The localization of a peroxidase associated with hard cuticle formation in an insect, Calpodes ethlius stoll, Lepidoptera, Hesperiidae

RNAi-Mediated Functional Analysis of Bursicon Genes Related to Adult Cuticle Formation and Tanning in the Honeybee, Apis mellifera

Bursicon is a heterodimeric neurohormone that acts through a G protein-coupled receptor named rickets (rk), thus inducing an increase in cAMP and the activation of tyrosine hydroxylase, the rate-limiting enzyme in the cuticular tanning pathway. In insects, the role of bursicon in the post-ecdysial tanning of the adult cuticle and wing expansion is well characterized. Here we investigated the roles of the genes encoding the bursicon subunits during the adult cuticle development in the honeybee, Apis mellifera. RNAi-mediated knockdown of AmBurs α and AmBurs β bursicon genes prevented the complete formation and tanning (melanization/sclerotization) of the adult cuticle. A thinner, much less tanned cuticle was produced, and ecdysis toward adult stage was impaired. Consistent with these results, the knockdown of bursicon transcripts also interfered in the expression of genes encoding its receptor, AmRk, structural cuticular proteins, and enzymes in the melanization/sclerotization pathway, thus evidencing roles for bursicon in adult cuticle formation and tanning. Moreover, the expression of AmBurs α, AmBurs β and AmRk is contingent on the declining ecdysteroid titer that triggers the onset of adult cuticle synthesis and deposition. The search for transcripts of AmBurs α, AmBurs β and candidate targets in RNA-seq libraries prepared with brains and integuments strengthened our data on transcript quantification through RT-qPCR. Together, our results support our premise that bursicon has roles in adult cuticle formation and tanning, and are in agreement with other recent studies pointing for roles during the pharate-adult stage, in addition to the classical post-ecdysial ones.

Genes involved in thoracic exoskeleton formation during the pupal-to-adult molt in a social insect model, Apis mellifera

Background

The insect exoskeleton provides shape, waterproofing, and locomotion via attached somatic muscles. The exoskeleton is renewed during molting, a process regulated by ecdysteroid hormones. The holometabolous pupa transforms into an adult during the imaginal molt, when the epidermis synthe3sizes the definitive exoskeleton that then differentiates progressively. An important issue in insect development concerns how the exoskeletal regions are constructed to provide their morphological, physiological and mechanical functions. We used whole-genome oligonucleotide microarrays to screen for genes involved in exoskeletal formation in the honeybee thoracic dorsum. Our analysis included three sampling times during the pupal-to-adult molt, i.e., before, during and after the ecdysteroid-induced apolysis that triggers synthesis of the adult exoskeleton.

Results

Gene ontology annotation based on orthologous relationships with Drosophila melanogaster genes placed the honeybee differentially expressed genes (DEGs) into distinct categories of Biological Process and Molecular Function, depending on developmental time, revealing the functional elements required for adult exoskeleton formation. Of the 1,253 unique DEGs, 547 were upregulated in the thoracic dorsum after apolysis, suggesting induction by the ecdysteroid pulse. The upregulated gene set included 20 of the 47 cuticular protein (CP) genes that were previously identified in the honeybee genome, and three novel putative CP genes that do not belong to a known CP family. In situ hybridization showed that two of the novel genes were abundantly expressed in the epidermis during adult exoskeleton formation, strongly implicating them as genuine CP genes. Conserved sequence motifs identified the CP genes as members of the CPR, Tweedle, Apidermin, CPF, CPLCP1 and Analogous-to-Peritrophins families. Furthermore, 28 of the 36 muscle-related DEGs were upregulated during the de novo formation of striated fibers attached to the exoskeleton. A search for cis-regulatory motifs in the 5′-untranslated region of the DEGs revealed potential binding sites for known transcription factors. Construction of a regulatory network showed that various upregulated CP- and muscle-related genes (15 and 21 genes, respectively) share common elements, suggesting co-regulation during thoracic exoskeleton formation.

