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First published online November 17, 2006
Journal of Experimental Biology 209, 4622-4637 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02554

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Morphology and action of the hind leg joints controlling jumping in froghopper insects

Malcolm Burrows

Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK

View larger version (67K): [in a new window]   Fig. 1. Drawing of a ventral view of Aphrophora to show the specialisations of the hind legs (green). The left hind leg is shown with the trochanter fully levated (cocked) about the coxa so that the femur is engaged with the coxal protrusion and the femoro-tibial joint is dorsal to the left middle leg but ventral to the thorax. The right leg is shown with the trochanter partly depressed about the coxa so that the coxal protrusion is exposed. Cross sections through the right femur and tibia show the profiles at five levels. The medial surfaces of both hind coxae are closely apposed. The tibia of a hind but not a middle leg has two ventral rows of spines at its joint with the tarsus and the two proximal segments of a hind tarsus have a single row of spines at their articulation with the next segment.

View larger version (68K): [in a new window]   Fig. 2. Angular movements possible at the five joints of a hind leg in Aphrophora. The pivot of each joint is indicated by a blue dot, and the broken lines indicate the positions of same parts of the joints in their different positions. In A,B,D,E one extreme position is shown in grey, the other in black. (A) The thoraco-coxal joint viewed laterally. The coxa can rotate by some 25° about the thorax. In this diagram the femur has also moved about the trochanter. (B-E) Ventral views. (B) The femur can rotate by some 30° about the trochanter. (C) Three positions of the coxo-trochanteral joint: almost fully levated, mid position, and almost fully depressed. The trochanter can levate and depress by some 75-100° about the coxa. (D) The tibia can flex and extend by some 170° about the femur. (E) The tarsus can levate and depress by some 45° about the tibia.

View larger version (65K): [in a new window]   Fig. 3. Attempted jump by a restrained Aphrophora viewed ventrally. All the legs are free to move but make no contact with any substrate. (A) Images captured at 1000 s-1 and with an exposure time of 0.5 ms, are arranged in three columns. Between frames -2110 and -2000 ms, the right hind leg is gradually swung anteriorly by a levation of the trochanter about the coxa. The femur now obscures the lateral protrusion of the coxa. The leg remains in this levated and cocked position for 2 s with no changes in the angles of coxo-trochanteral joint or other more distal joints. Toward the end of this period (between frames -2000 and -1 ms), the coxa moves forward about the thorax, indicated by the reduction in distance between its anterior edge and the posterior edge of the coxa of a middle leg (yellow horizontal lines). The joint between the trochanter and the coxa then depresses rapidly so that within 1 ms the whole leg has been extended (frames -1 ms and 0 ms). (B) Plot of the angular changes of the coxo-trochanteral and trochantero-femoral joints of the hind leg during the whole sequence. The vertical yellow bars indicate when the hind legs achieved their fully cocked position (left) and when the jump occurred (right).

View larger version (88K): [in a new window]   Fig. 4. Drawings and photographs of the structure and articulation of the trochanter and coxa of Lepyronia viewed ventrally. (A) Drawing showing the left trochanter fully levated and the right trochanter almost fully depressed. (B,C) Photographs with the joints in the same positions as in the drawings.

View larger version (97K): [in a new window]   Fig. 5. The articulation of the trochanter and coxa in Aphrophora viewed dorsally and posteriorly. (A) Drawing to show the trochanter of the right leg fully depressed and that of the left leg fully levated. A protrusion on the dorsal surface of the femur (asterisk) engages with the lateral protrusion of the coxa when the leg is fully levated. The trochanteral depressor muscle within the thorax is shown. Cuticular parts of the hind leg are coloured green. (B) Photograph to show the trochanter almost fully levated. The tendon of the trochanteral depressor muscle can be seen where the dorsal cuticle of the coxa has been removed; the broken lines show its full width. (C) The trochanter is fully levated so that the femoral protrusion is engaged with the coxal protrusion. (D) The trochanter is fully depressed about the coxa. (E) The trochanter is approximately in the middle of its range of movements. These photographs also show the insertion of a trochanteral levator muscle on a lateral cuticular protrusion of the trochanter and the insertion of the trochanteral depressor on heavily sclerotised medial cuticle of the trochanter. Cuticular reinforcing struts in the coxa extend anteriorly from the coxo-trochanteral pivot. Between these struts the tendon of the trochanteral depressor muscle is visible. (F) Photograph from a medial aspect to show the prominent femoral protrusion at the proximal end of the femur.

