The ultrastructure and function of the silk-producing basitarsus in the Hilarini (Diptera: Empididae)

New Species of Empidinae (Diptera) from San Rossore National Park, Italy, with the First Report on Leg Polymorphism in the Genus Hilara Meigen and Their DNA Barcoding Evidence

Altogether three species of Empidinae are described from San Rossore National Park, Italy: Empis (Euempis) sanrossorensis Barták sp. nov., Hilara polymorpha Barták sp. nov., and Rhamphomyia (Megacyttarus) sanrossorensis Barták sp. nov. Polymorphism in the shape of foreleg in Hilara is reported for the first time. The COI sequences for barcoding purposes and upcoming studies are provided.

Tropicohilara, a New Genus of Hilarini (Diptera: Empididae: Empidinae) from Brazil, with Descriptions of Six New Species

The genus Tropicohilaragen. nov. (Diptera: Empididae: Empidinae: Hilarini) is described and includes the following six new species from Brazil: Tropicohilara amazonensissp. nov. (type species, Brazil: Amazonas, Manaus); T. bahiensissp. nov. (Brazil: Bahia, Camacan); T. bellasp. nov. (Brazil: Pernambuco, Jaqueira); T. mineirasp. nov. (Brazil: Minas Gerais, Itamonte); T. paranaensissp. nov. (Brazil: Paraná, Piraquara); and T. sinclairisp. nov. (Brazil: Paraná, Morretes). The genus is presently recorded from the Amazonian and Atlantic Forest biomes. It differs from other hilarine genera by the following combination of characteristics: predominantly yellowish specimens; occiput somewhat conical in dorsal view; postpedicel elongate, male first fore tarsomere unmodified; hind tibia slightly shorter than hind femur; wing vein R2+3 with setae on ventral surface; male tergite 7 with a sclerotized band at posterior margin, tergite 8 reduced and upwardly directed so that terminalia can be flexed forward. A key to the species is provided.

The structural analysis of secretion in the freshwater mite Limnesia maculata (Acariformes, Limnesiidae) supports the idea of a new form of arthropod silk

The structural characteristics of silk secretion of the freshwater mite Limnesia maculata (O.F. Müller) (Acariformes, Limnesiidae) are described and analyzed for the first time based on light, atomic force and electron-microscopical approaches. The common dermal glands (14 pairs scattered over the body) produce silk mostly during the warm summer season. The process of silk secretion lasts from several hours to several days. The silk may appear like barely recognized clouds of a fine whitish substance. An individual silk thread is an indefinitely long uniform unbranched and non-stretchable tube, hollow or with a vesicular electron-dense residual content. In the silk bundle, threads may be freely interlaced, bent, curved or occasionally broken. The diameter of the tubes is in the range of 0.9-1.5 µm. The width of the tube walls varies greatly from 60 to 300 nm. Chaotically interlaced fine fibrils build the tube walls. On the external surface of the tube wall, these fibrils are loosely organized and frequently rising vertically, whereas on the internal side they are packed more tightly sometimes showing a mesh. The walls may reveal a layered structure or, contrary, are quite thin with through foramens. The revealed organization of silk in the freshwater mites is found to be the simplest among that of other arthropods. We propose a role of the silk in the capture of potential prey in the summer season. Silk in water mites significantly widens the wholesome area for the mites' life and gives them better chances in competition for potential resources.

Hilarini (Diptera: Empididae) from the Auckland Islands New Zealand, the Homoplastic Loss of the Male Silk-Producing Basitarsus in Two Insular Species, and Notes on the Empidoidea of the Subantarctic Islands

The New Zealand taxa of the superfamily Empidoidea (minus Dolichopodidae) are briefly summarized. Two species, Hilara ranui n. sp. and Hilarempis motumaka n. sp. (Diptera: Empididae: Empidinae), from the Auckland Islands of New Zealand, where they appear to be common, are described. Both species show a homoplastic loss or reversal of the swollen, silk-producing, fore basitarsi characteristic of males in their respective genera. The environmental conditions of the Subantartic islands and their selective pressure on empidoid taxa are discussed.

