The molt/intermolt cycle in the epidermis and other tissues of an insect Calpodes ethlius(Lepidoptera, hesperiidae)

The PEDtracker: An Automatic Staging Approach for Drosophila melanogaster Larvae

The post-embryonal development of arthropod species, including crustaceans and insects, is characterized by ecdysis or molting. This process defines growth stages and is controlled by a conserved neuroendocrine system. Each molting event is divided in several critical time points, such as pre-molt, molt, and post-molt, and leaves the animals in a temporarily highly vulnerable state while their cuticle is re-hardening. The molting events occur in an immediate ecdysis sequence within a specific time window during the development. Each sub-stage takes only a short amount of time, which is generally in the order of minutes. To find these relatively short behavioral events, one needs to follow the entire post-embryonal development over several days. As the manual detection of the ecdysis sequence is time consuming and error prone, we designed a monitoring system to facilitate the continuous observation of the post-embryonal development of the fruit fly Drosophila melanogaster. Under constant environmental conditions we are able to observe the life cycle from the embryonic state to the adult, which takes about 10 days in this species. Specific processing algorithms developed and implemented in Fiji and R allow us to determine unique behavioral events on an individual level—including egg hatching, ecdysis and pupation. In addition, we measured growth rates and activity patterns for individual larvae. Our newly created RPackage PEDtracker can predict critical developmental events and thus offers the possibility to perform automated screens that identify changes in various aspects of larval development. In conclusion, the PEDtracker system presented in this study represents the basis for automated real-time staging and analysis not only for the arthropod development.

Mutation of a cuticular protein, BmorCPR2, alters larval body shape and adaptability in silkworm, Bombyx mori

Cuticular proteins (CPs) are crucial components of the insect cuticle. Although numerous genes encoding cuticular proteins have been identified in known insect genomes to date, their functions in maintaining insect body shape and adaptability remain largely unknown. In the current study, positional cloning led to the identification of a gene encoding an RR1-type cuticular protein, BmorCPR2, highly expressed in larval chitin-rich tissues and at the mulberry leaf-eating stages, which is responsible for the silkworm stony mutant. In the Dazao-stony strain, the BmorCPR2 allele is a deletion mutation with significantly lower expression, compared to the wild-type Dazao strain. Dysfunctional BmorCPR2 in the stony mutant lost chitin binding ability, leading to reduced chitin content in larval cuticle, limitation of cuticle extension, abatement of cuticle tensile properties, and aberrant ratio between internodes and intersegmental folds. These variations induce a significant decrease in cuticle capacity to hold the growing internal organs in the larval development process, resulting in whole-body stiffness, tightness, and hardness, bulging intersegmental folds, and serious defects in larval adaptability. To our knowledge, this is the first study to report the corresponding phenotype of stony in insects caused by mutation of RR1-type cuticular protein. Our findings collectively shed light on the specific role of cuticular proteins in maintaining normal larval body shape and will aid in the development of pest control strategies for the management of Lepidoptera.

The formation of plasma membrane reticular systems in the oenocytes of an insect

Plasma membrane reticular systems (RSs) are infolds of the plasma membrane found in cells of several insect tissues that are not transporting epithelia. They form a subsurface reticular lymph space that may be involved in the loading and unloading of hemolymph carrier molecules. The development of a new RS during the fifth larval stadium has been studied in the oenocytes of Calpodes ethlius by scanning electron microscopy. The RS forms by the extension and progressive apical fusion of cell processes leaving a reticular lymph space below. Reticular system formation occurs in a front moving over the cell surface. The RS made in the 4th stadium persists through the moult to the 5th stage but diminishes for the next 3 days. A new intermoult RS then forms very quickly. Its time of formation follows the commitment ecdysteroid peak rather than the beginning of secretion by the wax glands. This new 5th stage RS is maintained during the period of intermoult synthesis, after which it declines and is nearly absent by the time of pupation.