View larger version (51K): [in a new window]   Fig. 6. Movements of the hind coxo-trochanteral joints of Aphrophora viewed posteriorly. The joint was moved from the fully levated position (A) through a partially depressed position (B) to the fully depressed position (C). Both the ventral and dorsal pivots of the joint are indicated by the blue arrows and are joined by the blue lines. The shorter blue lines indicate the insertions of the trochanteral depressor muscles. (D) Plot of the effective lever arms of the trochanteral levator and depressor muscles expressed as perpendicular distances between their insertion points and the axis of the pivot of the coxo-trochanteral joint over the full angular range of coxo-trochanteral joint movements.

View larger version (138K): [in a new window]   Fig. 7. Scanning electron micrographs of the proximal hind leg joints of Philaenus viewed ventrally. (A) The trochanter of the left hind leg is almost fully levated about the coxa and the right trochanter is fully depressed. The large white arrows indicate the pivot points of the proximal leg joints and the black arrows three prominent arrays of proprioceptive hairs. (B) Hair row on the right trochantin; midline is to the right. (C) The right coxo-trochantinal hair plate. (D) The right trochanteral hair plate.

View larger version (144K): [in a new window]   Fig. 8. Scanning electron micrographs of the lateral protrusions on the hind leg coxae viewed ventrally. (A) The trochantera of both hind legs are fully depressed to reveal the coxae. (B) Higher magnification to show the articulation of the coxa with the thorax and the coxal protrusion (boxed area in A) arising laterally. (C) The coxal protrusion is covered in swirls of microtrichia. (D) Higher power view of the microtrichia.

View larger version (133K): [in a new window]   Fig. 9. Scanning electron micrographs of the femoral protrusion of a left hind leg viewed dorsally. The different parts of this figure are shown at the same magnification as comparable parts of Fig. 8. (A) The trochantera of both hind legs are partially depressed and reveal the femoral protrusion and the coxal protrusion on the left hind leg. (B) Higher magnification of the femoral protrusion (boxed in A) shows that it is covered in microtrichia whereas the surrounding cuticle is smooth. (C) Microtrichia cover the femoral protrusion and closely spaced hairs are present anteriorly. (D) Higher power view of the microtrichia on the femoral protrusion.

View larger version (78K): [in a new window]   Fig. 10. Larval Philaenus lack microtrichia on the coxal protrusions and proximal femora of their hind legs. (A) The cuticle of the left coxal protrusion is smooth and has no microtrichia as viewed ventrally. (B) The right femur viewed dorsally has no protrusion at its proximal end and microtrichia are absent.

View larger version (64K): [in a new window]   Fig. 11. Depression movements of a hind leg in dead Cercopis. (A) Ventral view in which the left hind was forcibly moved into the cocked position so that the coxo-trochanteral joint was fully levated and the femoral protrusion engaged with the coxal protrusion. The leg then spontaneously depressed with the whole movement being completed in 2 ms. Note the rotation of the trochanter about the femur (yellow arrow) and the movement of the femur (blue line and arrow). (B) Two stage depression of the left hind leg in a second Cercopis. Ventral view. The first image (-17 ms) shows the leg in the fully levated position. The tibia was pushed posteriorly and the coxo-trochanteral joint depressed partially (image -2 ms) while the femur stayed in the same position. At 0 ms the femur moved and was accompanied by a further depression of the coxo-trochanteral joint. In the final image (+1 ms) the continuing movement of the femur exposed the coxal protrusion. Images were captured at 500 s-1 in A and at 1000 s-1 in B.

View larger version (48K): [in a new window]   Fig. 12. An audible click occurs when froghoppers jump. (A) During a free jump, a click occurred at the time of take-off. Images were captured at 1000 s-1. A cartoon of a froghopper is shown in the first frame to clarify the images. (B) Aphrophora was fixed ventral surface uppermost in PlasticeneTM and with a microphone 5 mm from the right hind leg. Movements of the hind legs were captured at 1000 s-1. The rapid simultaneous movement of both hind legs was accompanied by a click even though the hind legs did not contact anything during their movement. The left image of a ventral view shows the hind legs fully cocked and tucked between the overlying middle legs and the thorax. The right image shows both hind legs fully depressed 1 ms later. (C) Movements of a hind leg were recorded opto-electronically together with the sound, and 10 jump movements by one Aphrophora were superimposed. The first sound occurred only 0.05 ms after the start of the movement.