A post-ecdysial plasticization of the abdominal cuticle in Rhodnius

The ecdysis and emergence of fifth instar larvae of Rhodnius prolixus have been closely observed. At the time of ecdysis the cuticle of the head, legs, and wingpads is soft and readily deformed. it does not become sufficiently rigid for normal use until about 90 min later. The cuticle of the abdomen is however hard and inextensible at the time of ecdysis. From about 60 min onward this cuticle undergoes a plasticization; it is maximally extensible at about 180 min, thereafter becoming inextensible again. Unlike the plasticization of the abdominal cuticle which occurs after feeding, this post-ecdysial plasticization is not under direct nervous control. Although it seems that there is some temporal link with the darkening of the cuticle, it is considered unlikely that plasticization is a direct consequence of the tanning process. The significance of this post-ecdysial plasticization is not obvious.     

The mechanical properties of the abdominal cuticle of Rhodnius larvae.

1. The mechanical properties of loops of cuticle cut from the abdomens of 5th instar Rhodnius have been investigated. The cuticle shows pronounced viscoelastic behaviour. 2. Stress-relaxation tests show a continuously falling modulus over a wide range of times after the imposition of a strain. 3. Plasticized samples of cuticle show stress-relaxation curves which are shifted along the time axis towards earlier times by up to times 10-3. The modulus at any particular time after the imposition of strain is about 10 times lower than that of the unplasticized cuticle. 4. It is concluded that the mechanical properties of this cuticle are determined, at least for maintained stresses, largely by the matrix material. Chitin microfibrils may act as a reinforcing filler for short-term, rapid stresses. The cuticular macromolecules are probably not extensively cross-linked by primary bonds, though secondary interactions between them are probably important in the viscoelastic properties of the cuticle. 5. Plasticization probably involves a change in either the number or the strength of secondary interactions between the cuticle macromolecules, or both.     

The mechanism of extension of the pleural pleat in adult Rhodnius prolixus

Adult Rhodnius prolixus Stål increase the volume of the abdomen when feeding by stretching the lateral abdominal pleat. This paper shows that the observed extension may be entirely accounted for by unfolding corrugations in the pleat.     

The ultrastructure of the cuticle of Nematoda. II. The cephalic cuticle of Stilbonematinae (Adenophorea,Desmodoridae)

Summary  The ultrastructure of the cephalic cuticle of 6 of 7 representative genera of Stilbonematinae (Eubostrichus, Catanema, Laxus, Robbea, Leptonemella, Stilbonema) is investigated using SEM and TEM techniques. Additionally, one species of Spirinia (Spiriniinae) and one of Desmodora (Desmodorinae) were studied for outgroup comparison. Most of the studied species show modifications of the cephalic cuticle. Furthermore, at least four different pathways have been developed to reinforce the head within Stilbonematinae. Species with a coarsely annulated somatic cuticle (Leptonemella sp., Stilbonema majum, and Desmodora ovigera) developed a rigid, non-annulated cephalic capsule by modifying the main constructing element of the median zone, the ’’ring body.’’ In faintly annulated Laxus oneistus, the annulated cephalic capsule results from a newly inserted ’’block layer’’ between the median and basal zone. The non-annulated cephalic capsule of Robbea sp. is formed by both the block layer and the ring body element. The annulated capsule of Catanema sp. stems from a doubled number of fiber layers within the basal zone. In Spirinia sp., only the amphidial region is strengthened in what could be termed an amphidial shield. All forms with cephalic capsules show mechanisms to keep the oral region pliable. Only Eubostrichus topiarius lacks a reinforcement of the cephalic cuticle. A comparison with the literature is made to elucidate corresponding structures within the cephalic capsules of representatives of Desmodorida. It is demonstrated that the presence of a cephalic cuticle is of no systematic value above the genus level. Accepted: 3 March 1996     

Analysis of the chitin recognition mechanism of cuticle proteins from the soft cuticle of the silkworm, Bombyx mori