The development of branched silk gland nuclei

Nuclei in the giant polyploid silk gland cells of Calpodes ethlius grow by endomitosis and can develop hundreds of branches during larval life. The shape of the these nuclei is characteristic for each region of the gland. We have found shape to be correlated with arrangement of the nuclear matrix. Scanning electron microscopy showed nuclear matrices with shapes similar to those of feulgen stained nuclei. Profiles of isolated matrices seen by transmission electron microscopy had filaments aligned parallel to the long axis of nuclear branches. DNA stained by Hoechst had a similar parallel alignment within the branches. Nuclear shape may be maintained by a small number of components, since electrophoretic analysis showed only a few abundant polypeptides in the matrix fraction. Silk gland nuclei have some of the same nuclear matrix antigens found in smaller, more regularly shaped, eukaryote nuclei.

Surface membranes, Golgi complexes, and vacuolar systems

In the absence of fossils, the cells of vertebrates are often described in lieu of a general animal eukaryote model, neglecting work on insects. However, a common ancestor is nearly a billion years in the past, making some vertebrate generalizations inappropriate for insects. For example, insect cells are adept at the cell remodeling needed for molting and metamorphosis, they have plasma membrane reticular systems and vacuolar ferritin, and their Golgi complexes continue to work during mitosis. This review stresses the ways that insect cells differ from those of vertebrates, summarizing the structure of surface membranes and vacuolar systems, especially of the epidermis and fat body, as a prerequisite for the molecular studies needed to understand cell function. The objective is to provide a structural base from which molecular biology can emerge from biochemical description into a useful analysis of function.

L-Glutamate retrieved with the moulting fluid is processed by a glutamine synthetase in the pupal midgut of Calpodes ethlius

From apolysis until pupal ecdysis, the pharate pupa of the Brazilian Skipper (Calpodes ethlius) lies wrapped in a prepupal shell composed of the larval cuticle and an ecdysial space (ES) filled with enzyme-rich moulting fluid (MF). In the 4h before ecdysis the pharate pupa drinks the moulting fluid through its mouth and anus, and transfers the cuticular degradation products to its midgut (MG). At the same time, extra fluid passes across the body wall of the pharate pupa and flushes out the ES. The MF is recovered at an overall rate of 70µl/h and reabsorbed across the pharate pupal midgut at about 26µl/h. L-Glutamate was found to be the dominant amino acid in the moulting fluid. Total MF glutamate peaked at 850nmol about 8h before pupal ecdysis (P-8), but by ecdysis it had dropped to nearly zero as the MF became diluted with new fluid and was consumed. The drop in glutamate in the ES coincided with a rise in the glutamine content of the fluid in the midgut lumen. The highest rate of glutamine synthesis occurred in midguts isolated from pharate pupae actively drinking MF (P</=-4). The enzyme glutamine synthetase (GS) was found to be active in glutamate metabolism in the pharate pupal midgut. Glutamine synthesis in the midgut was L-glutamate-dependent and inhibited by two selective competitive inhibitors of GS activity, L-methionine sulfoximine (MSO) and glufosinate ammonium (GLA). Injection of GS inhibitors into the prepupal ES greatly reduced the glutamine content of the midgut epithelium by P+24. Although a corresponding increase in midgut glutamate levels was not seen, midgut serine levels in treated animals rose, suggesting that GS inhibitors shunted the MF-derived glutamate along an alternative metabolic pathway. GLA was much more toxic to pupae than MSO. Midgut GS appears to play a central role in the recycling of L-glutamate across the pupal MG epithelium at pupation.

The origin, transport and cleavage of the molt-associated cuticular protein CECP22 from Calpodes ethlius (Lepidoptera, Hesperiidae)

CECP22 (Calpodes ethlius Cuticular Protein 22 kDa) is a molt associated protein found in the cuticle of C. ethlius larvae and pupae. The mRNA for the CECP22 cuticular protein is expressed in the epidermis and fat body during the intermolt. The protein itself accumulates in intermolt hemolymph, but at molting, when the cuticle is being digested, it is also found in the cuticle of surface integument, tracheae, foregut and hindgut and in the molting fluid. CECP22 exists in two forms. The large form (19.17 kDa, pI 6.2) becomes smaller (16.1 kDa, pI 7.4) by cleavage at the proteolytic cleavage site (position 170) with amidation of the C-terminal. The small, more basic peptide, appears only at molting, first in the cuticle and then in the molting fluid. It is presumed to be the active form of an amidase involved in the earliest stages of cuticle degradation. The inactive form accumulates in the hemolymph during the long intermolt and probably represents an abundant source of precursor enzyme that can be provided to all cuticle containing organs for a precise initiation of cuticle degradation.

