The larval alimentary canal of the Antarctic insect, Belgica antarctica

Morphology and fine organization of the midgut of Gampsocleis gratiosa (Orthoptera: Tettigoniidae)

The morphology and ultrastructure of the midgut of Gampsocleis gratiosa (Orthoptera: Tettigoniidae) was examined by light and electron microscopy. The midgut consists of two bulbous gastric caeca and a tubular ventriculus. The general organization of the gastric caeca is similar to that of the ventriculus. They are composed of a peritrophic membrane, an epithelium, a basal lamina and muscle layer from the inside to outside. Three types of cells were identified: regenerative, principal, and endocrine. Regenerative cells occur in groups (called nidi) at the base of principal cells. Principal cells grow from regenerative cells. Rare endocrine cells are scattered throughout the epithelium. Principal cells exhibit intense secretory activity, and regional differences in their ultrastructure were observed along the entire midgut. The microvilli are longer than those in any other region in the posterior region of the midgut. Lysosomes, multivesicular bodies (MVBs), autophagosomes, abundant Golgi apparatuses and lipid droplets primarily occur in the gastric caeca. Three pathways of secretion (merocrine, apocrine and holocrine) occur within the midgut epithelium, and a distinctive type of apocrine bleb was found in the gastric caeca. Therefore, these gastric caeca may be evolving toward a special type of gland.

Compartmentalization of microbial communities that inhabit the hindguts of millipedes

The gut lumen of the arthropod detritivore provides hospitable and multifaceted environments for diverse assemblages of microbes. Many microbes, including trichomycetes fungi, bacteria, and archaea establish stable, adherent communities on the cuticular surface secreted by the hindgut epithelium. Regional differences in the surface topography within the hindgut of a given millipede are reflected in differing and diverse microbial assemblages. The spirostreptid millipede Cambala speobia is a detritivore found on the floors of Texas caves. This millipede species has a very circumscribed distribution in North America and a diet confined to the limited litter that accumulates on floors of these caves while the common julid millipede Cylindroiulus caeruleocinctus, an introduced European species, feeds on the diverse litter found in organic soils throughout North America. In both millipedes, the gut lumina are inhabited along their entire lengths by microbes, with the highest microbial densities in the hindguts. The anterior third of the hindgut with its distinctive six-fold symmetry is lined by cuticle having fine polarized scales, and the posterior-most third is lined by smooth cuticle. Trichomycetes only inhabit the anterior third of the hindgut, and scattered patches of filamentous bacteria along with their smaller adherent microbes occupy the posterior third. The densest populations of microbes inhabit the central region of the hindgut. Over the cuticular surface of this hindgut region, uniformly distributed indentations mark possible channels for nutrient and water exchange between the hindgut lumen and host hemolymph. Films of microbes are adherent to the cuticle that lines the hindgut while those microbes in the remainder of the gut (i.e., foregut + midgut) represent mostly unattached inhabitants.

Stress tolerance in a polyextremophile: the southernmost insect

Since biotic interactions within the simple terrestrial communities on the Antarctic Peninsula are limited compared with tropical and temperate regions, survival is largely dictated by the numerous abiotic challenges. Our research focuses on adaptations to environmental stresses experienced by the Antarctic midge (Belgica antarctica Jacobs, 1900), the southernmost free-living insect. Midge larvae can survive freezing and anoxia year-round. Not only can frozen larvae undergo rapid cold-hardening (RCH) at temperatures as low as –12 °C, but RCH develops more rapidly in frozen compared with supercooled larvae. Whether larvae overwinter in a frozen state or cryoprotectively dehydrated may depend on hydration levels within their hibernacula. Larvae constitutively up-regulate genes encoding heat shock proteins, as well as the antioxidant enzymes superoxide dismutase and catalase. Larvae accumulate osmoprotectants in response to freezing, desiccation, and exposure to seawater; exposure to one of these osmotic stressors confers cross-tolerance to the others. Molecular responses to dehydration stress include extensive genome-wide changes that include differential expression of aquaporins among tissues, upregulation of pathways associated with autophagy, inhibition of apoptosis, and downregulation of metabolism and ATP production.

Ultrastructural studies on the midgut of biting midge Forcipomyia nigra (Winnertz) (Diptera: Ceratopogonidae)

Biting midges belonging to the genus Forcipomyia are known to be hematophagous, predatory or saprophagous. Different stages of Forcipomyia nigra midges were investigated to provide a description of midgut ultrastructure. Larvae feeding on decaying organic matter possess simple, straight alimentary tracts whose middle regions are the longest. TEM studies of the larval midgut epithelium reveal that digestive cells show different ultrastructure depending on their age. The older cells with electron-dense cytoplasm degenerate while the younger ones with electron-lucent cytoplasm remain active in digestion. In saprophagous females, the ultrastructure of midgut epithelium changes according to the age of flies. Oogenesis induces degeneration of digestive cells and utilization of reserve material accumulated by them. The midgut epithelia of male midges consist of digestive and regenerative cells that show no evidence of cell degeneration as observed in females. Our results demonstrate differences between midgut digestive cells of males and females.

