral eyes. In this study, only the R-cells and neuropils of the median eyes were successfully stained. The R-cells terminate in two distinct visual neuropils. The first neuropil is located externally to the protocerebrum directly below the retina, the second neuropil lies in the cell body rind of the protocerebrum, and immediately adjacent is the arcuate body. This layout of the median eye visual system differs from Arachnopulmonata (Scorpiones + Tetrapulmonata). However, there are several similarities with Opiliones. In both, (1) the R-cells are connected to a first and second visual neuropil and not to any other region of the brain, (2) the first neuropil is not embedded in the cell body rind of the protocerebrum, it is rather external to the protocerebrum, (3) the second visual neuropil is embedded in the cell body rind, and (4) the second neuropil abuts the arcuate body. These findings may provide important new characters for the discussion on arachnid phylogeny.

Transcriptomes reveal expression of hemoglobins throughout insects and other Hexapoda

Insects have long been thought to largely not require hemoglobins, with some notable exceptions like the red hemolymph of chironomid larvae. The tubular, branching network of tracheae in hexapods is traditionally considered sufficient for their respiration. Where hemoglobins do occur sporadically in plants and animals, they are believed to be either convergent, or because they are ancient in origin and their expression is lost in many clades. Our comprehensive analysis of 845 Hexapod transcriptomes, totaling over 38 Gbases, revealed the expression of hemoglobins in all 32 orders of hexapods, including the 29 recognized orders of insects. Discovery and identification of 1333 putative hemoglobins were achieved with target-gene BLAST searches of the NCBI TSA database, verifying functional residues, secondary- and tertiary-structure predictions, and localization predictions based on machine learning. While the majority of these hemoglobins are intracellular, extracellular ones were recovered in 38 species. Gene trees were constructed via multiple-sequence alignments and phylogenetic analyses. These indicate duplication events within insects and a monophyletic grouping of hemoglobins outside other globin clades, for which we propose the term insectahemoglobins. These hemoglobins are phylogenetically adjacent and appear structurally convergent with the clade of chordate myoglobins, cytoglobins, and hemoglobins. Their derivation and co-option from early neuroglobins may explain the widespread nature of hemoglobins in various kingdoms and phyla. These results will guide future work involving genome comparisons to transcriptome results, experimental investigations of gene expression, cell and tissue localization, and gas binding properties, all of which are needed to further illuminate the complex respiratory adaptations in insects.

Arachnid Hemocyanins

Hemocyanin (Hc), a copper-containing extracellular multimeric protein, is the major protein component of hemolymph in different arachnid groups. Hc possesses 7 or 8 very well-characterized types of monomers with molecular weights ranging from 70 to 85 kDa, organized in hexamers or multiple of hexamers. The present chapter compiles the existing data with relation to the function of this protein in the arachnids. Hc has as main function the reversible transport of O₂, but it shows many secondary though not less important functions. With reference to this, it has been described that Hc can transport hydrophobic molecules (lipid-derived hormones and lipids) to the different organs, having a key role in the lipid transport system. In arachnids, like in other arthropods and invertebrates, Hc has phenoloxidase function which is related to different metabolic processes such as melanin formation and defense against pathogens. In addition, Hc has additional defensive functions since it can serve as precursor for the production of antimicrobial peptides. In short, the evolution of this protein has led to the development of multiple functions essential for organisms possessing this protein.

Morphology of the tracheal system of camel spiders (Chelicerata: Solifugae) based on micro-CT and 3D-reconstruction in exemplar species from three families

We studied the tracheal system of exemplar species representing three families of Solifugae Sundevall, 1833, i.e., Galeodes granti Pocock, 1903, Ammotrechula wasbaueri Muma, 1962 and Eremobates sp., using μCT-imaging and 3D-reconstruction. This is the first comparative study of the tracheal system of Solifugae in 85 years and the first using high-resolution nondestructive methods. The tracheal system was found to be structurally similar in all three species, with broad major tracheae predominantly in the prosoma as well as anastomoses (i.e., connections between tracheal branches from different stigmata) in the prosoma and opisthosoma. Differences among the three species were observed in the presence or absence of cheliceral air sacs, the number of tracheae supplying the heart, and the ramification of major tracheae in the opisthosoma. The structure of the tracheal system with its extensive branches and some anastomoses is assumed to aid rapid and efficient gas exchange in the respiratory tissues of these active predators. The large diameter of cheliceral tracheae (air sacs) of taxa with disproportionally heavier chelicerae suggests a role in weight reduction, enabling solifuges to reach greater speeds during predation. The air sacs may also permit more rapid and efficient gaseous exchange, necessary to operate the musculature of these structures, thereby improving their use for predation in an environment where prey is scarce.