Site of synthesis, tissue distribution, and lipophorin transport of hydrocarbons in Blattella germanica (L.) nymphs

The site of hydrocarbon (HC) synthesis and the amount of HC in various tissues were investigated in relation to developmental stage in the last larval stadium of the German cockroach, Blattella germanica. Abdominal integument linearly incorporated [1-(14)C]propionate into HC for at least 6h in vitro, whereas other body parts synthesized little or no HC. The third through sixth abdominal sternites and tergites were the principal sites of synthesis. High rates of HC synthesis resulted in a fivefold increase in internal HC during the last stadium. We examined the distribution of HC in the hemolymph, fat body, and the developing imaginal cuticle. Hemolymph HC titer was relatively constant at approximately 8&mgr;g/&mgr;l. However, as hemolymph volume increased from 5 to 11&mgr;l in the first 4days of the last stadium, HC content increased and then remained stable the remainder of the stadium. Lipophorin, immunoprecipitated with adult lipophorin polyclonal antibodies, was the only HC carrier protein in nymphal hemolymph and its HC profile was identical to that of hemolymph and similar to that of the epicuticle. The concentration and total amount of hemolymph lipophorin increased until 3days before adult eclosion and declined immediately after ecdysis. The HC content of non-biosynthetic integument (legs, pronotum) doubled during formation of the imaginal cuticle, as did the HC content of sternites, which synthesize HC. HC content of fat body, however, increased threefold during the same period, suggesting that the fat body serves as a storage site for HC during cuticle formation. We conclude that in the last stadium HC is synthesized by abdominal oenocytes, loaded onto hemolymph lipophorin, and transported to fat body and both nymphal and imaginal cuticle. Hydrocarbons associate with the imaginal integument several days before eclosion.

A cuticular protein from the moulting stages of an insect

A 22 kDa peptide was purified from prepupal cuticles of 5th instar Calpodes ethlius caterpillars. It was absent earlier in the stadium and from the egg and adult, i.e. it is related to cuticle turnover rather than cuticle structure. It was present at larval and metamorphic moults, showing that it is related to moulting not just metamorphosis. The cDNA corresponding to the 22 kDa peptide was isolated by antibody screening of an epidermal cDNA expression library. Hybridization to Calpodes genomic DNA showed that the gene was present as a single copy. The deduced amino acid sequence is not like any of the sequences of cuticular structural proteins that have been published, but has a 47 amino acid sequence similar to bacteriophage T7 N-acetylmuramoyl-L-alanine amidase (34% identical, 51% similar). The amino acid sequence, the timing of expression in development, and the similarity between the substrate of the bacteriophage amidase and components of insect cuticle, all suggest that the 22 kDa protein may have a role in cleaving chitin-peptide bonds as a prerequisite for digestion of the cuticle by chitinases and proteases.

Caterpillars have evolved lungs for hemocyte gas exchange

Since insect blood usually lacks oxygen-carrying pigments it has always been assumed that respiratory needs are met by diffusion in the gas-filled lumen of their tracheal systems. Outside air enters the tracheal system through segmentally arranged spiracles, diffuses along tubes of cuticle secreted by tracheal epithelia and then to tissues through tracheoles, thin walled cuticle tubes that penetrate between cells. The only recognized exceptions have been blood cells (hemocytes), which are not tracheated because they float in the hemolymph. In caterpillars, anoxia has an effect on the structure of the hemocytes and causes them to be released from tissues and to accumulate on thin walled tracheal tufts near the 8th (last) pair of abdominal spiracles. Residence in the tufts restores normal structure. Hemocytes also adhere to thin-walled tracheae in the tokus compartment at the tip of the abdomen. The specialized tracheal system of the 8th segment and tokus may therefore be a lung for hemocytes, a novel concept in insect physiology. Thus, although as a rule insect tracheae go to tissues, this work shows that hemocytes go to tracheae.

