Morphology and function of Malpighian tubules and associated structures in the cockroach, Periplaneta americana

The ultrastructure of the spermatheca of Mordellistena brevicauda (Coleoptera, Tenebrionoidea) and the associated bacterial cells

The ultrastructural study on the female reproductive system of the beetle M. brevicauda (Mordellidae) confirmed the positive correlation between the length of the sperm and the size of the female seminal receptacle (Spermatheca). The spermatheca of the species is characterized by an apical bulb-like structure where the spermathecal duct forms numerous folds filled with sperm. At this level many bacterial cells are present intermingled with the duct folds. Some are organized in large structures, such as bacteriomes, while other are single bacteriocytes. The latter are often found near the basal lamina of duct epithelium. In addition, some bacteria are visible in the cytoplasm of the duct epithelial cells. Interestingly, bacterial cells have never been observed in the duct lumen. The possible function of the bacterial cells is discussed.

Histological Findings in Captive Madagascar Hissing Cockroaches (Gromphadorhina portentosa) and a Literature Review

Madagascar hissing cockroaches (MHC, Gromphadorhina portentosa) are members of the Blaberidae (giant cockroaches) family of the Insecta class. They are native to the African island of Madagascar where they live within leaf litter on the rainforest floor. Due to their large size, relative tameness, and general easy keeping, they have become popular in classrooms, zoological collections, museums, research laboratories, and as private exotic pets; however, descriptions of diseases of MHC in the literature are rare. The objective of this study is to describe and characterize postmortem histological findings in 18 captive MHC from a single zoological collection. In this retrospective study, 18 (4 females and 14 males) adult MHC necropsies were submitted to Northwest ZooPath between 2016 and 2020 for evaluation. The main organs with histological lesions were chitinous gut (foregut and/or hindgut; n = 17), tracheae (n = 15), fat body (n = 14), ventriculus (midgut) (n = 13), body wall (n = 12), Malpighian tubules (n = 12), and hemolymphatic sinuses (n = 12). All animals had inflammatory lesions affecting various organs. Inflammatory lesions typically consisted of aggregates of hemocytes with variable amounts of melanization and/or encapsulation. Bacterial, fungal, and parasitic infections were common and variably associated with hemocytic inflammation. Many of these organisms may represent symbiotic organisms of the MHC that cause opportunistic infections. This study contributes to the current knowledge of pathological findings and disease response of MHC and reviews diseases reported in multiple cockroach species.

Dehydration Dynamics in Terrestrial Arthropods: From Water Sensing to Trophic Interactions

Since the transition from water to land, maintaining water balance has been a key challenge for terrestrial arthropods. We explore factors that allow terrestrial arthropods to survive within a variably dry world and how they shape ecological interactions. Detection of water and hydration is critical for maintaining water content. Efficient regulation of internal water content is accomplished by excretory and osmoregulatory systems that balance water intake and loss. Biochemical and physiological responses are necessary as water content declines to prevent and repair the damage that occurs during dehydration. Desiccation avoidance can occur seasonally or daily via a move to more favorable areas. Dehydration and its avoidance have ecological impacts that extend beyond a single species to alter trophic interactions. As climate changes, evolutionary and ecological processes will be critical to species survival during drought.

The ultrastructure of sperm and female sperm storage organs in the water strider Gerris lacustris L. (Heteroptera) and a possible example of genital coevolution