Insect cuticle is composed mainly of chitin, a polymer of N-acetylglucosamine, and chitin-binding cuticle proteins. Four major cuticle proteins, BMCP30, 22, 18, and 17, have been previously identified and purified from the larval cuticle of silkworm, B. mori. We analyzed the chitin-binding activity of BMCP30 by use of chitin-affinity chromatography. The pH optimum for the binding of BMCP30 to chitin is 6.4, which corresponds to hemolymph pH. Competition experiments using chitooligosaccharides suggested that BMCP30 recognizes 4-6 mer of N-acetylglucosamine in chitin fiber as a unit for binding. The comparison of the binding properties of BMCP30 with those of BMCP18 showed that their binding activities to chitin are similar in a standard buffer but that BMCP30 binds to chitin more stably than BMCP18 in the presence of urea. BMCPs possess the RR-1 form of the R&R consensus, about 70 amino acids region conserved widely among cuticle proteins mainly from the soft cuticle of many insect and arthropod species. Analysis of the binding activity using deletion mutants of BMCPs revealed that this type of conserved region also functions as the chitin-binding domain, similarly to the RR-2 region previously shown to confer chitin binding. Thus, the extended R&R consensus is the general chitin-binding domain of cuticle proteins in Arthropoda.     

On the mechanism of formation of N-acetyldopamine quinone methide in insect cuticle

The mechanism of formation of quinone methide from the sclerotizing precursor N-acetyldopamine (NADA) was studied using three different cuticular enzyme systems viz. Sarcophaga bullata larval cuticle, Manduca sexta pharate pupae, and Periplaneta americana presclerotized adult cuticle. All three cuticular samples readily oxidized NADA. During the enzyme-catalyzed oxidation, the majority of NADA oxidized became bound covalently to the cuticle through the side chain with the retention of o-diphenolic function, while a minor amount was recovered as N-acetylnorepinephrine (NANE). Cuticle treated with NADA readily released 2-hydroxy-3′,4′-dihydroxyacetophenone on mild acid hydrolysis confirming the operation of quinone methide sclerotization. Attempts to demonstrate the direct formation of NADA-quinone methide by trapping experiments with N-acetylcysteine surprisingly yielded NADA-quinone-N-acetylcysteine adduct rather than the expected NADA-quinone methide-N-acetylcysteine adduct. These results are indicative of NADA oxidation to NADA-quinone and its subsequent isomerization to NADA-quinone methide. Accordingly, all three cuticular samples exhibited the presence of an isomerase, which catalyzed the conversion of NADA-quinone to NADA-quinone methide as evidenced by the formation of NANE—the water adduct of quinone methide. Thus, in association with phenoloxidase, newly discovered quinone methide isomerase seems to generate quinone methides and provide them for quinone methide sclerotization.     

Ultrastructural changes in intersegmental cuticle during rotation of the terminal abdominal segments in a mosquito

The terminal abdominal segments of male Aedes aegypti rotate 180° within 24 hr after adult emergence, rotation occurring in the intersegmental membrane between abdominal segments VII and VIII. The ultrastructure of this rotating membrane is compared with non-rotating intersegmental membranes at different developmental stages. The deposition of cuticle in both the rotating and non-rotating intersegments appears ultrastructurally similar, and follows the sequential pattern described for other insects. Shortly after adult emergence, however, disruptive changes occur in the membrane cuticle that are more pronounced in non-rotating intersegments. This disruption occurs initially 1 hr after adult emergence and becomes maximal within 3 hr. Disruption appears to occur by the addition of fluid to the cuticle and results in a ten-fold increase in cuticle thickness in non-rotating intersegments but only a two-fold increase in thickness in the rotating intersegment. While in the disrupted condition, the non-rotating intersegmental membranes become extensively folded whereas the cuticle in the rotating intersegment becomes stretched. During rotation, strain forces in the rotating intersegment result in a reorientation of microfibers in the cuticle from parabolic to parallel. This reorientation is presumably brought about by plastic flow.     

Expanding abdominal cuticle in the bug Rhodnius and the tick Boophilus

The abdominal cuticles of Rhodnius prolixus (fifth instar) and Boophilus microplus (adult female) expand dramatically and rapidly during feeding. In the unfed stage of both species the epicuticle of the abdomen is deeply folded, and when rapid stretching takes place the epicuticle unfolds and the underlying procuticle stretches so that the thickness of the cuticle is halved. The cuticles contained only trace amounts of protein soluble in water and aqueous KCl but substantial quantities were extracted with 7 M aqueous urea. The proteins were analysed for their amino acid composition and investigated by gel electrophoresis and isoelectric focusing.In solubility, amino acid composition, molecular weight distribution, and isoelectric points, the proteins isolated from both species resembled one another closely. They had higher molecular weights and higher isoelectric points than did the proteins from flexible, non-stretching cuticles and unlike them had high alanine and histidine and low aspartic acid and glutamic acid contents. Their amino acid composition was very similar to that of the whole cuticle. The proteins were not associated with neutral sugars. Both the Rhodnius and Boophilus cuticles had low chitin contents, 11·2 and 3·8% respectively (on a water-free basis). The composition of the cuticles and the properties of the proteins are discussed in relation to the stretching which they undergo.     

