Semi-lunar Process Research Articles

Non-invasive characterization of the elastic protein resilin in insects using Raman spectroscopy

Resilin is an extremely efficient elastic protein found in the moving parts of insects. Despite many years of resilin research, we are still only just starting to understand its diversity, native structures, and functions. Understanding differences in resilin structure and diversity could lead to the development of bioinspired elastic polymers, with broad applications in materials science. Here, to better understand resilin structure, we offer a novel methodology for identifying resilin-rich regions of the insect cuticle using non-invasive Raman spectroscopy in a model species, the desert locust (Schistocerca gregaria). The Raman spectrum of the resilin-rich semilunar process of the hind leg was compared with that of nearby low-resilin cuticle, and reference spectra and peaks assigned for these two regions. The main peaks of resilin include two bands associated with tyrosine at 955–962 and 1141–1203 cm−1 and a strong peak at 1615 cm−1, attributed to the α-Amide I group associated with dityrosine. We also found the chitin skeletal modes at ~485–567 cm−1 to be significant contributors to spectra variance between the groups. Raman spectra were also compared to results obtained by fluorescence spectroscopy, as a control technique. Principal component analysis of these resulting spectra revealed differences in the light-scattering properties of resilin-rich and resilin-poor cuticular regions, which may relate to differences in native protein structure and relative abundance.

Biomimetic layered fiber-reinforced Ti–Al composites: Effects of various parameters on their strength and ductility through finite element analysis

In this study, layered n-layer fiber-reinforced (L-nLFR) Ti–Al composites mimicking portion II of the locust semilunar process (SLP) with excellent mechanical properties were designed, and the finite element method was used to determine the influences of the total volume content of Ti (VTi), volume content of layered Ti (VLTi), and number of fiber layers (n) on the specific bending strength and fracture bending strain of the L-nLFR Ti–Al composite structures. The results showed that increasing VTi from 0.1 to 0.6 and VLTi from 0 to VTi significantly increases the specific bending strength but has a non-monotonic effect on the fracture bending strain of the structures. Increasing n from 1 to 2 significantly increases the specific bending strength but reduces the fracture bending strain, and the increase of n from 2 to 5 can reduce the damage evolution rate. Compared to multilayer Ti–Al composite structures in previous studies with approximately equal specific bending strengths, the fracture bending strain of the L-nLFR structure is 15%-20% larger. The biomimetic layered fiber-reinforced Ti-Al composites designed in this study can provide a new choice for engineering composite materials.

Microstructure, elastic modulus, and energy storage properties of portion II in the locust semi-lunar process

A locust stores the energy needed for jumping in its semi-lunar processes (SLPs) at the end of the hind leg femur, and portion II of the SLP plays a key role in the energy storage–release process. In this paper, scanning electron microscopy and transmission electron microscopy tests were conducted on portion II to determine its microstructure. The elastic moduli of different directions of portion II were tested by atomic force microscopy. Test data demonstrate that portion II is a layered structure formed by lamination of composite layers with a thickness of 1.09 ± 0.44 µm and chitin layers with a thickness of 0.10 ± 0.02 µm, and the composite layer is a fiber-reinforced structure. The elastic moduli of the composite layers along the fiber direction and perpendicular to the fiber direction are 11.32 ± 1.09 GPa and 10.36 ± 2.64 GPa, respectively. Furthermore, the influence of the change in the thickness of the composite layer, the volume ratio of the composite layer, and the fiber volume content in the composite layer on the maximum von Mises stress and strain energy of portion II was analyzed using the finite element method. The analysis results show that the strain energy corresponding to the actual parameters of portion II is close to the maximum. Under this premise, the maximum von Mises stress is close to the minimum. This suggests that the actual parameters give portion II almost the largest energy storage and then the longest fatigue life.

