Getting Into The Groove: Understanding The Planthopper's Natural Gear System

Rohini Bhattacharjya

30th September, 2020 

ABSTRACT:

Insects greatly rely on their hind legs to exhibit powerful yet agile movements in their environment. The hindlegs of champion jumping insects, froghoppers and planthoppers, move counter-rotationally in approximately the same near-horizontal plane beneath the body ( M. Burrows, J. Exp. Biol. 213, 469–478 (2010), G. P. Sutton, M. Burrows, J. Exp. Biol. 213, 1406–1416 (2010)). It was demonstrated by Burrows and Sutton (Burrows & Sutton. 2013. Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect. Science) that a pair of enmeshed gears aid in the propulsion of the flightless planthopper insect Issus coleoptratus (P.F. Whitehead & R.S. Key Observations on British Issus (Hemiptera, Issidae) with reference to development, periodicity, and ecology).

These planthoppers, as nymphs project a row of cuticular gear (cog) teeth in their inner hindlegs which interlock and operate mechanically to help in their characteristic jumping movement.. The most rapid take-off occurred in 2 ms [2.01 T 0.1 ms (mean T SEM) for eight nymphs] with a velocity of 3.9 m/s (mean 2.2 T 0.56m/s, n =8). The two propulsive hindlegs started moving within 30 ms of each other. At the final molt to adulthood, this synchronization mechanism is jettisoned.

INTRODUCTION

 

Gears are unusual in animals with little reports of use as a fully functioning mechanical system. However, it was found out that the flightless planthopper Issus coleoptratus has a set of gears in their inner hindlegs that engage and help them in their primarily ballistic movement.  Two arrangements of the hindlegs are mainly found in insects; those of grasshoppers and fleas move in separate planes at the side of the body; those of the champion jumping insects, froghoppers and planthoppers, move counter-rotationally in a horizontal plane beneath the body( M. Burrows, J. Exp. Biol. 213, 469–478 (2010), G. P. Sutton, M. Burrows, J. Exp. Biol. 213, 1406–1416 (2010)). In the latter, synchronous movements of the hindlegs are necessary to avoid rapid spinning in the yaw plane.  These planthoppers often propel rapidly in a time-span as short as 2ms. The two propulsive hindlegs started moving within 30 ms of each other. Such precise synchrony would be difficult to achieve by 1-ms-long neural spikes.  Thus this system of gears serves to overcome this lag and promote the efficiency of movement in these planthoppers.

MORPHOLOGY OBSERVED

Young planthoppers (nymphs) possess gear like teeth in the inner appendage of their propulsive hindlegs. The hind coxae are opposed at the ventral midline and gain propulsive thrust from the large thoracic muscles which rotate the trochanterae of the hind legs about the coxae. The gear-like projections are found on this trochanterae. They occur as a curved strip with the gear of one trochanter enmeshed into another.  Each gear strip is  350 nm to 400nm, contained 10-12 teeth, and had a radius of curvature of 200nm. Both the trochanterae had an equal number of teeth resulting in a gear ratio of 1:1. However, these gears are not found in the front or middle legs. Also,  these gears persist only up until the final molt of adulthood.  This maturation is occasioned by the gradual hormonal changes which occur during the larval-to-adulthood transition. Adults have been found out to be greater jumpers than the nymphs, reaching greater velocity in their flight. They greatly rely on the frictional contact between the more proximal trochanterae to synchronize their hindleg movements. Their improved performance may be attributed to other factors and not necessarily the shedding of their larval gear mechanism. (Burrows & Sutton. 2013. Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect. Science)

OPERATIONAL MECHANISM

The action of the gears in jumping was captured by Burrows and Sutton et al in high-speed videos  (about 5000 frames per second)  of a nymph restrained on its back but with its legs free. The characteristic, synchronized movements of the hindlegs were induced by gently touching the abdomen. In preparation for a jump, the hindlegs were raised (elevated) at their coxo-trochanteral joints so that both hindlegs were rotated forward. In the propulsive jumping movements, both hindlegs were depressed at the coxo-trochanteral joints with angular velocities as high as 200,000°/s . 