Structure of the abdominal receptor responsive to internally applied pressure in the blood-feeding insect,Rhodnius prolixus

Summary The sensory receptor responsive to pressure applied internally to the ventral abdominal body wall of the blood-feeding insects, Rhodnius prolixus, is a single sense cell containing, at its distal end, a cilium enclosed within a scolopale, a densely staining structure characteristic of insect scolopidial sensilla. A small spherical structure lies within a dilation near the midregion of the cilium, and contains nine heavily staining bodies, the position of each corresponding to a pair of microtubules in the cilium. Proximal to the dilation, the microtubules are organized in a ring of nine pairs with one microtubule of each pair associated with dyneinlike arms. Dastal to the dilation a central pair of microtubules is present, but dyneinlike arms are absent. The scolopale cell, which gives risc to the scolopale, has cytoplasmic invaginations that form an elaborate array of extracellular compartments surrounding the body wall of the sense cell. These compartments may serve to dampen high frequency vibrations permitting the receptor to respond to pressure exerted by touch, an attribute in keeping with the receptor's proposed function of detecting abdominal distension related to the size and movement of the stomach.     

The tanning of insect cuticle—A critical review and a revised mechanism

Two mechanisms to account for the stiffening of cuticle at tanning were proposed in 1940. The quinone tanning theory has been almost universally accepted; that of dehydration almost universally neglected. Calculations and tests on the mechanical properties of cuticle under differing conditions suggest that covalent cross-linking, even if it exists, is insufficient to account for the degree of stiffening of cuticle at sclerotisation. Dehydration will induce sufficient secondary bonds to account for the stiffness and insolubility of ‘tanned’ cuticle in the complete absence of covalent cross-links. It is suggested that the mechanism of sclerotisation is driven by quinones and other chemicals which are secreted into the cuticle at sclerotisation and cause highly controlled dehydration. Covalent cross-linking may still occur, but must be considered to be of secondary importance and unproven in all cuticles other than resilin.     

The biochemical basis of tolerance to malathion in Rhodnius prolixus.

1. LC50 of malathion, fenitrothion and lindane were determined in R. prolixus and T. infestans. R. prolixus was shown to be tolerant to malathion. 2. The penetration rate of (14C)-malathion into R. prolixus and T. infestans was similar. 3. Acetylcholinesterase from R. prolixus heads was 3.3-fold less sensitive to inhibition by malaoxon than the similar enzyme of T. infestans. 4. R. prolixus showed more activity of GSH-S-transferases against DCNB than T. infestans. 5. The in vitro degradation of (14C)-malathion demonstrated that R. prolixus is more active than T. infestans in carboxyester splitting to give alpha and beta monoacids. 6. The synergism of TPP and TOCP on malathion toxicity was higher in R. prolixus than in T. infestans. 7. Esterase activity against alpha and beta naphthyl acetates proved to be much lower in R. prolixus homogenates than in T. infestans homogenates. An inverse result was observed when PTA was the substrate.     

The development of the cuticle in Phormium tenax

Summary As seen in the scanning electron microscope the surface wax of leaves of Phormium tenax L. consists of vertical, plate-like crystals. These increase in size and number and undergo a change in form during development. The abaxial surface has a dense covering of wax crystals, but none are present on the ridges over vascular tissues. Numerous papillae are found between these ridges in later stages of development. On the adaxial surface both wax crystals and papillae are present only around infrequent stomata.When viewed in section normal to the leaf surface the cuticle is first apparent as a thin, lamellate layer. Another layer containing a reticulum of electrondense material increases in thickness beneath the lamellae during development. This layer eventually becomes the most extensive component of the cuticle. Both the adaxial and abaxial cuticles show a similar pattern of development.