Natural arrangement of micro-strips reduces shear strain in the locust cuticle during power amplification

As one of the key components in power amplification of locust, semi-lunar process (SLP) cuticle has five different portions where its portion II stores a large amount of strain energy when locusts kick. The SLP portion II is composed of regular chitin micro-strips that non-uniformly arranged in its transverse plane. However, it is still unknown whether the natural arrangement of micro-strips affects the power amplification of the SLP cuticle. To address this, SLP portion II samples of adult locusts were mechanically tested along three orthogonal directions, corresponding to the length, width, and thickness axes of the micro-strips, respectively. Four different arrangements of micro-strips in the portion II were compared using finite element method. It was found that the micro-strip set of the portion II is orthotropic elastic and its elastic modulus is maximal in the length axis of the micro-strip. The inclusion of the micro-strips offers more strain energy than the isotropic portion II (without micro-strips). Compared to the horizontally- and vertically-arrangements of the micro-strips, their natural arrangement does not affect the strain energy but reduces the in-plane shear strain in the portion II. This research helps us deeply understand the power amplification mechanism of locusts and may guide optimization of catapult device design in bioinspired robots.

Impact of Different Developmental Instars on Locusta migratoria Jumping Performance.

Ontogenetic locomotion research focuses on the evolution of locomotion behavior in different developmental stages of a species. Unlike vertebrates, ontogenetic locomotion in invertebrates is poorly investigated. Locusts represent an outstanding biological model to study this issue. They are hemimetabolous insects and have similar aspects and behaviors in different instars. This research is aimed at studying the jumping performance of Locusta migratoria over different developmental instars. Jumps of third instar, fourth instar, and adult L. migratoria were recorded through a high-speed camera. Data were analyzed to develop a simplified biomechanical model of the insect: the elastic joint of locust hind legs was simplified as a torsional spring located at the femur-tibiae joint as a semilunar process and based on an energetic approach involving both locomotion and geometrical data. A simplified mathematical model evaluated the performances of each tested jump. Results showed that longer hind leg length, higher elastic parameter, and longer takeoff time synergistically contribute to a greater velocity and energy storing/releasing in adult locusts, if compared to young instars; at the same time, they compensate possible decreases of the acceleration due to the mass increase. This finding also gives insights for advanced bioinspired jumping robot design.

Time-scale mechanical behaviors of locust semi-lunar process cuticles under power amplification for rapid movements

The semi-lunar process (SLP) is a key component in the power amplification of locusts to achieve rapid movements. Its mechanical properties determine the amount of the power amplification and the subsequent locomotion performance. As previously reported, the SLP cuticle endures physiological dynamic loadings. However, the time-scale mechanical properties of the SLP are still unknown, especially under stress relaxation and cyclic loadings. In this paper, the SLP cuticles of adult desert locusts (Schistocerca gregaria) were studied using stress relaxation and cyclic tests, with loadings corresponding to the physiological loading conditions of the power amplification. The SLP cuticle was found to show pronounced stress relaxation behavior with the resultant force and an evident time shift between the maximal displacement and the maximal resultant force. The number of loading cycles before mechanical failure (life cycle number) increases when the SLP cuticle is cyclically loaded by a lower stress level. Moreover, the failure strength of the SLP at low cycles equals the physiological stress level in the power amplification, implying that the healing of the cuticle might contribute to the successful performance of numerous jumps in the course of the adult locust life. This study not only deepens our understanding of the power amplification mechanism of locust locomotion but also provides valuable knowledge for the design optimization of bioinspired jumping robots and elastic energy storage devices.

Mechanical characterization of the key portions in locust semi-lunar processes under different strain rates

The excellent rapid jumping and kicking of locusts are largely attributed to the power amplification mechanisms due to the semi-lunar processes (SLP) at their distal metathoracic femurs, especially dorsal-core (i.e., portion II) and ventral-core parts (i.e., portion III). The physiological range of strain rates at the two portions of locust SLP is quite broad in the periods of energy storage and release (approximately three orders). However, it still remains elusive how the mechanical properties of the two SLP portions change with the strain rate. We identified the elastic moduli and material compositions of SLP portions II and III by using nanoindentation and confocal laser scanning microscope. Apparent and creep-corrected reduced elastic moduli were calculated to represent the total energy absorption and storage, respectively. The results revealed that both portions II and III exhibit strain rate-sensitive elastic moduli, regardless of water content. The efficiency of elastic energy storage is only 51–70% in the case of low strain rate. This work can deepen our understanding in the energy storage and release mechanisms in locust locomotion and further provide guidelines for biomimetic design of power amplification apparatus in jumping robots.