Ontogenesis of corneal surface ultrastructure in nocturnal Lepidoptera

The morphogenesis of epicorneal structures in nocturnal Lepidoptera was studied with light and electron microscopy. During the first 4-5 days after pupation, microvilli (with their tips hexagonally distributed) arose gradually from the corneagenous cell surface. At the time of onset of moulting (about 5 days after pupation), patches of lamellar elements appeared distal to the tips of the microvilli. There was one patch for each microvillus from which the patch was separated by a narrow cleft. The cleft was traversed by a few thin bridges which seemed to originate in the microvillus. The bridges were interpreted to be extracellular continuations of intramicrovillar filaments and to insert on the proximal surface of the patch. At about 5 1/2 days after pupation, the patches were seen to be composed of two outer electron-dense lines and a less distinct, inner and thicker dense line. The patches bulged markedly, their concavity turned towards the microvillar tip. A number of discrete bridges extended between the microvillus and the base of the patch, which now appeared as a low dome. The bases of the domer later coalesced to form a continuous lamellar 'membrane' system ( epicorneal lamina , ECL), and the concavity of the domes increased, forming successively deeper lamina evagmations (LE) which strictly retained their spatial relationship to the tips of the microvilli (MV) throughout the ontogenesis. Growth of the ECL evaginations to form an array of successively higher cupoles—and, finally, the complete nipple anlage-was suggested to take place by addition of new material at all points of the LE surface within the palisade of MV/LE bridges. The latter were proposed to act as structures of constraint preventing the ECL to buckle randomly and causing the evaginations to develop in a regular fashion. The results were compared with those described in reports on the morphogenesis of the body cuticle of insects. It was proposed that different types of corneal surface protuberances (corneal nipples of various heights; low protrusions in regular or irregular arrangement) as well as some types of surface lpturing in the body cuticle of insects may be produced on the basis of the same mechanism as the one described for the formation of the full-sized nipples of nocturnal Lepidoptera

F-actin forms transient perinuclear shells at the mitosis-interphase transition

Intermediate filaments and microtubules are known to be involved in establishing and maintaining nuclear shape. F-actin may also be involved in determining nuclear shape, since we have found it associated with reforming nuclei very briefly after cell division. We stained cells from vertebrate tissue cultures (3T3 and NRK-49F) and epidermal cells from an insect with rhodamine-phalloidin and Hoechst #33342 to localize F-actin in relation to the nucleus. We found that F-actin forms shells only around nuclei during reorganization in late mitosis and early interphase. We suggest that perinuclear F-actin shells may be generally present in eukaryotes, but that they are easily missed because of their delicacy and transience.

The cuticular localization of integument peptides from particular routing categories

The distribution of integument peptides in relation to chitin and structural features has been studied in the surface epidermis of the caterpillar of Calpodes ethlius by immunoblotting and immunogold labelling using antibodies prepared to peptides isolated from lamellate endocuticle or from hemolymph. The intermoult cuticle consists of an epicuticle, an endocuticle of many chitin containing lamellae, and a chitin containing assembly zone directly above the apical epidermal microvilli and the perimicrovillar space. During the intermoult, the epidermis secretes peptides constitutively, that is, secretory vesicles containing peptides exocytose without accumulating, traverse the perimicrovillar space and form lamellae in the assembly zone. At moulting, the epidermis deposits ecdysial droplets in addition. These interrupt the last few lamellae which later go on to become the perforated ecdysial membrane. The integument is involved with four routing classes of peptide. Secretion is apical into the cuticle (C), basal into the hemolymph (H), bidirectional (BD), or transported to the cuticle across the epidermis from the hemolymph (T). Some peptides change their routing at moulting. There are several patterns of localization. (1) C and BD cuticular peptides occur mainly in chitin containing lamellate cuticle. (2) Some are also present in epicuticle, and are therefore not obligatorily linked to chitin or matrix between chitin fibers. Cuticular peptides that also occur in the hemolymph are glycosylated, whereas most that are only secreted apically into the cuticle are not. All BD but few C peptides carry alpha-D-glucose/alpha-D-mannose. Some C and BD peptides carry N-acetyl glucosamine. (3) C36 extracted from cuticle has most N-acetyl glucosamine and colocalizes with chitin rather than the protein matrix. It is therefore probably the main link between chitin fibers and the matrix. (4) H235 is barely detectable at the apical cell surface during the intermoult but is abundant at moulting around and below the ecdysial droplets. (5) T66 occurs in intermoult lamellate cuticle. At moulting, alone among the peptides examined, it is in ecdysial droplets. Intermoult C and BD peptides are not in ecdysial droplets but continue to be present in the ecdysial membrane, suggesting that constitutive secretion is independent from the exocytosis of transported moult peptides. T66 differs from most hemolymph peptides in that it does not carry N-acetyl glucosamine or alpha-D-glucose/alpha-D-mannose. (6) Weakly reacting BD peptides (and some H peptides barely detectable in cuticle) localize near the apical surface. Their distribution therefore favours apical secretion and retrieval as a mechanism for basal secretion.