Conclusions

These findings help reveal molecular aspects of rigid thoracic exoskeleton formation during the ecdysteroid-coordinated pupal-to-adult molt in the honeybee.

Ecdysteroid-dependent expression of the tweedle and peroxidase genes during adult cuticle formation in the honey bee, Apis mellifera

Cuticle renewal is a complex biological process that depends on the cross talk between hormone levels and gene expression. This study characterized the expression of two genes encoding cuticle proteins sharing the four conserved amino acid blocks of the Tweedle family, AmelTwdl1 and AmelTwdl2, and a gene encoding a cuticle peroxidase containing the Animal haem peroxidase domain, Ampxd, in the honey bee. Gene sequencing and annotation validated the formerly predicted tweedle genes, and revealed a novel gene, Ampxd, in the honey bee genome. Expression of these genes was studied in the context of the ecdysteroid-coordinated pupal-to-adult molt, and in different tissues. Higher transcript levels were detected in the integument after the ecdysteroid peak that induces apolysis, coinciding with the synthesis and deposition of the adult exoskeleton and its early differentiation. The effect of this hormone was confirmed in vivo by tying a ligature between the thorax and abdomen of early pupae to prevent the abdominal integument from coming in contact with ecdysteroids released from the prothoracic gland. This procedure impaired the natural increase in transcript levels in the abdominal integument. Both tweedle genes were expressed at higher levels in the empty gut than in the thoracic integument and trachea of pharate adults. In contrast, Ampxd transcripts were found in higher levels in the thoracic integument and trachea than in the gut. Together, the data strongly suggest that these three genes play roles in ecdysteroid-dependent exoskeleton construction and differentiation and also point to a possible role for the two tweedle genes in the formation of the cuticle (peritrophic membrane) that internally lines the gut.

Insect cuticular sclerotization: a review

Different regions of an insect cuticle have different mechanical properties, partly due to different degrees of stabilization and hardening occurring during the process of sclerotization, whereby phenolic material is incorporated into the cuticular proteins. Our understanding of the chemistry of cuticular sclerotization has increased considerably since Mark Pryor in 1940 suggested that enzymatically generated ortho-quinones react with free amino groups, thereby crosslinking the cuticular proteins. The results obtained since then have confirmed the essential features of Pryor's suggestion, and the many observations and experiments, which have been obtained, have led to a detailed and rather complex picture of the sclerotization process, as described in this review. However, many important questions still remain unanswered, especially regarding the precise regional and temporal regulation of the various steps in the process.

The egg-shell of Drosophila melanogaster III. Covalent crosslinking of the chorion proteins involves endogenous hydrogen peroxide

Utilizing two cytochemical methods, namely, diaminobenzidine for the assay of peroxidases and cerium(III) chloride for the localization of hydrogen peroxide it was found that the enzyme exists in two out of the five egg-shell layers: the innermost choronic layer and the endochorion. In addition, hydrogen peroxide which acts as a substrate for the enzyme in vitro enabling the formation of covalent bonding between the egg-shell proteins, was found to be produced at the follicle cell plasma membrane during the last stage of oogenesis. It is concluded that hydrogen peroxide is an endogenous, programmed product of the follicle cells, responsible for the action of peroxidase in order to oxidize the tyrosyl residues producing di-tyrosine and tri-tyrosine bonds between the chorion polypeptides.