The ultrastructure of the midgut epithelium in millipedes (Myriapoda, Diplopoda)

The midgut epithelia of the millipedes Polyxenus lagurus, Archispirostreptus gigas and Julus scandinavius were analyzed under light and transmission electron microscopies. In order to detect the proliferation of regenerative cells, labeling with BrdU and antibodies against phosphohistone H3 were employed. A tube-shaped midgut of three millipedes examined spreads along the entire length of the middle region of the body. The epithelium is composed of digestive, secretory and regenerative cells. The digestive cells are responsible for the accumulation of metals and the reserve material as well as the synthesis of substances, which are then secreted into the midgut lumen. The secretions are of three types - merocrine, apocrine and microapocrine. The oval or pear-like shaped secretory cells do not come into contact with the midgut lumen and represent the closed type of secretory cells. They possess many electron-dense granules (J. scandinavius) or electron-dense granules and electron-lucent vesicles (A. gigas, P. lagurus), which are accompanied by cisterns of the rough endoplasmic reticulum. The regenerative cells are distributed individually among the basal regions of the digestive cells. The proliferation and differentiation of regenerative cells into the digestive cells occurred in J. scandinavius and A. gigas, while these processes were not observed in P. lagurus. As a result of the mitotic division of regenerative cells, one of the newly formed cells fulfills the role of a regenerative cell, while the second one differentiates into a digestive cell. We concluded that regenerative cells play the role of unipotent midgut stem cells.

The fine structure of the midgut epithelium in a centipede, Scolopendra cingulata (Chilopoda, Scolopendridae), with the special emphasis on epithelial regeneration

Scolopendra cingulata has a tube-shaped digestive system that is divided into three distinct regions: fore-, mid- and hindgut. The midgut is lined with a pseudostratified columnar epithelium which is composed of digestive, secretory and regenerative cells. Hemocytes also appear between the digestive cells of the midgut epithelium. The ultrastructure of three types of epithelial cells and hemocytes of the midgut has been described with the special emphasis on the role of regenerative cells in the protection of midgut epithelium. The process of midgut epithelium regeneration proceeds due to the ability of regenerative cells to proliferate and differentiate according to a circadian rhythm. The regenerative cells serve as unipotent stem cells that divide in an asymmetric manner. Additionally, two types of hemocytes have been distinguished among midgut epithelial cells. They enter the midgut epithelium from the body cavity. Because of the fact that numerous microorganisms occur in the cytoplasm of midgut epithelial cells, we discuss the role of hemocytes in elimination of pathogens from the midgut epithelium. The studies were conducted with the use of transmission electron microscope and immunofluorescent methods.

Alimentary Canal of the Adult Blow Fly, Chrysomya megacephala (F.) (Diptera: Calliphoridae)-Part I: Ultrastructure of Salivary Glands

The salivary gland ultrastructure of the adult male blow fly, Chrysomya megacephala (F.) (Diptera: Calliphoridae), was investigated at the ultrastructural level using light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The salivary glands are paired structures composed of a single median deferent duct bifurcated into two long, narrow efferent ducts connected to the coiled tubular glands. The SEM image of the gland surface revealed that the basal lamina is relatively smooth in general, but the whole surface appeared as a trace of rough swollen insertion by intense tracheal ramification. Ultrastructurally, the salivary gland is enclosed within the basal lamina, and interdigitation cytoplasmic extensions were apparent between the adjacent gland cells. The basement membrane appeared infoldings that is similar to the complex of the labyrinth channel. The cytoplasm characteristic of the gland revealed high activity, based on the abundance of noticeable secretory granules, either singly or in an aggregated reservoir. In addition, mitochondria were found to intersperse among rich parallel of arrays rough endoplasmic reticulum. Thick cuticle, which was well-delineated and electron dense, apically lined the gland compartments, with discontinuity of the double-layer cuticle revealing a trace of secretion discharged into the lumen. Gross anatomy of the adult salivary gland was markedly different from that of the third instar of the same species, and structural dissimilarity is discussed briefly.

Function and immuno-localization of aquaporins in the Antarctic midge Belgica antarctica

Aquaporin (AQP) water channel proteins play key roles in water movement across cell membranes. Extending previous reports of cryoprotective functions in insects, this study examines roles of AQPs in response to dehydration, rehydration, and freezing, and their distribution in specific tissues of the Antarctic midge, Belgica antarctica (Diptera, Chironomidae). When AQPs were blocked using mercuric chloride, tissue dehydration tolerance increased in response to hypertonic challenge, and susceptibility to overhydration decreased in a hypotonic solution. Blocking AQPs decreased the ability of tissues from the midgut and Malpighian tubules to tolerate freezing, but only minimal changes were noted in cellular viability of the fat body. Immuno-localization revealed that a DRIP-like protein (a Drosophila aquaporin), AQP2- and AQP3 (aquaglyceroporin)-like proteins were present in most larval tissues. DRIP- and AQP2-like proteins were also present in the gut of adult midges, but AQP4-like protein was not detectable in any tissues we examined. Western blotting indicated that larval AQP2-like protein levels were increased in response to dehydration, rehydration and freezing, whereas, in adults DRIP-, AQP2-, and AQP3-like proteins were elevated by dehydration. These results imply a vital role for aquaporin/aquaglyceroporins in water relations and freezing tolerance in B. antarctica.