Patterns of DNA synthesis and mitotic activity during the intermoult of Daphnia

Among insects, the epidermal cell cycle pattern is related to the type of ontogenetic development. In taxa undergoing complete metamorphosis, cells are commonly maintained in the G2 stage of interphase between bouts of cell division. In crustaceans, as in insects exhibiting incomplete metamorphosis, it is believed that cells ordinarily remain in G1 for much of the intermoult, with DNA replication occurring late in the moult cycle followed closely by cell division. The present study reveals a differing pattern of epidermal cell division in two distantly related members of the cladoceran crustacean genus Daphnia. Cell cycle kinetics were examined in the last juvenile instar of each species using DNA content determinations and estimates of mitotic frequency. These analyses confirm that each epidermal cell possessed the diploid DNA amount, completed a single cell cycle, and remained in G1 for the majority of the instar. However, DNA replication occurred shortly after moulting and was followed by intense mitotic activity so that cell proliferation was restricted to a short period soon after ecdysis. Cell densities during the instar increased by approximately 60 and 100% for D. pulex and D. magna, respectively.

The planes of division in twin cell doublets

The fifth stage larval epidermis of Calpodes ethlius (Lepidoptera, Hesperiidae) is a syncytium of doublets where sibling cells remain connected by residual midbodies between mitoses. These twins resemble one another more than their other neighbours in features such as the shape and number of nucleolar particles, the number of actin bundles, the position of condensed chromosomes in female cells and the timing of mitosis. Although the patterns of arrangement of structures may be similar in twinned nuclei they differ in their orientation. We have now found that twins also differ in the orientation of their division planes with respect to the previous plane of division. Similarity of structural pattern but not orientation is most easily explained if nuclei, together with determinants for the plane of division, are free to rotate in the plane of the epithelium.

The timing of division in twin cell doublets

The fifth stage larval epidermis of Calpodes ethlius (Lepidoptera, Hesperiidae) is a syncytium of doublets where sibling cells are twins connected by residual midbodies between divisions. Twin cells resemble one another more than their other neighbours in such features as the shape and number of nucleolar particles, the number of actin bundles and the position of condensed female chromosomes. We have now found that they also resemble one another in the timing of cell division in preparation for pupation. Twins are more likely to divide together than at random. The paired timing of mitosis is presumed to be a result of the twins sharing a common cytoplasm.

Rapid de novo formation of gap junctions between insect hemocytes in vitro: a freeze-fracture, dye- transfer and patch-clamp study