Chlorinated tyrosine derivatives in insect cuticle

A method for quantitative measurement of 3-monochlorotyrosine and 3,5-dichlorotyrosine in insect cuticles is described, and it is used for determination of their distribution in various cuticular regions in nymphs and adults of the desert locust, Schistocerca gregaria. The two chlorinated tyrosine derivatives were present in all analyzed regions in mature adult locusts, the highest concentrations were found in the sclerotized cuticle of femur and tibia, but significant amounts were also present in the unsclerotized arthrodial membranes. Small amounts of the two amino acids were obtained from pharate, not-yet sclerotized cuticle of adult femur and tibia, the amounts increased rapidly during the first 24 h after ecdysis and more slowly during the next two weeks. Control analyses using stable isotope dilution mass spectrometry have confirmed that the chlorinated tyrosines are not artifacts formed during sample hydrolysis. Mono- and dichlorotyrosine are also present in cuticular samples from other insect species, such as the beetle, Tenebrio molitor, the moth Hyalophora cecropia, the cockroach Blaberus craniifer, and the bug Rhodnius prolixus, but not in the sclerotized puparial cuticle of the blowfly, Calliphora vicina, or in sclerotized ootheca from the cockroach, Periplaneta americana. Cuticular sclerotization and formation of chlorotyrosines occur simultaneously in locust legs; sclerotized cuticles tend to have a higher content of chlorotyrosines than unsclerotized cuticles, but it is concluded that the chlorotyrosines are not just a by-product from the sclerotization process.

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.

Tyrosine cross-linking of extracellular matrix is catalyzed by Duox, a multidomain oxidase/peroxidase with homology to the phagocyte oxidase subunit gp91phox

High molecular weight homologues of gp91phox, the superoxide-generating subunit of phagocyte nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase, have been identified in human (h) and Caenorhabditis elegans (Ce), and are termed Duox for "dual oxidase" because they have both a peroxidase homology domain and a gp91phox domain. A topology model predicts that the enzyme will utilize cytosolic NADPH to generate reactive oxygen, but the function of the ecto peroxidase domain was unknown. Ce-Duox1 is expressed in hypodermal cells underlying the cuticle of larval animals. To investigate function, RNA interference (RNAi) was carried out in C. elegans. RNAi animals showed complex phenotypes similar to those described previously in mutations in collagen biosynthesis that are known to affect the cuticle, an extracellular matrix. Electron micrographs showed gross abnormalities in the cuticle of RNAi animals. In cuticle, collagen and other proteins are cross-linked via di- and trityrosine linkages, and these linkages were absent in RNAi animals. The expressed peroxidase domains of both Ce-Duox1 and h-Duox showed peroxidase activity and catalyzed cross-linking of free tyrosine ethyl ester. Thus, Ce-Duox catalyzes the cross-linking of tyrosine residues involved in the stabilization of cuticular extracellular matrix.

Structural and biochemical analysis of the parasite Gordius villoti (Nematomorpha, Gordiacea) cuticle

The cuticle of the nematomorpha Gordius villoti is a proteinaceous extracellular structure that covers the body during the endoparasitic life in the hemocoelic cavity of insect hosts, and of the free-living adult animals. The ultrastructure of the cuticle has a complex spatial organization with several parallel layers of large diameter fibers, interposed thinner fibrous elements and honeycomb-shaped matrix surrounding the fibers. When adult isolated cuticles were partially solubilized by several compounds, the structure revealed a strong insolubility and the main fibers were always observable. HPLC and spectrophotometric assays carried out to investigate the presence of tyrosine cross-linking, indicated such a mechanism as a key-element in the hardening process of the cuticle. Such data strongly suggest that the Gordius cuticle contains dityrosine compounds, whose formation is probably mediated by endogenous peroxidase activity.

Mutants of Methylobacterium extorquens and Paracoccus denitrificans deficient in c-type cytochrome biogenesis synthesise the methylamine-dehydrogenase polypeptides but cannot assemble the tryptophan-tryptophylquinone group

Five mutants of Methylobacterium extorquens and four mutants of Paracoccus denitrificans that have a general defect in c-type cytochrome synthesis also failed to assemble an active methylamine dehydrogenase. In all cases methanol dehydrogenase, another periplasmic enzyme, was fully active. All nine mutant strains accumulated both the heavy and light subunits of methylamine dehydrogenase to essentially wild-type levels. In all nine mutants, the heavy-subunit and light-subunit polypeptides were proteolytically processed, suggesting that translocation to the periplasm had occurred; in the case of the P. denitrificans mutants, a periplasmic location for the heavy and light subunits was confirmed experimentally. While specific quinone staining of the methylamine dehydrogenase light subunit in wild-type M. extorquens and P. denitrificans strains could readily be demonstrated, the light subunit polypeptides accumulated by the mutants did not quinone stain, indicating that the methylamine dehydrogenase prosthetic group, tryptophan tryptophylquinone, is not assembled in the absence of functional c-type cytochromes.