The fine structural organization of the male and the female inner reproductive apparatuses of the water-strider Gerris lacustris was studied. The sperm of the species shows a long helicoidal acrosome provided with longitudinal tubules, and a short nucleus. The flagellum is characterized by crescent mitochondrial derivatives and a 9 + 9 + 2 axoneme, as occurs in all Heteroptera. The female reproductive apparatus is characterized by an extremely long spermathecal duct, filled with sperm, which plays the role of the main sperm storage organ. The duct has a thin epithelium surrounded by a complex of secretory and duct-forming cells. The spermathecal duct flows into the gynatrial sac. This region, together with the fertilization chamber, exhibits a simple epithelium with deep apical plasma membrane invaginations, and it does not show conspicuous secretions. The basal cell region shows plasma membrane infoldings forming thin cytoplasmic bands hosting mitochondria and large intercellular spaces. This organization is typical of epithelia active in fluid reabsorption. Two lateral large gynatrial glands open into the gynatrial sac. Such glands also exhibit secretory and duct forming cells. The same structure of these glands is also present along the proximal region of the fecundation canal. The duct forming cells of these regions have very wide ducts with peculiar cuticular finger-like structures at their opening into the gland duct lumen. The results of the present study suggest the occurrence of a coevolution between the sperm and the spermathecal duct lengths.

Novel glucosinolate metabolism in larvae of the leaf beetle Phaedon cochleariae

Plants of the Brassicales are defended by a binary system, in which glucosinolates are degraded by myrosinases, forming toxic breakdown products such as isothiocyanates and nitriles. Various detoxification pathways and avoidance strategies have been found that allow different herbivorous insect taxa to deal with the glucosinolate-myrosinase system of their host plants. Here, we investigated how larvae of the leaf beetle species Phaedon cochleariae (Coleoptera: Chrysomelidae), a feeding specialist on Brassicaceae, cope with this binary defence. We performed feeding experiments using leaves of watercress (Nasturtium officinale, containing 2-phenylethyl glucosinolate as major glucosinolate and myrosinases) and pea (Pisum sativum, lacking glucosinolates and myrosinases), to which benzenic glucosinolates (benzyl- or 4-hydroxybenzyl glucosinolate) were applied. Performing comparative metabolomics using UHPLC-QTOF-MS/MS, N-(phenylacetyl) aspartic acid, N-(benzoyl) aspartic acid and N-(4-hydroxybenzoyl) aspartic acid were identified as major metabolites of 2-phenylethyl-, benzyl- and 4-hydroxybenzyl glucosinolate, respectively, in larvae and faeces. This suggests that larvae of P. cochleariae metabolise isothiocyanates or nitriles to aspartic acid conjugates of aromatic acids derived from the ingested benzenic glucosinolates. Myrosinase measurements revealed activity only in second-instar larvae that were fed with watercress, but not in freshly moulted and starved second-instar larvae fed with pea leaves. Our results indicate that the predicted pathway can occur independently of the presence of plant myrosinases, because the same major glucosinolate-breakdown metabolites were found in the larvae feeding on treated watercress and pea leaves. A conjugation of glucosinolate-derived compounds with aspartic acid is a novel metabolic pathway that has not been described for other herbivores.

The effect of cold acclimation on active ion transport in cricket ionoregulatory tissues

Cold-acclimated insects defend ion and water transport function during cold exposure. We hypothesized that this is achieved via enhanced active transport. The Malpighian tubules and rectum are likely targets for such transport modifications, and recent transcriptomic studies indicate shifts in Na+-K+ ATPase (NKA) and V-ATPase expression in these tissues following cold acclimation. Here we quantify the effect of cold acclimation (one week at 12°C) on active transport in the ionoregulatory organs of adult Gryllus pennsylvanicus field crickets. We compared primary urine production of warm- and cold-acclimated crickets in excised Malpighian tubules via Ramsay assay at a range of temperatures between 4 and 25°C. We then compared NKA and V-ATPase activities in Malpighian tubule and rectal homogenates from warm- and cold-acclimated crickets via NADH-linked photometric assays. Malpighian tubules of cold-acclimated crickets excreted fluid at lower rates at all temperatures compared to warm-acclimated crickets. This reduction in Malpighian tubule excretion rates may be attributed to increased NKA activity that we observed for cold-acclimated crickets, but V-ATPase activity was unchanged. Cold acclimation had no effect on rectal NKA activity at either 21°C or 6°C, and did not modify rectal V-ATPase activity. Our results suggest that an overall reduction, rather than enhancement of active transport in the Malpighian tubules allows crickets to maintain hemolymph water balance during cold exposure, and increased Malpighian tubule NKA activity may help to defend and/or re-establish ion homeostasis.