Structures, properties, and energy-storage mechanisms of the semi-lunar process cuticles in locusts.

Locusts have excellent jumping and kicking abilities to survive in nature, which are achieved through the energy storage and release processes occurring in cuticles, especially in the semi-lunar processes (SLP) at the femorotibial joints. As yet, however, the strain energy-storage mechanisms of the SLP cuticles remain unclear. To decode this mystery, we investigated the microstructure, material composition, and mechanical properties of the SLP cuticle and its remarkable strain energy-storage mechanisms for jumping and kicking. It is found that the SLP cuticle of adult Locusta migratoria manilensis consists of five main parts that exhibit different microstructural features, material compositions, mechanical properties, and biological functions in storing strain energy. The mechanical properties of these five components are all transversely isotropic and strongly depend on their water contents. Finite element simulations indicate that the two parts of the core region of the SLP cuticle likely make significant contributions to its outstanding strain energy-storage ability. This work deepens our understanding of the locomotion behaviors and superior energy-storage mechanisms of insects such as locusts and is helpful for the design and fabrication of strain energy-storage devices.

Increased muscular volume and cuticular specialisations enhance jump velocity in solitarious compared with gregarious desert locusts, Schistocerca gregaria.

The desert locust, Schistocerca gregaria, shows a strong phenotypic plasticity. It can develop, depending upon population density, into either a solitarious or gregarious phase that differs in many aspects of behaviour, physiology and morphology. Prominent amongst these differences is that solitarious locusts have proportionately longer hind femora than gregarious locusts. The hind femora contain the muscles and energy-storing cuticular structures that propel powerful jumps using a catapult-like mechanism. We show that solitarious locusts jump on average 23% faster and 27% further than gregarious locusts, and attribute this improved performance to three sources: first, a 17.5% increase in the relative volume of their hind femur, and hence muscle volume; second, a 24.3% decrease in the stiffness of the energy-storing semi-lunar processes of the distal femur; and third, a 4.5% decrease in the stiffness of the tendon of the extensor tibiae muscle. These differences mean that solitarious locusts can generate more power and store more energy in preparation for a jump than can gregarious locusts. This improved performance comes at a cost: solitarious locusts expend nearly twice the energy of gregarious locusts during a single jump and the muscular co-contraction that energises the cuticular springs takes twice as long. There is thus a trade-off between achieving maximum jump velocity in the solitarious phase against the ability to engage jumping rapidly and repeatedly in the gregarious phase.

Development and deposition of resilin in energy stores for locust jumping.

Locusts jump by using a catapult mechanism in which energy produced by slow contractions of the extensor tibiae muscles of the hind legs is stored in distortions of the exoskeleton, most notably (1) the two semi-lunar processes at each knee joint and (2) the tendons of the extensor muscles themselves. The energy is then suddenly released from these stores to power the rapid, propulsive movements of the hind legs. The reliance on the mechanical storage of energy is likely to impact on jumping because growth occurs by a series of five moults, at each of which the exoskeleton is replaced by a new one. All developmental stages (instars) nevertheless jump as a means of forward locomotion, or as an escape movement. Here, I show that in each instar, resilin is added to the semi-lunar processes and to the core of the extensor tendons so that their thickness increases. As the next moult approaches, a new exoskeleton forms within the old one, with resilin already present in the new semi-lunar processes. The old exoskeleton, the tendons and their resilin are discarded at moulting. The resilin of the semi-lunar processes and tendons of the new instar is initially thin, but a similar pattern of deposition results in an increase of their thickness. In adults, resilin continues to be deposited so that at 4weeks old the thickness in the semi-lunar processes has increased fourfold. These changes in the energy stores accompany changes in jumping ability and performance during each moulting cycle.