Cytochemical localization of a D-amino acid oxidizing enzyme in peroxisomes of Drosophila melanogaster

A peroxide generating oxidase is demonstrated cytochemically in the peroxisomes of adult and larval Drosophila melanogaster, Oregon R and Rosy-506 strains. This enzyme activity is demonstrable using D-pipecolate or D-proline, but not L-proline, as substrate and is inhibited by kojic acid. Thus this enzyme shares cytochemical characteristics with vertebrate D-amino acid oxidase.

Ultrastructural localization of phenoloxidases in cuticle and haemopoietic tissue of the blowfly Lucilia cuprina

The ultrastructural localization of two types of biochemically characterized phenol oxidase activity is described in the larva of the sheep blowfly, Lucilia cuprina. Cuticular tyrosinase activity (enzyme A) is seen in epicuticular filaments and procuticle. Procuticle activity can be detected only after a presumed process of activation takes place in damaged cuticle. By using either the dopamine reaction or inducing melanization by hot-water treatment, tyrosinase is readily shown in haemopoietic tissue which, in L. cuprina, occurs subdermally as well as being associated with the dorsal vessel. The adaptation of the diaminobenzidine technique, used to stain laccase in electrophoretic gels, to ultrastructural cytochemistry has made it possible to demonstrate enzymic activity probably due to laccase (enzyme B). The laccase activity is present in the inner epicuticle of late wandering third instar larvae (about to pupariate) but is not present in the epicuticle of younger larvae.

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.

Autoradiographic and fine structural study of chitin deposition in the cuticle of a barnacle using [3H]-D-glucosamine incorporation

[3H]-D-Glucosamine was injected into the rostral sinus of Balanus eburneus (barnacle) and the distribution of labelled chitin in the cuticle was studied with autoradiography and electron microscopy. When the pattern of labelling was examined in different body regions of the same organism where thickness of fully formed cuticle varied, it was observed that the rate of chitin deposition varied, being greater in thick than in thin regions. The density of Ag grains overlying cuticle was also greater in the thick regions. When the pattern of labelling was examined in regions of cuticle, comparable in thickness, taken from a series of organisms sacrificed at different time points a comparable value for the rate of chitin deposition was obtained. In addition, asynchrony in deposition of cuticle in different body regions of the same organism as well as uptake of the label by substances other than chitin, i.e. glycogen and glycoproteins were described.

Tyrosine storage vacuoles in insect fat body

The watery vacuoles first described from larval insect fat body (Chironomus, Voinov, 1927; Aedes, Wigglesworth, 1942; Rhodnius, Wigglesworth, 1967) have been studied in 4th and 5th stage Calpodes larvae. The vacuoles arise at the beginning (E + 6-24 hr) of the 4th stadium from plasma membrane infolds that separate from the cell surface as provacuoles less than 1 micron in diameter. These provacuoles grow and fuse with one another through the intermolt until about half the volume of each fat body cell is occupied by a single, large vacuole. The vacuoles begin to disappear at molting. Their membrane is either incorporated into the plasma membrane by exocytosis or fragmented into vesicles that fuse to become lamellar bodies where the membranes are presumably digested. All the vacuoles have gone by a few hours after ecdysis. The tyrosine content of the fat body increases and decreases in proportion to the size of the vacuoles. As the vacuoles decrease at molting the titre of tyrosine in the hemolymph is transiently elevated at the time when there is most demand for phenolics for cuticle stabilization. Crystals having the form of tyrosine crystallize out from vacuoles separated from the fat body. In fat body extracts separated by thin layer chromatography, similar crystals occur only in the eluates from spots corresponding to tyrosine. The vacuoles are therefore presumed to be tyrosine stores used in cuticle stabilization at molting. They correspond to a type of aqueous storage compartment that is well known in plants but hitherto little recognized in animal cells.