Morphology and ultrastructure of the epithelial femoral gland in cicadas (Hemiptera: Cicadidae)

Exocrine glands in the legs of social insects are found throughout all leg segments, but studies of exocrine glands in legs of solitary insects are very limited. We discovered a novel gland at the apex of the fore, mid and hind femurs from six representative species of Cicadidae, which we propose to name as the epithelial femoral gland. The epithelial femoral gland is located between the paired apodemes and the articulation membrane within the apex of the femur, which faces the proximal articulation region of the tibia. The epithelial femoral gland in the midlegs is less developed than that in the fore- and hindlegs within a species. The glandular cells belong to class-1, which contain a large amount of rough endoplasmic reticulum, secretory vesicles and Golgi bodies, indicating that these cells may produce a proteinaceous secretion. Details of the epithelial femoral gland at the ultrastructural level suggest that it may function to produce nourishing substances to the joint between femur and tibia. The less developed epithelial femoral gland in the midlegs and the slight difference in the glands between fore- and hindlegs within a species could be related to the functional differentiation of the corresponding legs in cicadas. Further studies of exocrine glands in the legs of cicadas and other Cicadomorpha insects may improve our understanding of the structural and functional divergence of legs in hemipteran insects.

Jurassic scorpionflies (Mecoptera) with swollen first metatarsal segments suggesting sexual dimorphism

BACKGROUND

Sexual dimorphism is widespread in insects. The certain specialized structures may be used as weapons in male-male combats or as ornaments to enhance mating opportunities.

RESULTS

We report striking swollen first tarsal segments in two families, four genera and six species of scorpionflies from the Middle Jurassic Yanliao Biota of Northeastern China. Swollen tarsal segments are restricted to male specimens and to hind leg tarsi. The geometric morphometric analyses reveal that the degree of swelling within the orthophlebiid species possessing swollen first metatarsal segments is species-specific, which can be used as a diagnostic character for taxonomic and phylogenetic studies.

CONCLUSIONS

The new findings indicate that swollen first metatarsal segments are relatively common in the family Orthophlebiidae during the Middle Jurassic. The tarsal swellings are considered to be sexually dimorphic, potentially associated with sexually display by males and/or camouflage of a "nuptial gift" in the mating process.

Fine structure of the silk spinning system in the caddisworm, Hydatophylax nigrovittatus (Trichoptera: Limnephilidae)

Silk is produced by a variety of insects, but only silk made by terrestrial arthropods has been examined in detail. To fill the gap, this study was designed to understand the silk spinning system of aquatic insect. The larvae of caddis flies, Hydatophylax nigrovittatus produce silk through a pair of labial silk glands and use raw silk to protect themselves in the aquatic environment. The result of this study clearly shows that although silk fibers are made under aquatic conditions, the cellular silk production system is quite similar to that of terrestrial arthropods. Typically, silk production in caddisworm has been achieved by two independent processes in the silk glands. This includes the synthesis of silk fibroin in the posterior region, the production of adhesive glycoproteins in the anterior region, which are ultimately accumulated into functional silk dope and converted to a silk ribbon coated with gluey substances. At the cellular level, each substance of fibroin and glycoprotein is specifically synthesized at different locations, and then transported from the rough ER to the Golgi apparatus as transport vesicles, respectively. Thereafter, the secretory vesicles gradually increase in size by vesicular fusion, forming larger secretory granules containing specific proteins. It was found that these granules eventually migrate to the apical membrane and are exocytosed into the lumen by a mechanism of merocrine secretion.