Tracing the evolutionary origins of insect renal function

Knowledge on neuropeptide receptor systems is integral to understanding animal physiology. Yet, obtaining general insight into neuropeptide signalling in a clade as biodiverse as the insects is problematic. Here we apply fluorescent analogues of three key insect neuropeptides to map renal tissue architecture across systematically chosen representatives of the major insect Orders, to provide an unprecedented overview of insect renal function and control. In endopterygote insects, such as Drosophila, two distinct transporting cell types receive separate neuropeptide signals, whereas in the ancestral exopterygotes, a single, general cell type mediates all signals. Intriguingly, the largest insect Order Coleoptera (beetles) has evolved a unique approach, in which only a small fraction of cells are targets for neuropeptide action. In addition to demonstrating a universal utility of this technology, our results reveal not only a generality of signalling by the evolutionarily ancient neuropeptide families but also a clear functional separation of the types of cells that mediate the signal.

The evolution of neuropeptide signalling in insects is poorly understood. Here the authors map renal tissue architecture in the major insect Orders, and show that while the ancient neuropeptide families are involved in signalling in nearly all species, there is functional variation in the cell types that mediate the signal.

Physiology of environmental adaptations and resource acquisition in cockroaches

Cockroaches are a group of insects that evolved early in geological time. Because of their antiquity, they for the most part display generalized behavior and physiology and accordingly have frequently been used as model insects to examine physiological and biochemical mechanisms involved with water balance, nutrition, reproduction, genetics, and insecticide resistance. As a result, a considerable amount of information on these topics is available. However, there is much more to be learned by employing new protocols, microchemical analytical techniques, and molecular biology tools to explore many unanswered questions.

The tiptop/teashirt genes regulate cell differentiation and renal physiology in Drosophila

The physiological activities of organs are underpinned by an interplay between the distinct cell types they contain. However, little is known about the genetic control of patterned cell differentiation during organ development. We show that the conserved Teashirt transcription factors are decisive for the differentiation of a subset of secretory cells, stellate cells, in Drosophila melanogaster renal tubules. Teashirt controls the expression of the water channel Drip, the chloride conductance channel CLC-a and the Leukokinin receptor (LKR), all of which characterise differentiated stellate cells and are required for primary urine production and responsiveness to diuretic stimuli. Teashirt also controls a dramatic transformation in cell morphology, from cuboidal to the eponymous stellate shape, during metamorphosis. teashirt interacts with cut, which encodes a transcription factor that underlies the differentiation of the primary, principal secretory cells, establishing a reciprocal negative-feedback loop that ensures the full differentiation of both cell types. Loss of teashirt leads to ineffective urine production, failure of homeostasis and premature lethality. Stellate cell-specific expression of the teashirt paralogue tiptop, which is not normally expressed in larval or adult stellate cells, almost completely rescues teashirt loss of expression from stellate cells. We demonstrate conservation in the expression of the family of tiptop/teashirt genes in lower insects and establish conservation in the targets of Teashirt transcription factors in mouse embryonic kidney.

The versatile stellate cell - more than just a space-filler

Most epithelia contain multiple cell types that interact to perform the roles required of the tissue. In insect epithelia, the apical plasma membrane V-ATPase dominates ion-transport models, and (as in vertebrates) is usually found in specialized intercalated cell types or regions. The Malpighian tubules of several insect Orders contain not just a mitochondrion-rich principal cell expressing high levels of V-ATPase, but a smaller, intercalated "type II", "secondary" or "stellate" cell. Recent data show that this cell type plays a key role in control of chloride and water flux across the tissue, but also may play other, still unsuspected dynamic roles.