Locusts use a composite of resilin and hard cuticle as an energy store for jumping and kicking

Locusts jump and kick by using a catapult mechanism in which energy is first stored and then rapidly released to extend the large hind legs. The power is produced by a slow contraction of large muscles in the hind femora that bend paired semi-lunar processes in the distal part of each femur and store half the energy needed for a kick. We now show that these energy storage devices are composites of hard cuticle and the rubber-like protein resilin. The inside surface of a semi-lunar process consists of a layer of resilin, particularly thick along an inwardly pointing ridge and tightly bonded to the external, black cuticle. From the outside, resilin is visible only as a distal and ventral triangular area that tapers proximally. High-speed imaging showed that the semi-lunar processes were bent in all three dimensions during the prolonged muscular contractions that precede a kick. To reproduce these bending movements, the extensor tibiae muscle was stimulated electrically in a pattern that mimicked the normal sequence of its fast motor spikes recorded in natural kicking. Externally visible resilin was compressed and wrinkled as a semi-lunar process was bent. It then sprung back to restore the semi-lunar process rapidly to its original natural shape. Each of the five nymphal stages jumped and kicked and had a similar distribution of resilin in their semi-lunar processes as adults; the resilin was shed with the cuticle at each moult. It is suggested that composite storage devices that combine the elastic properties of resilin with the stiffness of hard cuticle allow energy to be stored by bending hard cuticle over only a small distance and without fracturing. In this way all the stored energy is returned and the natural shape of the femur is restored rapidly so that a jump or kick can be repeated.

Neuromechanical simulation of the locust jump

The neural circuitry and biomechanics of kicking in locusts have been studied to understand their roles in the control of both kicking and jumping. It has been hypothesized that the same neural circuit and biomechanics governed both behaviors but this hypothesis was not testable with current technology. We built a neuromechanical model to test this and to gain a better understanding of the role of the semi-lunar process (SLP) in jump dynamics. The jumping and kicking behaviors of the model were tested by comparing them with a variety of published data, and were found to reproduce the results from live animals. This confirmed that the kick neural circuitry can produce the jump behavior. The SLP is a set of highly sclerotized bands of cuticle that can be bent to store energy for use during kicking and jumping. It has not been possible to directly test the effects of the SLP on jump performance because it is an integral part of the joint, and attempts to remove its influence prevent the locust from being able to jump. Simulations demonstrated that the SLP can significantly increase jump distance, power, total energy and duration of the jump impulse. In addition, the geometry of the joint enables the SLP force to assist leg flexion when the leg is flexed, and to assist extension once the leg has begun to extend.

Role of the semi-lunar process in locust jumping

itor> Meeting abstracts - A single PDF containing all abstracts in this Supplement is available here http://www. biomedcentral.co m/content/pdf/14 71-2202-8-S2-in fo.pdf

Proprioceptors monitoring forces in a locust hind leg during kicking form negative feedback loops with flexor tibiae motor neurons.

In preparation for jumping and kicking, a locust slowly generates large forces in the femoral muscles of its hind legs and stores them in elastic distortions of the tendons and femoral cuticle. At the femoro-tibial joints, the semi-lunar processes are bent, the cuticle of the dorsal distal femur is crumpled, and the femur is expanded in a mediolateral direction. We have analysed whether these distortions are monitored by sense organs and whether the information they provide is used to limit the forces generated and thus prevent structural damage to the joint. The two sensory neurons comprising the lump receptor lie in a groove in the ventral part of the distal femur. The sensory neurons spike if force is applied to the flexor tendon when the joint is fully flexed, but not when it is extended. They also spike as the tendon of the flexor muscle slides into the ventral femoral groove when the tibia is fully flexed during the co-contraction phase of kicking. Their spike frequency correlates with the extent of bending of a semi-lunar process that provides a quantifiable measure of the joint distortions. If the tibia is not fully flexed, however, then muscle contractions still cause distortions of the joint but these are not signalled by sensory spikes from the lump receptor. The lump receptor, therefore, does not respond primarily to the joint distortions but to the movements or force in the flexor tendon. Contractions of the flexor tibiae muscle caused by spikes in individual flexor motor neurons can evoke spikes in sensory neurons from the lump receptor when the joint is fully flexed. In turn, the sensory neurons cause a hyperpolarisation in particular flexor motor neurons in a polysynaptic negative feedback loop. The lump receptor could, therefore, regulate the output of the flexor motor neurons and, thus, limit the amount of force generated during co-contraction. It may also contribute to the inhibition of the flexors at the end of co-contraction that allows rapid kicking movements to occur.