Gene expression analysis in the larval silk gland of the eri silkworm Samia ricini

Insects produce silk for a range of purposes. In the Lepidoptera, silk is utilized as a material for cocoon production and serves to protect larvae from adverse environmental conditions or predators. Species in the Saturniidae family produce an especially wide variety of cocoons, for example, large, golden colored cocoons and those with many small holes. Although gene expression in the silk gland of the domestic silkworm (Bombyx mori L.) has been extensively studied, considerably fewer investigations have focused on members of the saturniid family. Here, we established expression sequence tags from the silk gland of the eri silkworm (Samia ricini), a saturniid species, and used these to analyze gene expression. Although we identified the fibroin heavy chain gene in the established library, genes for other major silk proteins, such as fibroin light chain and fibrohexamerin, were absent. This finding is consistent with previous reports that these latter proteins are lacking in saturniid silk. Recently, a series of fibrohexamerin-like genes were identified in the Bombyx genome. We used this information to conduct a detailed analysis of the library established here. This analysis identified putative homologues of these genes. We also found several genes encoding small silk protein molecules that are also present in the silk of other Lepidoptera. Gene expression patterns were compared between eri and domestic silkworm, and both conserved and nonconserved expression patterns were identified for the tested genes. Such differential gene expression might be one of the major causes of the differences in silk properties between these species. We believe that our study can be of value as a basic catalogue for silk gland gene expression, which will yield to the further understanding of silk evolution.

Three-dimensional reconstruction on cell level: case study elucidates the ultrastructure of the spinning apparatus of Embia sp. (Insecta: Embioptera)

Spinning is a phenomenon not only present in spiders, but also in many other arthropods. The functional morphology and complexity of spinning organs is often poorly understood. Their elements are minute and studying them poses substantial methodological difficulties. This study presents a three-dimensional reconstruction of a silk gland of Embia sp. on cellular level, based on serial sections acquired with serial block-face scanning electron microscopy (SBFSEM) to showcase the power of this method. Previous studies achieved either high resolution to elucidate the ultrastructure or satisfying three-dimensional representations. The high-resolution achieved by SBFSEM can be easily used to reconstruct the three-dimensional ultrastructural organization of cellular structures. The herein investigated spinning apparatus of Embioptera can be taken as an example demonstrating the potential of this method. It was possible to reconstruct a multinucleated silk gland containing 63 nuclei. We focused on the applicability of this method in the field of morphological research and provide a step-by-step guide to the methodology. This will help in applying the method to other arthropod taxa and will help significantly in adapting the method to other animals, animal parts and tissues.

The spinning apparatus of webspinners--functional-morphology, morphometrics and spinning behaviour

Webspinners (Insecta: Embioptera) have a distinctly unique behaviour with related morphological characteristics. Producing silk with the basitarsomeres of their forelegs plays a crucial role in the lives of these insects--providing shelter and protection. The correlation between body size, morphology and morphometrics of the spinning apparatus and the spinning behaviour of Embioptera was investigated for seven species using state-of-the-art methodology for behavioural as well as for morphological approaches. Independent contrast analysis revealed correlations between morphometric characters and body size. Larger webspinners in this study have glands with greater reservoir volume, but in proportionally smaller tarsi relative to body size than in the smaller species. Furthermore, we present a detailed description and review of the spinning apparatus in Embioptera in comparison to other arthropods and substantiate the possible homology of the embiopteran silk glands to class III dermal silk glands of insects.

The comparative morphology of epidermal glands in Pentatomoidea (Heteroptera)

The Heteroptera show a diversity of glands associated with the epidermis. They have multiple roles including the production of noxious scents. Here, we examine the cellular arrangement and cytoskeletal components of the scent glands of pentatomoid Heteroptera in three families, Pentatomidae (stink bugs), Tessaratomidae, and Scutelleridae (shield-backed bugs or jewel bugs). The glands are; (1) the dorsal abdominal glands, (2) the tubular glands of the composite metathoracic gland, and (3) the accessory gland component of the composite metathoracic gland. The dorsal abdominal glands are at their largest in nymphs and decrease in size in adults. The metathoracic gland is an adult-specific gland unit with a reservoir and multiple types of gland cells. The accessory gland is composed of many unicellular glands concentrated in a sinuous line across the reservoir wall. The lateral tubular gland is composed of two-cell units. The dorsal abdominal glands of nymphs are composed of three-cell units with a prominent cuticular component derived from the saccule cell sitting between the duct and receiving canal. The cuticular components that channel secretion from the microvilli of the secretory cell to the exterior differ in the three gland types. The significance of the numbers of cells comprising gland units is related to the role of cells in regenerating the cuticular components of the glands at moulting in nymphs.