Segmental specializations in the Malpighian tubules of the fire ant Solenopsis saevissima Forel 1904 (Myrmicinae): an electron microscopical study

The Malpighian tubules of workers of the fire ant Solenopsis saevissima (Myrmicinae) were analyzed by scanning and transmission electron microscopy in order to determine their functional organization and association with the hindgut epithelium. The ants showed six Malpighian tubules with three segments morphologically and structurally different. The proximal segment was long and its cells showed abundant smooth endoplasmic reticulum and lipid droplets, which suggest their role in lipid secretion. The mid segment was long and undulated and it was composed by the cells that showed the typical features of ion transporting epithelia. The distal segment, short and flattened, adheres to the rectum wall. The cells of this segment showed the basal lamina fused to that of the rectum, it is probable that this part of the tubule may play a role in ion and water uptake from the feces.

Light and electron microscopic study of the hindgut of the ant (Formica nigricans, hymenoptera): I. Structure of the ileum

The study of the ileum of the ant Formica nigricans by light and electron microscopy revealed the existence of three differentiated regions: proximal, middle, and distal ileum. The middle region constitutes most of the length of the organ. Its wall is made up by a folded simple epithelium lined by a cuticle, which is surrounded by an inner circular muscle layer and various external longitudinal muscle fibers adjacent to the hemolymph. A subepithelial space is present between the epithelium and the circular muscle layer. Epithelial cells show extensive infoldings of the apical, and to a lesser extent the basolateral plasma membrane. Apical infoldings are characterized by the presence of 10-nm particles (portasomes) covering the cytoplasmic side of the membrane. Mitochondria are abundant throughout the cytoplasm, although they mainly are present underneath the apical infoldings. Lateral borders of epithelial cells display an apical junctional complex, mainly constituted by a long and convoluted pleated septate junction. These features support the view that epithelial cells in the middle ileum are specialized in ion solutes and water transport. The proximal ileum connects with the ampulla into which the Malpighian tubules drain. As opposed to the middle ileum, epithelial cells of the proximal ileum show less developed basolateral infoldings, and the apical plasma membrane is devoid of portasomes and only occasionally invaginates. These features suggest that the proximal ileum plays no relevant role in ion and water transport. The distal ileum penetrates into the rectal sac, forming a valve-like structure; this region presumably controls the amount of urine reaching the rectum.

Fine structure of larval malpighian tubules and rectal sac in the tick Ornithodoros (Pavlovskyella) erraticus (Ixodoidea: Argasidae)

The fine structure of the Malpighian tubules (Mts) and rectal sac (rs) is described in the larval tick Ornithodoros (Pavlovskyella) erraticus before and after feeding up to molting. Mts consist of structurally different pyramidal and cuboidal cells along the entire length of the tubule. In unfed ticks, the two types of cell are characterized by apical microvilli and a few basal membrane infoldings. The abundant pyramidal cells contain glycogen particles, lipid droplets, lysosomelike structures, and rickettsialike microorganisms. After feeding but before molting, pyramidal cells loose glycogen particles and become very dense and dramatically reduced in size. These cells are possibly involved in the formation of guanine crystalloids as an excretory product. In contrast, cuboidal cells, filled with glycogen particles, free ribosomes, and mitochondria in unfed larvae, grow steadily after feeding; their cytoplasm becomes rich in lipid droplets in addition to showing an increase in glycogen particles. Lipid and glycogen could be the source of energy required for water and ion reabsorption in which cuboidal cells are probably involved. The paired-lobe rs consists of one type of cuboidal cells with basal membrane infoldings and a brush-border microvilli covered by a fuzzy coat of glycocalyx. These cells grow rapidly after feeding; they have functional features indicating extensive, selective reabsorption of essential components from excretory products.