Gap junctions form between insect hemocytes (blood cells) when they encapsulate foreign objects in the hemocoel (body cavity). In this study we show that hemocytes from cockroach (Periplaneta americana) form gap-junctions rapidly in vitro. Freeze-fracture replicas of hemocyte aggregates fixed 5 minutes after bleeding contain gap-junctional plaques. Dye passage was detected between carboxyfluorescein diacetate- labelled and unlabelled hemocytes within 3 minutes of bleeding, when the cells made contact as they flattened rapidly onto coverslips. When double whole-cell voltage-clamp was used to measure gap-junction formation between cells which were pushed together, electrical coupling was detected within one second of cell-cell contact. To prevent extensive flattening, cells were plated onto lipophorin-coated coverslips. Junctional conductance increased in staircase fashion with steps corresponding to an average single channel conductance of 345 pS. Assuming all channels to have this conductance, the maximal accretion rate of channels to the growing junction was one channel per second. Junctional currents and dye-coupling were detected in the absence of Ca2+, indicating that involvement of Ca2+-dependent adhesion molecules is not a prerequisite for gap-junction formation in hemocytes. Hemocytes from distantly related insects (cockroach and moth) form functional gap junctions with each other, suggesting sequence homology among gap- junction proteins in insects. The function of rapid gap-junction formation between hemocytes during encapsulation and wound healing in vivo are discussed.

Juvenile hormone action to suppress gene transcription and influence message stability

Proteins normally expressed in high abundance only at larval-pupal metamorphosis in Trichoplusia ni were examined in a comparative analysis of the role and level of hormonal control of their expression. Some related proteins in the hemocyanin-superfamily (i.e., an acidic protein [AJHSP1] and two basic proteins [BJHSP1, BJHSP2]) were shown by nuclear run-on analysis to be specifically transcriptionally suppressed by juvenile hormone (JH), while transcription of another member of that family which is also metamorphosis-associated (arylphorin) was not specifically sensitive to JH. The stability of the mRNA for those members transcriptionally down-regulated by JH appeared to decrease under high JH conditions. While each protein was resorbed to some extent by the prepupal fat body, only the two basic proteins were quantitatively cleared from prepupal hemolymph. The JH-sensitive proteins studied appear to be encoded in single copy genes not immediately juxtaposed in the genome. These and previous studies now permit a more comprehensive understanding of the different combinations of mechanisms involving transcription, mRNA stability, translation, and protein clearance that operate to regulate these metamorphosis-associated proteins.

The relative positions of condensed chromosomes are maintained between divisions in the epidermis of Calpodes ethlius

The larval epidermis of Calpodes ethlius (Lepidoptera, Hesperiidae) is a syncytium of doublets where sibling cells are twins that remain connected by residual midbodies between mitoses. Twins resemble one another more than their other neighbours in such structural features as the shape and number of nucleolar particles and the number of actin bundles. We have now found that they also resemble one another in the position of the condensed chromosomes that occur in female cells. Female lepidopteran cells contain one or more particles of condensed chromatin, depending on their ploidy. In the epidermis, nuclei with two condensed chromosomes are found in pairs and are separated by the same distances. However, clones of cells with multiple condensed chromosomes are not all alike, suggesting that chromosomes are repositioned at mitosis. Separation distances between chromosomes remain the same between but not through cell divisions, suggesting that determinants for nuclear structure are conserved through interphase and relaxed at mitosis. Although the condensed chromosomes of sibling nuclei resemble one another in their separation, they differ in their orientation, as would be expected if whole nuclei rotate in the plane of the epithelium.

Larval cuticular morphogenesis in the tobacco hornworm, Manduca sexta, and its hormonal regulation

The sequential synthesis and deposition of larval cuticular proteins was followed during the final larval molt and the final larval instar of the tobacco hornworm Manduca sexta and correlated with changes in cuticular structure. On the final day of feeding (Day 3) before the onset of metamorphosis many endocuticular proteins were no longer synthesized and new isoelectric variants of 27,000-Da polypeptides were deposited into the cuticle coincident with the formation of lamellae 5- to 10-fold thinner than those previously deposited. Application of a juvenile hormone analog methoprene on Day 1 prevented this change in protein synthesis and in lamellar structure by preventing the observed rise in the intermolt ecdysteroid titer on Day 2. These changes could be induced in vitro by 25-100 ng/ml 20-hydroxyecdysone in the absence of juvenile hormone. Thus, the intermolt change in the lamellar assembly process appears to result from hormone-induced changes in cuticular protein synthesis.