The morphogenesis of spermathecae and spermathecal glands in Drosophila melanogaster

Sperm storage in female insects is important for reproductive success and sperm competition. In Drosophila melanogaster females, sperm viability during storage is dependent upon secretions produced by spermathecae and parovaria. Class III dermal glands are present in both structures. Spermathecal glands are initially comprised of a three-cell unit that is refined to a single secretory cell in the adult. It encapsulates an end-apparatus joining to a cuticular duct passing secretions to the spermathecal lumen. We have examined spermatheca morphogenesis using DIC and fluorescence microscopy. In agreement with a recent study, cell division ceases by 36 h after puparium formation (APF). Immunostaining of the plasma membrane at this stage demonstrates that gland cells wrap around the developing end-apparatus and each other. By 48-60 h APF, the secretory cell exhibits characteristic adult morphology of an enlarged nucleus and extracellular reservoir. A novel finding is the presence of an extracellular reservoir in the basal support cell that is continuous with the secretory cell reservoir. Some indication of early spermathecal gland formation is evident in the division of enlarged cells lying adjacent to the spermathecal lumen at 18 h APF and in cellular processes that bind clusters of cells between 24 and 30 h APF.

Chitin in the silk gland ducts of the spider Nephila edulis and the silkworm Bombyx mori

Here we report the detection and localisation of chitin in the cuticle of the spinning ducts of both the spider Nephila edulis and the silkworm Bombyx mori. Our observations demonstrate that the duct walls of both animals contain chitin notwithstanding totally independent evolutionary pathways of the systems. We conclude that chitin may well be an essential component for the construction of spinning ducts; we further conclude that in both species chitin may indicate the evolutionary origin of the spinning ducts.

Alleged silk spigots on tarantula feet: electron microscopy reveals sensory innervation, no silk

Several studies on tarantulas have claimed that their tarsi could secrete fine silk threads which would provide additional safety lines for maintaining a secure foot-hold on smooth vertical surfaces. This interpretation was seriously questioned by behavioral experiments, and more recently morphological evidence indicated that the alleged spigots ("ribbed hairs") were not secretory but most likely sensory hairs (chemoreceptors). However, since fine structural studies were lacking, the sensory nature was not proven convincingly. By using transmission electron microscopy we here present clear evidence that these "ribbed hairs" contain many dendrites inside the hair lumen - as is the case in the well-known contact chemoreceptors of spiders and insects. For comparison, we also studied the fine structure of regular silk spigots on the spinnerets and found them distinctly different from sensory hairs. Finally, histological studies of a tarantula tarsus did not reveal any silk glands, which, by contrast, are easily found within the spinnerets. In conclusion, the alleged presence of silk spigots on tarantula feet is refuted.

Comparison of silk glands of diapause and non-diapause larval Sitodiplosis mosellana

The wheat midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), is one of the most serious pests of wheat worldwide. It overwinters as cocooned larvae in diapause and non-diapause forms. The cocoon is made of silk from the salivary glands. The silk glands, therefore, play an important role in the S. mosellana diapause. In the present study, the ultra-structures of the silk glands between diapause and non-diapause larvae were examined by electro and light-microscopically. The silk glands consist of 156 cells organized like moniliform particles. Although silk gland cells of both diapause and non-diapause larvae contain developed organelles, including the endoplasmic reticulum, dictyosome, mitochondria, and lipid droplet, the organelles in non-diapause larvae are more developed than those in diapause larvae. These morphological characteristics of the silk glands in the diapause and non-diapause larvae can be used to distinguish the diapause status of the larvae.