Fine structure of the Malpighian tubules of chironomus larva in relation to glycogen storage and fate of hemoglobin

The larval Malpighian tubules of Chironomus tentans were studied using light and electron microscopy. The tubules are composed of two cell types: primary and stellate cells. Both cell types lack muscles, tracheoles, and laminate crystals in the cytoplasm and mitochondria in the microvilli. The primary cells exhibit long, wide basal membrane infoldings associated with mitochondria. They have a number of canaliculi and long, closely packed microvilli. The stellate cells possess shorter interconnecting basal infoldings and shorter, well-spaced microvilli. Both cell types are linked by septate and gap junctions. They have cytoplasmic processes and pedicels which enclose narrow slits between them and that are apposed to a basal lamella. In the 'fed' larva, the cells are stuffed with glycogen which is depleted in the 'starved' larva. Both cell types are involved in the vesicular transport of biliverdin. The presence of coated vesicles, tubular elements and various forms of lysosomes in the primary cells suggests they transport and break down functional hemoglobin. Structural modification of basal infoldings, canaliculi and microvilli is strongly correlated with increased secretory activity of the Malpighian tubules in 'fed' versus 'starved' larva.

Microbial aspects of the cockroach hindgut

The cockroach hindgut has a complex, active microbiota, a large portion of which is associated with the chitinous gut wall. It provides a different environment from that of termites and other insects which are dependent on their hindgut microbiota for the digestion of cellulose. The pH of the midgut of Eublaberus posticus was not as high as it is in insects with a primarily cellulolytic nutrition. The hindgut of E. posticus was highly methanogenic, normal adults producing typically 10-25 mumol of methane per hour. The hindgut contained large amounts of the storage products polyphosphate and poly-beta-hydroxybutyrate. Dilution series on nonselective medium yielded 100 times more obligately anaerobic colonies than facultatively anaerobic colonies. The most common facultative isolates were Klebsiella oxytoca, Citrobacter freundii and Enterobacter agglomerans. Treatment of E. posticus with metronidazole caused a dedifferentiation of the different regions of the hindgut. One region of the hindgut is characterized by its visibly black color, a unique microbiota, and electron dense deposits in electron micrographs. Chemical determinations showed high concentrations of ferrous and sulfide ions in the region. X-ray microprobe analysis showed that some of the electron dense deposits consisted of iron, sulfur and lower amounts of copper, aluminium, and chromium associated with ruthenium red staining material. Spectra of other deposits revealed only silicon, which was not associated with ruthenium red.

Ultrastructural evidence for paracellular fluid flow in the Malpighian tubules of a larval mayfly

The ultrastructure of the Malpighian tubules of larvae of the Mayfly Ecdyonurus dispar (Ephemeroptera) is described. There are about 60 tubules, which consist of four distinct regions. The most proximal section (region I) appears to be responsible for fluid secretion. A unique feature is the presence of channels leading off the main lumen, which end close to the basal border of the cells. Microvilli are confined to these channels in region I. Region II is a short spiral region, the cells of which possess long basal folds and associated mitochondria. Region III is a simple conducting tube leading to one of six collecting ducts (region IV) arranged radially around the gut. In each collecting duct there are two cell types present. Type 2 cells are relatively simple, but give rise to numerous, long, microvilli-like projections. Type 1 cells possess long basal folds, and curious membrane whorls in the apical zone. Evidence is presented which suggest that water movements into region I takes place via the paracellular route. Region II is probably a reabsorptive region, but the function of region IV, based on ultrastructural evidence is more difficult to elucidate.

A clarification of the two types of invertebrate pleated septate junction

It is confirmed that there are two distinct variations septate junction. The first of these, the 'lower invertebrate pleated septate junction', is described fully using conventional thin section, lanthanum tracer and freeze-fracture techniques. The second type, the well-known pleated septate junction characteristic of the molluscs and arthropods, is renamed the 'mollusc-arthropod pleated septate junction', and is described briefly to allow easier comparison between the two variations. As both types have not been studied in a range of invertebrate phyla the results can be used as a basis for discussing their respective phylogenetic positions. The lower invertebrate pleated septate junction occurs in several groups in the minor phyla immediately above the Coelenterata and in the lower phyla of both the deuterostome and proterostome lineages. The mollusc-arthropod pleated septate junction is restricted to the Mollusca and Arthropoda as its name implies.