Lenticles: innervated secretory structures that are expressed at every other larval moult

Lenticles are dome-shaped circles or ovals of cuticle with a dark rim. They occur with a precise segmental arrangement in the larvae and pupae of lycaenid and hesperiid butterflies. In Calpodes ethlius (Lepidoptera, Hesperiidae) each lenticle is secreted by a pair of large polyploid epidermal cells. The dark rim or annulus is formed from a ring-shaped cell. The dome, which consists of an epicuticle with a perforate intermediate layer like a pepper-pot, is formed by a central goblet cell. Between the perforate intermediate layer and the cell surfaces there is a cavity that contains material presumed to be secretion. Both cells have elaborate basal plasma membrane reticular systems and the apical microvilli associated with an extensive smooth endoplasmic reticulum that is typical of lipid secreting cells. In addition, there is a plasma membrane reticular system in the ring cell and between it and the goblet cell that contains the endings of nerves having neurosecretory vesicles. Lenticles thus have a structure appropriate for an innervated organ of lipid secretion. However, in their development, lenticles arise from bristles that are presumed to be sensory. Lenticles or their precursors are segmentally arranged in the five larval instars and the pupa, but the pattern changes at each moult. The cells that form a lenticle at one moult have a rest period at the next one when they only secrete surface cuticle. Many lenticles are paired in their cycle of development, with only one of the pair making a lenticle at a particular moult. For example, the dorsal and lateral lenticles alternate in position between anterior and posterior. The second and fourth instar segments have anterior and the third and fifth instars have posterior lenticles. In the first instar the cells that will make lenticles for the second and third instars both make bristles. Lenticles are thus formed by cells that not only change their response to ecdysone qualitatively by switching from bristle to lenticle but also alternate in their later responses, switching back and forth at alternate moults between the formation of a lenticle and the secretion of surface cuticle.

The induction and distribution of an insect ferritin--a new function for the endoplasmic reticulum

Three insect tissues have particular roles as filters to maintain the fluid composition of the hemolymph. Water and ions enter and leave through the midgut. The pericardial cells filter circulating hemolymph. Malpighian tubules, often with the rectum, allow resorption from a hemolymph filtrate that passes to the hindgut. All three tissues have plasma membrane infolds making a reticulum on their hemolymph surfaces, and all three have RER leading to SER extensions into their reticula. SER is a catch-all description for membranes lacking ribosomes in the pre-Golgi complex set of compartments of the vacuolar system. Some kinds of SER are well known for their role in housing enzymes for steroid metabolism and for detoxification. The SER ramifying within the plasma membrane reticular systems of tissues concerned with hemolymph filtration contains ferritin, suggesting that this SER has another, different function. In contrast to vertebrate cells, where ferritin is confined to the cytosol and lysosomes, we have found that in Calpodes and perhaps in most insects, ferritin occurs in the vacuolar system and not in the cytosol. Ferritin occurs naturally in the RER and SER of cells at the hind end of the midgut, in pericardial cells and in the yellow region of the Malpighian tubules. Additional ferritin is induced by loading the gut or hemolymph with iron. Overloading with iron causes ferritin secretion to the gut lumen. We propose that the SER in these cells functions in iron homeostasis by holding ferritin for loading and unloading as it moves to and from the reticulum at the cell surface where it can be maximally exposed to extracellular fluid flow.

A function for plasma membrane reticular systems

The basal surface in transporting epithelia is infolded in a way that encourages the formation of standing gradients. Many insect cells have a similar infolded reticular system (RS) although they are clearly not transporting epithelia. These cells are like one another metabolically in that they sequester lipid from hemolymph lipophorins (lipid transporting proteins). Dietary lipids enter the hemolymph from the midgut RS which may be an adaptation for lipophorin loading. The plasma membrane reticular system of tissues metabolizing lipids (fat body, wax glands, oenocytes, lenticles) may be an adaptation for lipophorin reception and unloading. Cationic ferritin (pI 8.5) shows all RSs are covered by a lamina functioning as a negatively charged sieve. The basal plasma membrane leading to the RS is also negatively charged. The RS is a container with charged entrances that would be expected to affect the composition of the contents. Midgut cells release lipid particles into their RS. The particles are positively charged since in tracer studies they associate with anionic but not cationic ferritin. Lipophorins are anionic. The electrostatic binding of lipid to lipophorin would make it less anionic and more likely to leave the RS when loaded, thus carrying lipid to the hemolymph. Conversely, at the destination RS, loaded lipophorin would penetrate more easily than unloaded. A change in charge with unloading would be expected to alter the equilibrium between entering and leaving lipophorin, causing protein concentration in the RS of lipid receiving tissues as has been observed in the fat body. Reticular systems may thus be reaction vessels for interactions between carrier proteins and their load.