Morphology and histology of secretory setae in terrestrial larvae of biting midges of the genus Forcipomyia (Diptera: Ceratopogonidae)

Apneustic larvae of the genus Forcipomyia possess unique secretory setae located on the dorsal surface along the body in two rows, one pair on each thoracic and abdominal segment and two pairs on the head. Morphological and histological studies of secretory setae in fourth instar larvae of Forcipomyia nigra (Winnertz) and Forcipomyia nigrans Remm indicate they are modified mechanoreceptors (sensilla trichodea) in which the trichogen cell is a glandular cell producing a hygroscopic secretion. The cytoplasm of the glandular trichogen cell fills the lumen of a secretory seta, which shows one or more pores on the apex. The cytoplasm contains numerous microtubules responsible for transportation of proteinaceous vesicles, and an extremely large polyploid nucleus typical of gland cells. The main role of the hygroscopic secretion is to moist the body and thus facilitate cuticular respiration.

Extreme male leg polymorphic asymmetry in a new empidine dance fly (Diptera: Empididae)

A new dance fly (Empididae: Empidinae) with hugely modified male fore tarsus, either on the right, left, both or neither sides, is described from Japan. Such massive polymorphic asymmetry occurring with so high an incidence in a population is previously unreported. In view of the courtship behaviour of other Empidinae, we hypothesize that the oversized tarsus is a secondary sexual character employed by males for attracting females. Alternative hypotheses are also discussed. We suggest that this extraordinary new species is a potential model for the study of mating biology in Empidinae and the evolution of mating systems in general.

Insect silk: one name, many materials

Silks play a crucial role in the survival and reproduction of many insects. Labial glands, Malpighian tubules, and a variety of dermal glands have evolved to produce these silks. The glands synthesize silk proteins, which become semicrystalline when formed into fibers. Although each silk contains one dominant crystalline structure, the range of molecular structures that can form silk fibers is greater than any other structural protein group. On the basis of silk gland type, silk protein molecular structure, and the phylogenetic relationship of silk-producing species, we grouped insect silks into 23 distinct categories, each likely to represent an independent evolutionary event. Despite having diverse functions and fundamentally different protein structures, these silks typically have high levels of protein crystallinity and similar amino acid compositions. The substantial crystalline content confers extraordinary mechanical properties and stability to silk and appears to be required for production of fine protein fibers.

Comparison of embiopteran silks reveals tensile and structural similarities across Taxa

Embioptera is a little studied order of widely distributed, but rarely seen, insects. Members of this group, also called embiids or webspinners, all heavily rely on silken tunnels in which they live and reproduce. However, embiids vary in their substrate preferences and these differences may result in divergent silk mechanical properties. Here, we present diameter measurements, tensile tests, and protein secondary structural analyses of silks spun by several embiid species. Despite their diverse habitats and phylogenetic relationships, these species have remarkably similar silk diameters and ultimate stress values. Yet, ultimate strain, Young's modulus, and toughness vary considerably. To better understand these tensile properties, Fourier transformed infrared spectroscopy was used to quantify secondary structural components. Compared to other arthropod silks, embiid silks are shown to have consistent secondary structures, suggesting that commonality of amino acid sequence motifs and small differences in structural composition can lead to significant changes in tensile properties.