Jumping and kicking in the false stick insect Prosarthria teretrirostris: kinematics and motor control.

The false stick insect Prosarthria teretrirostris looks and behaves like a real stick insect but can jump and kick rapidly and powerfully like a locust, to which it is more closely related. It has an elongated body with slender hind legs that are some 2.5 times longer than the front and middle legs. A male with a body 67 mm long and weighing 0.28 g can jump 90 cm with a take-off angle of 40 degrees and velocity of 2.5 ms(-1), requiring an energy expenditure of 850 microJ. The body is accelerated at 165 ms(-2) for only 30 ms. The larger and heavier females (mean body length 104 mm and weighing 1.5 g) can jump on average a distance of 49 cm. During jumping, the tibiae of the hind legs are extended in 30 ms with maximum rotational velocities of 11.5 degrees per ms, but during kicking, when there is no body weight to support, extension is complete in 7 ms with rotational velocities as high as 48 degrees per ms. The short time available to accelerate the body indicates that the movements are not powered by direct muscle contractions and that there must be storage of elastic energy in advance. The motor patterns responsible for generating the necessary forces in the hind legs for jumping and kicking are similar and consist of three phases; an initial flexion of the tibia is followed by a co-contraction of the small flexor and large extensor tibiae muscles lasting several hundred milliseconds while the tibia remains fully flexed. Finally, the flexor motor neurons stop spiking so that the tibia is able to extend rapidly. The small semi-lunar processes at the femoro-tibial joints are not distorted, so that they cannot act as energy stores. Some 7% of the energy is stored transiently by bending the thin tibiae during the initial acceleration phase of a jump and releasing it just before take-off. The jumping and kicking mechanisms of Prosarthria teretrirostris have features in common with those used by locusts but also have their own characteristics. The evolution of jumping in Orthoptera is discussed in this context.

The kinematics and neural control of high-speed kicking movements in the locust.

The tibiae of locust hind legs can be extended fully in a kick in 3 ms with peak angular velocities of at least 80 degrees ms(-1). If the mass of the distal part of the leg is halved, then the extension is complete in less than 1 ms with angular velocities of more than 200 degrees ms(-1). The high velocities and the associated power are generated by a preceding storage of energy and its sudden release produced by a specific motor pattern and specialisations of the femoro-tibial joints. To understand the dynamics of these rapid movements and the interrelations between joint mechanics and the motor pattern, kicks were analysed with high-speed video images coupled to simultaneous intracellular recordings from identified leg motor neurones. The first movement is a full tibial flexion followed by co-contraction of the extensor and flexor tibiae muscles for 0.3-1 s, during which the distal end of the femur is flattened dorso-ventrally and expanded laterally. The two semi-lunar processes on the distal femur are bent when the fast extensor tibiae motor neurone spikes so that their tips move ventrally by up to 0.6 mm. The inward projections of these processes into the femur form the proximal part of the hinge joint with the tibia, so that the pivot of the joint also changes and the tibia therefore moves proximally and ventrally, widening the gap between it and the femur. Extension of the tibia begins on average 34 ms after the flexor motor neurones are inhibited at the end of the co-contraction phase. The tibia then begins to extend slowly, reaching peak velocities only when it has extended by 60-70 degrees. The semi-lunar processes do not start to unfurl until the tibia has extended by 55 degrees, so they cannot provide the initial energy for extension. An audible click is produced when the semi-lunar processes unfurl. The peak velocity of tibial extension is correlated with the amount of bending of the semi-lunar processes and with the number of fast extensor motor spikes, but the same amount of semi-lunar bending can be produced by both short and long co-contractions. When the tibia reaches full extension, inertial forces may cause it to bend by as much as 33 degrees at a plane of weakness in the proximal tibia, thus allowing further extension of the distal end.