Lamellar bodies are the intracellular site of membrane turnover in insect fat body

Analysis of the time course of highly cationic ferritin uptake by fat body cells has shown that the tracer bound to the plasma membrane and was pinocytosed by coated vesicles. The first sites of intracellular accumulation were multivesicular bodies which became filled with ferritin between 30-60 min after cells were exposed to the tracer. At no time during the experiments were any parts of the Golgi complex labeled by the tracer. By 60 min, the ferritin was increasingly found in lamellar bodies. The different types of 'light' and 'dark' multivesicular bodies suggest that lamellar bodies form from multivesicular bodies as they fill with tracer. The occurrence of lamellar bodies in many different cell types suggests an important role in membrane dynamics.

Charged sieving by the basal lamina and the distribution of anionic sites on the external surfaces of fat body cells

The distribution of anionic sites on the basal lamina has been examined with highly cationic ferritin. The penetration of ferritins, with a range of charges from anionic to highly cationic, through the basal lamina into the spaces between fat body cells in an insect is correlated with the charge of the tracer. The anionic sites of the basal lamina may therefore affect the composition of the lymph that bathes the fat body cells. There was more cationic ferritin bound to the plasma membrane reticular reticular system than to the lateral plasma membranes, suggesting that there may be regional differences in surface charge.

Epidermal feet in pupal segment morphogenesis

Epidermal cells in insect integumental epithelia develop branched cytoskeletal extensions or feet at their base that are similar in appearance to the processes put out by cells in tissue culture. We have developed a procedure to show the feet that gives an effect as if thousands of cells randomly arranged in the epithelium had each been injected with lead salt visualized as black lead sulphide. The procedure depends upon the fact that after brief glutaraldehyde fixation, tannic acid only penetrates some cells where it mordants lead ions and binds osmium. Individual cells visualized in this manner show their outlines as if they are separate in a tissue culture although they are part of a closely packed epithelium. The feet are metamorphic structures formed after pupal commitment and are necessary for metamorphic changes in segment shape. In Calpodes larvae the feet are orientated axially in the direction of the segmentally repeating gradient and may extend for several cell diameters. They extend under the influence of low titres of 20-hydroxyecdysone such as those occurring in the intermoult. When stimulated by high titres like those in pre-pupae, the feet contract at the same time as the segments shorten to pupal proportions. We believe that cell processes like the epidermal feet are ubiquitous but that they have often been overlooked because of the difficulty of demonstrating the outlines of single cells that are united in epithelia.

Electrical activity of neurosecretory axons from the brain of Rhodnius prolixus: relation of changes in the pattern of activity to endocrine events during the moulting cycle

Ongoing electrical activity was recorded from the axons of neurosecretory cells from the brain of 5th instar Rhodnius prolixus throughout the moulting cycle. Dramatic changes in both the frequency and pattern of electrical activity occur at specific times during the cycle, enabling the timing of release of neurohormones from the brain to be inferred. The level of transport of stainable secretion appears to be closely coupled to the electrical activity at all times. Activity is low in unfed Rhodnius, but within minutes of the insect taking a blood meal, there is a rapid appearance of bursting activity from a number of units, in conjunction with apparently continuous firing components. The bursting activity declines at about 2 h remaining low for the following 5 days, at which time there is a resurgence in the bursting pattern for a few hours. Both peaks of bursting activity immediately precede increases in haemolymph titer of ecdysones, suggesting that release of prothoracotropic hormone occurs on these two occasions. The continuously firing components initiated at feeding maintain a high level for 5 days indicating release of other brain neurosecretions. Intense electrical activity in the form of bursting activity and high apparently continuous pattern resumes shortly before ecdysis and continues until 12 h after. The relationship of neurohormone release at this time to bursicon and ecdysis behavior is discussed.