Crosslinking of theDrosophila chorion involves a peroxidase

TheDrosophila chorion contains an endogenous peroxidase activity which remains inactive until late stage 14 when it catalyzes the crosslinking of the chorionic proteins. Using explanted follicles developing in vitro, premature, but otherwise normal crosslinking can be induced with hydrogen peroxide and normal crosslinking can be prevented with peroxidase inhibitors. Inhibition or premature activation of the shell peroxidase allows characterization of chorionic filament specific proteins and establishes new criteria for the identification of eggshell components.

Apolysis and the turnover of plasma membrane plaques during cuticle formation in an insect

The apical plasma membranes of Calpodes epidermal cells have small fattened areas or plaques with an extra density upon their cytoplasmic face. The plaques are typically at the tips of microvilli. The are present during the deposition of fibrous cuticle and the cuticulin layer. Since the plaques are close (less than 15nm) to the sites where these kinds of cuticle first appear, they are presumed to have a role in their synthesis and/or deposition and orientation. When fifth stage larval cuticle deposition ceases prior to pupation, the plaques are lost as the area of the apical plasma membrane is reduced. The plaques pass from the surface into pinocytosis vesicles and multivesicular bodies where they are presumably digested. The loss of plaques occurs as the blood level of moulting hormone reaches a peak at the critical period after which the prothoracic glands are no longer needed for pupation. Apolysis or separation of the epidermis from the old cuticle is the stage when plaques are absent, the old ones have been lost but the new ones have yet to form. After the critical period, the epidermis prepared for pupation with a phase of elevated RNA synthesis at the end of which plaques and microvilli reform in time to secrete the new cuticulin layer and later the fibrous cuticle of the pharate pupa. There is a new generation of plaques for each moult and succeeding intermoult and each generation is involved in two kinds of cuticle deposition before involution and redifferentiation.

Hardening of the sea urchin fertilization envelope by peroxidase-catalyzed phenolic coupling of tyrosines

Within minutes after its elevation from the egg surface, the sea urchin fertilization envelope (FE) becomes "hardened" by a reaction that renders it resistant to agents that solubilize, denature or degrade most proteins. Peroxidase activity is released into the surrounding seawater from Stronglyocentrotus purpuratus eggs during fertilization. Evidence from several sources indicate that the catalytic action of the peroxidase is responsible for hardening the FE through the phenolic coupling of tyrosyl residues of the FE proteins. First, the peroxidase is localized within the hardened FE and within the crystalline FE precursor material released from egg cortical granules during the fertilization reaction. Second, a direct correlation is established between the effectiveness of compounds in inhibiting the cortical granule peroxidase (CGP) and their effectiveness in inhibiting hardening of the FE. Third, the CGP catalyzes the cross-linking of tyrosines in solution, a reaction known to be catalyzed by horseradish peroxidase (HRP). Fourth, acid hydrolysates of hardened FEs contain cross-linked tyrosines that are identified by comparing their chromatographic ultraviolet absorption and fluorescent characteristics to those known for cross-linked tyrosines formed by HRP. Finally, when eggs are fertilized in the presence of 125I, the CGP heavily labels proteins of the FE and of the crystalline FE precursor material released with the enzyme from the cortical granules. The iodide label reflects the localization of the CGP and may reflect the sites of peroxidase-generated tyrosyl phenyl radicals involved in the tyrosine coupling reaction. Maximal iodide labeling occurs during the first 5 min period following fertilization, corresponding to the period of FE hardening.