Characterization of silk spun by the embiopteran, Antipaluria urichi

Silks are renowned for being lightweight materials with impressive mechanical properties. Though moth and spider silks have received the most study, silk production has evolved in many other arthropods. One insect group that has been little investigated is Embioptera (webspinners). Embiopterans produce silk from unique tarsal spinning structures during all life stages. We characterize the molecular and mechanical properties of Antipaluria urichi (Embioptera) silk through multiple approaches. First, we quantify the number of silk secretory structures on their forelimbs and the tensile properties of Antipaluria silk. Second, we present silk protein (fibroin) transcripts from an embiopteran forelimb protarsomere cDNA library. We describe a fibroin that shares several features with other arthropod silks, including a subrepetitive core region, a non-repetitive carboxyl-terminal sequence, and a composition rich in glycine, alanine, and serine. Despite these shared attributes, embiopteran silk has several different tensile properties compared to previously measured silks. For example, the tensile strength of Antipaluria silk is much lower than that of Bombyx mori silk. We discuss the observed mechanical properties in relation to the fibroin sequence, spinning system, and embiopteran silk use.

Silks produced by insect labial glands

Insect silks are secreted from diverse gland types; this chapter deals with the silks produced by labial glands of Holometabola (insects with pupa in their life cycle). Labial silk glands are composed of a few tens or hundreds of large polyploid cells that secrete polymerizing proteins which are stored in the gland lumen as a semi-liquid gel. Polymerization is based on weak molecular interactions between repetitive amino acid motifs present in one or more silk proteins; cross-linking by disulfide bonds may be important in the silks spun under water. The mechanism of long-term storage of the silk dope inside the glands and its conversion into the silk fiber during spinning is not fully understood. The conversion occurs within seconds at ambient temperature and pressure, under minimal drawing force and in some cases under water. The silk filament is largely built of proteins called fibroins and in Lepidoptera and Trichoptera coated by glue-type proteins known as sericins. Silks often contain small amounts of additional proteins of poorly known function. The silk components controlling dope storage and filament formation seem to be conserved at the level of orders, while the nature of polymerizing motifs in the fibroins, which determine the physical properties of silk, differ at the level of family and even genus. Most silks are based on fibroin beta-sheets interrupted with other structures such as alpha-helices but the silk proteins of certain sawflies have predominantly a collagen-like or polyglycine II arrangement and the silks of social Hymenoptera are formed from proteins in a coiled coil arrangement.

An Australian webspinner species makes the finest known insect silk fibers

Aposthonia gurneyi, an Australian webspinner species, is a primitive insect that constructs and lives in a silken tunnel which screens it from the attentions of predators. The insect spins silk threads from many tiny spines on its forelegs to weave a filmy sheet. We found that the webspinner silk fibers have a mean diameter of only 65 nm, an order of magnitude smaller than any previously reported insect silk. The purpose of such fine silk may be to reduce the metabolic cost of building the extensive tunnels. At the molecular level, the A. gurneyi silk has a predominantly beta-sheet protein structure. The most abundant clone in a cDNA library produced from the webspinner silk glands encoded a protein with extensive glycine-serine repeat regions. The GSGSGS repeat motif of the A. gurneyi silk protein is similar to the well-known GAGAGS repeat motif found in the heavy fibroin of silkworm silk, which also has beta-sheet structure. As the webspinner silk gene is unrelated to the silk gene of the phylogenetically distant silkworm, this is a striking example of convergent evolution.

An independently evolved Dipteran silk with features common to Lepidopteran silks

Male hilarine flies (Diptera: Empididae: Empidinae) present prospective mates with silk-wrapped gifts. The silk is produced by specialised cells located in the foreleg basitarsus of the fly. In this report, we describe 2.3 kbp of the silk gene from a hilarine fly (Hilara spp.) that was identified from highly expressed mRNA extracted from the prothoracic basitarsus of males. Using specific primers, we found that the silk gene is expressed in the basitarsi and not in any other part of the male fly. The silk gene from the basitarsi cDNA library matched an approximately 220 kDa protein from the silk-producing basitarsus. Although the predicted silk protein sequence was unlike any other protein sequence in available databases, the architecture and composition of the predicted protein had features in common with previously described silks. The convergent evolution of these features in the Hilarini silk and other silks emphasises their importance in the functional requirements of silk proteins.