Definitive evidence for the existence of tight junctions in invertebrates

Extensive and unequivocal tight junctions are here reported between the lateral borders of the cellular layer that circumscribes the arachnid (spider) central nervous system. This account details the features of these structures, which form a beltlike reticulum that is more complex than the simple linear tight junctions hitherto found in invertebrate tissues and which bear many of the characteristics of vertebrate zonulae occludentes. We also provide evidence that these junctions form the basis of a permeability barrier to exogenous compounds. In thin sections, the tight junctions are identifiable as punctate points of membrane apposition; they are seen to exclude the stain and appear as election- lucent moniliform strands along the lines of membrane fusion in en face views of uranyl-calcium-treated tissues. In freeze-fracture replicas, the regions of close membrane apposition exhibit P-face (PF) ridges and complementary E-face (EF) furrows that are coincident across face transitions, although slightly offset with respect to one another. The free inward diffusion of both ionic and colloidal lanthanum is inhibited by these punctate tight junctions so that they appear to form the basis of a circumferential blood-brain barrier. These results support the contention that tight junctions exist in the tissues of the invertebrata in spite of earlier suggestions that (a) they are unique to vertebrates and (b) septate junctions are the equivalent invertebrate occluding structure. The component tight junctional 8- to 10-nm-particulate PF ridges are intimately intercalated with, but clearly distinct from, inverted gap junctions possessing the 13-nm EF particles typical of arthropods. Hence, no confusion can occur as to which particles belong to each of the two junctional types, as commonly happens with vertebrate tissues, especially in the analysis of developing junctions. Indeed, their coexistance in this way supports the idea, over which there has been some controversy, that the intramembrane particles making up these two junctional types must be quite distinct entities rather than products of a common precursor.

An ultrastructural study and stereological analysis of the colon wall in the cockroach Periplaneta americana

The colonic epithelium has been examined for ultrastructural evidence of physiological activity. The cells show extensive folding of the apical plasma membrane, associated with mitochondria and an internal coating of particles about 120 A diameter. Anteriorly many apical infoldings are dilated at the tip to form substantial extracellular spaces up to 0.8 micron wide. Narrow intercellular channels are present, opening to the haemolymph side of the epithelium. Pinocytosis is seen frequently at the basal surface. The surface densities of apical plasma membranes were not significantly different in the posterior mid-gut and colon. Similarly the volume densities of mitochondria were equal in the colonic epithelium and rectal pads, but the surface density of outer mitochondrial membranes was greater in the colon. It is suggested that the colon may absorb organic solutes from the gut lumen.

Developmental changes in Malpighian tubule cell structure

Structural changes which occur in the Malpighian tubule yellow region primary cells during larval-pupal-adult development of the skipper butterfly Calpodes ethlius are described. The developmental changes in cell structure are correlated with functional changes in fluid transport (Ryerse, 1978a) in a way which supports osmotic gradient models of fluid secretion. Larval tubules are specialized for fluid secretion with deep basal infolds and elongate mitochondria-containing apical microvilli which provide channels in which osmotic gradients could be set up. The Malpighian tubule cells are extensively remodelled at pupation when fluid transport is switched off, but they persist intact through metamorphosis. At this time, the basement membrane doubles in thickness, the mitochondria are retracted from the microvilli and are isolated for degradation in autophagic vacuoles, and both apical and basal plasma membranes are internalized via coated vesicles for degradation in multivesicular bodies, which results in the shortening of the microville and the disappearance of the basal infolds. Mitochondria are re-inserted into the microvilli, and the basal infolds re-form in pharate adult stage Malpighian tubules when fluid secretion resumes. Adult tubules are similar in general structure to larval tubules and contain mitochondria in the microvilli and basal infolds. However, they differ from larval tubules in that they are capable of very rapid fluid transport, have a reduced tubule diameter and tubule wall thickness, a much thicker basement membrane and peripherally associated tracheoles. Mineral concretions of calcium phosphate accumulate in larval tubules, persist through metamorphosis and decline in number in adults, suggesting they serve some anabolic role.