Cytochemistry of late ovarian chambers of Drosophila melanogaster

Late ovarian chambers of Drosophila melanogaster have been examined by ultrastructural cytochemistry in an attempt to characterize some of the transformations which precede the completion of oogenesis. From stage 11 onward peroxidase activity is present in the endoplasmic reticulum of both nurse cells and oocyte, as well as in the egg-covering precursors of the columnar follicle cells. Catalase activity is restricted to the very last stages of oogenesis (stage 13-14) and appears to be located in membrane-bound organelles of the ooplasm which are continuous with the endoplasmic reticulum. Because of the presence of catalase as well as by their structural appearance, these organelles are to be identified as microperoxisomes. Catalase activity becomes cytochemically detectable in the ooplasm somehow in coincidence with the formation of glycogen. Furthermore, glycogen is first formed in intimate association with alpha-1 yolk platelets. On the basis of these findings it is suggested that glycogen synthesis occurs by a process of gluconeogenesis.

Investigative studies of the dermatitis caused by the larva of the brown-tail moth (Euproctis chrysorrhoea Linn.) II. Histopathology of skin lesions and scanning electron microscopy of their causative setae

Two different aspects related to the dermatitis caused by the so-called nettling hairs of the larva of the brown-tail moth, Euproctis chrysorrhoea L., are documented. The first part describes the sequence of histopathologic changes associated with the inflammatory process in the human skin induced by epicutaneous application untreated (UT-N) and heat treated (HT-N) nettling hairs. The penetration of untreated and heat treated nettling hairs into the epidermis is evident from their presence in 12 out of 29 UT-N lesions and 5 out of 10 HT-N lesions respectively. The introduction of nettling hairs into the skin leads to damage and bulla formation of the surface epithelium and early inflammatory signs consisting of edematous changes of the dermis and pervascular infiltration of neutrophils, eosinophils and mononuclear leucocytes. After few hours the intensity of the dermal infiltrate has increased and spongiotic changes appear. After 48--72 h the perivascular infiltrate is mainly composed of mononuclear cells, while along with increased spongiosis the damaged surface epithelium may show repair. The traumatic changes of the surface epithelium in HT-N lesions appear less prominent in comparison with the UT-N lesions. The second part describes the findings obtained by scanning electron microscopy of the nettling hairs. Evidence is brought forward to support the view that the offending setae may be looked upon as tapering, hollow microcapillaries which are open at both ends. The present findings are consistent with the idea that the nettling hairs may serve as microneedles from which irritant substances may be liberated when penetrating into the skin, and that the resulting inflammatory reactions are attributable to combined mechanical and toxic effects.

The fine structure of a parasitic ciliate Terebrospira during ingestion of the exoskeleton of a shrimp Palaemonetes

The ciliated protozoan, Terebrospira chattoni, invades the exoskeleton of the shrimp, Palaemonetes pugio, eating out long galleries parallel to the surface of the exoskeleton. Solubilization of the exoskeleton occurs around an area of the elaborately infolded surface membrane at the anterior of the organism. Dissolved products of the digestion of the exoskeleton are taken into the body by the formation of coated vesicles at pores in the membrane. The surface membrane that is taken in by pinocytosis is apparently recycled by the introduction into the membrane of organelles implicated in membrane recycling in other ciliates. Acid phosphatase can be demonstrated on the surface membrane as well as in the endocuticle around the organism.

Some ultrastructural observations on the integument of a pentastomid

The cuticle of the cephalobaenid pentastomid Reighardia sternae is described at various stages of the moult-intermoult cycle. The intermoult cuticle comprises four layers: an outer epicuticle; an underlying dense layer, the protein epicuticle; a fibrillar endocuticle; and a denser subcuticle. The overall similarity between the structure and composition of these layers and those of insects is discussed. However, the orientation of the chitin-protein fibres in the endocuticle does not show the rotating structure characteristic of many arthropod species, but this does appear in the sclerotized hooks. It is suggested that this comparatively loose, poorly oriented endocuticular structure produces a highly extensible cuticle which is precisely adapted to the specialize, endoparasitic habit of this species. Events at ecdysis, particularly the secretion of moulting fluid and the deposition of cuticulin, follow the insect pattern precisely. The phyletic significance of these observations is discussed.