It was found out both during the preparatory and propulsive movements, the gear teeth were engaged to mechanically constrain the hind trochanter to move within 21 ms of each other. 

Two further experiments by Burrows and Sutton et al showed that gears ensure synchronous movements of the hindlegs. First, when the hind legs were cocked in readiness for jumping, experimental manipulation of the tendon of the large jumping muscle of one hindleg led to the synchronous and rapid movement of the other leg, even in a dead animal.

The mechanical nature of the system became even more apparent when the gears occasionally failed to engage at the start of the propulsive phase of a jump so that some teeth spun past each other. After these few misses, one tooth engaged with a tooth on the other hind leg, which then depressed rapidly, delayed only by a few microseconds relative to the first hindleg. This shows that gears play a crucial role in maintaining close synchrony.

When one leg moves first at the start of a jump, its gear teeth will engage with and transmit power to the other stationary leg inducing it to move. 

The left and right power-producing muscles are innervated by independent sets of two motor neurons each,  but all four motor neurons carry highly synchronized spike patterns that should help to ensure that the same amount of force is generated in each leg. 

This neural mechanism assists the synchrony of the leg movements but cannot deliver the level of synchrony measured during jumping. Thus, the primary role of the gears is to ensure that the hindlegs move synchronously within microseconds of each other.

DISCUSSION

Often several so-called man-made designs and inventions have been found to previously exist in nature. These include examples as basic as the Greek key motif found in secondary protein structures or like the screw in the femora of beetles. Elsewhere in the animal kingdom, apparently ornamental cogs occur on the shell of the cogwheel turtle Heosemys spinosa and on the pronotum of the wheel bug Arilus cristatus (Hemiptera, Reduviidae). The hearts of crocodilians have a cogwheel valve that closes during each heartbeat and can increase the resistance in the pulmonary outflow. In some insects, a row of regularly spaced protrusions works like clockwork escapement mechanisms to produce sound.  Perhaps we can seek inspiration for future technological advancement by studying in greater detail, these intricate, ingenious systems shaped by millions of years of evolution. It is hoped that the science behind these technological marvels might become a little less confusing as nature has already seemed to have trodden the same exploratory path we are taking in pursuit of a new technological discovery. Perhaps we aren’t as special as we believe ourselves to be, yet this could motivate us to find solutions to our problems more easily than before.

Fig. 1. Gears on the hind trochantera of Issus nymphs. (A) Nymphs viewed from the side. (B) Gears on the left and right hind trochantera are viewed posteriorly. (C) Scanning electron micrograph of the partially elevated articulation between hind trochantera and coxae and the engagement of gears on the two sides as viewed ventrally. (D) Higher magnification of the interdigitation of the gears. (E). Diagram showing the radius of curvature of the trochanter (rgear), the angular placement of the teeth, and how the gears enmesh. (F) Profile of a gear tooth in Issus (left) compared with a man-made involute gear tooth (right). The radius of the curvature of the fillet (rfillet) is indicated.

Fig. 2. Engagement of the gears during jumping. (A). Three images, at the times, indicated (captured at a rate of 5000 images per second) show the

levation of the hind trochantera and proximal femora into their cocked position in preparation for a jump. The trochanter starts moving at the time

0 ms and cocking is completed 80 ms later. (B) Drawings of the progressive movements of the gears and joints during cocking. The horizontal black arrows indicate the correspondence between the frames in (A) and the drawings. (C) Rapid and synchronized depression of the two hindlegs that power jumping .Four images starting from the first detectable depression movement of the hind trochantera (time 0 ms). Full depression was completed 1.8 ms later. The curved, black arrows show the direction of movement of each hindleg and the curved, open arrows, the direction of gear rotation.

REFERENCES:

Burrows & Sutton. 2013. Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect.

http://dx.doi.org/10.1126/science.1240284

About the Author

Rohini, majoring in Microbiology from St. Xavier's College, Kolkata, is a self-proclaimed thalassophile , she likes to read and create jazz compositions on the piano in her spare time.