Regional specialization in the Malpighian tubules of the New Zealand glow-worm Arachnocampa luminosa (Diptera: mycetophilidae). The structure and function of type I and II cells

The Malpighian tubules of the glow-worm Arachnocampa luminosa are divided into four morphologically distinct regions (Parts 1--4) each comprised of a different cell type (Types I--IV). The ultrastructure of Type II cells is indicative of a transport function. The basal cell surface is highly invaginated and at the apical surface the lumen is lined with microvilli about 80% of which contain mitochondria. Spherites contained in these cells are formed from small vesicles produced by the Golgi apparatus. They have a central uric acid core enclosed by laminations of phosphates of calcium and magnesium. Cells of Part 2 of the tubule secrete a fluid high in potassium (173 mM) and low in sodium (18 mM). The cell is 30 mV negative and the lumen 44 mV positive to the bathing solution. This is consistent with the proposal of an apical cation pump. The secretion produced by Part 2 of the tubules is modified by the Type I cells by the reabsorption of potassium (162 mM) and the addition of sodium (24 mM) to the primary excretory fluid. Type I cells are 20 mV negative and the lumen 22 mV positive with respect to the bathing medium. From ultrastructural observations, Type I cells exhibit features characteristic of transporting cells thought to have an absorptive function. The basal and apical cell surfaces are extensively folded, and mitochondria are found in bands above the basal infoldings and below the microvilli. Mitochondria do not penetrate the microvilli. On comparative grounds, the fine structure of Type I cells suggest that they reabsorb ions from the tubule lumen. Energy for these processes may come from the breakdown of lipids by microperoxisomes contained within these cells. Alternatively, the fluid produced by Part 2 of the tubule may be modified passively by diffusional processes across Type I cells.

The ultrastructure of the posterior midgut caecum of Pachygrapsus crassipes (Decapoda, Brachyura) adapted to low salinity

The effects of salinity adaptation and of composition and tonicity of fixatives upon the ultrastructure of the posterior midgut caecum (PMC) of Pachygrapsus crassipes have been studied. The PMC epithelium consists of a single layer of columnar cells with a microvillous border. The apical cytoplasm contains numerous mitochondria, lysosomes, and much smooth endoplasmic reticulum. Rough endoplasmic reticulum and Golgi apparatus are situated in the perinuclear cytoplasm. This epithelium resembles other transporting epithelia in that the basal cytoplasm has an extensive system of branched tubules formed from invaginations of the lateral and basal plasma membrane. Numerous mitochondria are associated with the basal tubular system. To determine the possible contribution of the PMC to the osmoregulatory ability of Pachygrapsus, the ultrastructure of the PMC from animals adapted to 40, 50, and 100 and 150% sea water was investigated. Enlargement of basal tubules and intercellular spaces at low salinity, suggestive of fluid-transport activity, was found to be an artifact of fixation. The most consistent response when animals were acclimated to dilute salinities was that some basal mitochondria assume a more complex shape, usually appearing as rings in cross sections of the caecum. A hypothesis concerning the functional significance of these mitochondria is proposed.

Fine structure and x-ray microanalysis of mineralized concretions in the malpighian tubules of the housefly, Musca domestica

The epithelium and the lumen of the Malpighian tubules of the housefly contains mineralized dense bodies called concretions. The morphological characteristics, mode of origin, nature of the sequestered elements and the age-associated changes in the distribution of concretions are reported. There are three types of concretions in the cytoplasm, which have been designated as type A, type B, and type C. Type A concretions are membrane-bound spherical structures which may arise by the gradual intravacuolar accumulation of dense material. Type B concretions appear to be related to multivesicular bodies. Type C concretions are heteromorphic and morphologically resemble the residual bodies. They show a positive localization of acid phosphatase reaction product. X-ray microanalysis of intracytoplasmic and intraluminal concretions revealed the presence of phosphorus, sulphur, chlorine, calcium, iron, zinc and copper. There was no evidence suggesting the extrusion of the intracytoplasmic concretions into the lumen of the Malpighian tubules. There is an age-associated increase in the distribution of type C concretions. It is hypothesized that the sequestration of metal ions within the concretions may provide a means for the effective excretion of these elements.