The Development of the Locust Jumping Mechanism : II. Energy Storage and Muscle Mechanics

ABSTRACT In an escape jump, the adult locust extensor tibiae muscle produces more than twice the specific energy output of the fourth instar muscle. If forced to jump repeatedly, the extensor muscles of hoppers and adults produce the same specific energy output. In escape jumping, specific energy production increases with age within an instar, but this does not seem to be the case in repeated jumping over a 10-min period. Adult locusts have thicker cuticle, including thicker semilunar processes, than hoppers. They also have relatively larger extensor apodemes. The cuticle thickness of all instars increases with age up to apolysis or maturity. The fourth instar extensor muscle has a lower angle of pennation and relatively longer muscle fibres than the adult muscle. The stiffness of the semilunar processes increases with age both within and between instars,’so that the mature adult leg has the highest energy storage capacity. To produce the required amount of energy storage, the adult muscle fibres must shorten slightly more than those of the fourth instar, but they must produce more than twice the force per unit area. The strain on the apodeme is similar in adults and hoppers, and greater at the distal than at the proximal end.

The Development of the Locust Jumping Mechanism : I. Allometric Growth and its Effect on Jumping Performance

ABSTRACT The jump range of locusts of all instars varies throughout an instar, reaching a peak at apolysis in hoppers, and about 3 weeks after the final moult in adults. Mature adult locusts have an escape jump range three times that of young hoppers. During a period of repeated jumping, mature adults have an average jump range three times that of the hoppers. Allometric growth of the metathoracic leg occurs during development, and accounts for 25 % of the increase in range between the fourth instar and adult, the remaining increment being due to absolute size increase. The allometric changes do not, however, affect the mechanics of the femoro-tibial joint. The extensor apodeme to tarsus lever ratio remains at 27:1 in the fourth, fifth and adult stages. The extension of the joint would be expected to follow a similar pattern in the fourth instar and adult stages. The change in form of the metathoracic leg of the locust also means that the adult femur accommodates more muscle, i.e. 6·3 % of the total body weight in the adult male, as opposed to 4·3 % in the fourth instar male. Therefore relatively more force and energy may be available for use in a jump. The lengthening of the semilunar processes may be expected to have an effect on the energy storage capacity of the leg. In an escape jump, all instars use the same specific power output, which is more than the published figures for maximum specific power output (250Wkg-1, Weis-Fogh & Alexander, 1977, and 860Wkg-1, Pennycuick & Parker, 1966). This suggest that energy storage is necessary in locust hoppers.

The energetics of the jump of the locust Schistocerca gregaria.

The anatomy of the metathoracic leg is redescribed with particular reference to storage of energy in cuticular elements and the way in which the stored energy is used in jumping. The jump of adult male locusts requires an energy of 9 mJ and that of the female requires 11 mJ. The semilunar processes of each metafemur store 4 mJ at a stress of 15 N, and the extensor tibiae apodeme stores a further 3 mJ at the same stress. The total stored energy in both metathoracic legs is 14 mJ. The extensor tibiae muscle produces a maximum isometric force of over 15 N at 30 degrees C and, when loaded with the extensor apodeme and semilunar processes, attains this force in 0.3 sec with a strain of 0.8 mm. The peak power output is 36 mW or 0.45 W.g-1. The peak isometric force is attained when the tibia is fully flexed and the force falls as the tibia extends. The extensor tibiae muscle A band is 5.5 mum long and the peak force is over 0.75 N.m-2. The peak velocity of shortening is 7 mm.sec-1 or about 1.75 lengths/sec at 30 degrees C. The tensile strength of the extensor apodeme is 0.6 kN.mm-2 and Young's modulus is 19 kN.mm-2. The safety factor does not exceed 1.2 and the safety factor of the semilunar processes and tibial cuticle is little higher. The jump impulse lasts 25-30 msec. A velocity of 3.2 m.sec-1 is reached after a peak acceleration of 180 m.sec-2. The peak power output is 0.75 W at close to maximum velocity. Energy losses in rotating the femur and tibia are small and it is shown that the leg is able to extend at 7 times the normal rate with losses of about 20%. Most of the stored energy is converted to kinetic energy as the animal jumps. A model is based on the relaxation of a spring that has the properties of the elastic elements of the locust leg into a lever with the same kinematics as the locust leg produces a force-distance curve similar to that measured for locust jumps. The major part of the jump energy is stored before the jump.