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First published online March 9, 2004
Journal of Experimental Biology 207, 1273-1286 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00888

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Stick insect locomotion in a complex environment: climbing over large gaps

Bettina Blaesing^* and Holk Cruse

Faculty of Biology, University of Bielefeld, Postbox 100131, D-33501 Bielefeld, Germany

View larger version (19K): [in a new window]   Fig. 1. Consecutive photographs (from top to bottom) of Aretaon asperrimus male climbing across a gap of 50 mm width between two cardboard footbridges; each picture illustrates one of the sections of the gap-crossing sequence. EFL, exploration and front leg gap-crossing steps; IFM, interval between front leg and middle leg gap-crossing steps; ML, middle leg gap-crossing steps; IMH, interval between middle leg and hind leg gap-crossing steps; HL, hind leg gap-crossing steps.

View larger version (17K): [in a new window]   Fig. 2. (1) Tentative step, (2) gap-crossing step, (3) normal walking step and (4) short step, shown schematically. PEP, posterior extreme position; AEP, anterior extreme position; fAEP, fictive anterior extreme position; swing, initial swing movement (green); search, subsequent searching movement (red).

View larger version (26K): [in a new window]   Fig. 3. Histograms of swing amplitudes of all steps (with exception of gap-crossing steps and tentative steps) observed during undisturbed walking (left) and gap crossing (right). (A) Front legs, (B) middle legs, (C) hind legs, (D) pooled data of all leg pairs. The broken lines mark the threshold of 8.5 mm that separates short steps from normal walking steps (as indicated).

View larger version (22K): [in a new window]   Fig. 4. Examples of stepping patterns. (A) Gap crossing, (B) undisturbed walking. Grey bars, ground contacts of antennae; coloured bars, swing movements of legs. FL, front leg (red); ML, middle leg (green); HL, hind leg (blue); l, left; r, right; ac, first contact of the antennae with the second footbridge; gc, gap-crossing step; te, tentative step; sh, short step. Normal walking steps are not marked. In A, the defined sections of gap-crossing behaviour (see Fig. 1) are separated by vertical lines.

View larger version (37K): [in a new window]   Fig. 5. Ethogram of gap-crossing behaviour; elements of behaviour used in the flow chart diagram (Fig. 10) are printed red. Ant, antenna; FL, front leg; ML, middle leg; HL, hind leg; fbr_1, first footbridge; fbr_2, second footbridge; norm, normal walking step; AEP, anterior extreme position; fAEP, fictive anterior extreme position; swing, initial swing movement; search, subsequent searching movement.

View larger version (26K): [in a new window]   Fig. 6. Velocity profiles of forward movement of the body overground during a time window of 6 s at the beginning of the gap-crossing sequence. (A) t=0 (broken line) taken as initial `non-contact' of the front leg (tarsus crosses footbridge level while swinging into the gap for the first time); (B) same data as in A, but t=0 (broken line) taken as initial `non-contact' of the antenna. Heavy lines, arithmetic mean; thin lines, single trials; red dots in the upper panel mark `non-contacts' of the antenna (t=0 in B), illustrating their temporal relation to corresponding `non-contacts' of the front leg. Left, 30 mm trials; right, 50 mm trials; N=7 trials in each gap width, bin width=200 ms.

View larger version (26K): [in a new window]   Fig. 7. Box and whisker plots of duration (A), body advance (B) and body velocity (C) of sections of the gap-crossing sequences (note the different scales for body velocity and duration). Red, IFM and IMH; black, EFL, ML, HL (see Fig. 1 for abbreviations); arithmetic means are added as squares. Left, 30 mm trials (N=7 animals, n=15 trials); right, 50 mm trials (N=5 animals, n=10 trials); mean walking velocity (30.0±3.4 mm s–1) calculated from ten sequences of undisturbed walking is added as a horizontal broken line in B. Negative duration values of interval sections indicate overlapping of gap-crossing steps of neighbouring leg pairs (30 mm: 5 cases in IFM, 5 cases IMH; 50 mm: 4 cases in IMH).

View larger version (20K): [in a new window]   Fig. 8. Average frequency of different step types in the sections EFL to HL (see Fig. 1 for abbreviations). (A) Front legs, (B) middle legs, (C) hind legs, left: 30 mm trials (N=7 animals, n=15 trials), right: 50 mm trials (N=5 animals, n=10 trials). Black columns, normal walking steps; red columns, short steps; green columns, tentative steps; blue columns, gap-crossing steps.

View larger version (19K): [in a new window]   Fig. 9. Swing direction and temporal distribution of short steps. Swing direction (AEP relative to PEP) is defined in four groups (f, forward; b, backward; o, outward; i, inward; coordinate system is shown in the inset figure below). Temporal distribution is defined according to the five sections EFL to HL (see Fig. 1 for abbreviations); short steps caused by the levator or TOT-reflex and by clinging to the second footbridge have not been taken into account.

View larger version (23K): [in a new window]   Fig. 10. Flow chart diagrams of gap-crossing behaviour. Arrows connect any two elements of behaviour that were performed in direct transition (bigrams), pointing from the first to the second element. The size of arrow (see key) represents the 2-values of the corresponding bigrams (explained in the text; see Materials and methods). Arrows marked by c connect behavioural elements performed by limbs of contralateral body segments (e.g. left antenna and right front leg). Only bigrams that occurred more than twice and with higher probability than expected in a random distribution (P0.001) have been included in the flow chart. `Typical' combinations that represent steps described in the text are schematically displayed on the right side: (A) tentative step, (B) gap-crossing step, (C) gap-crossing step without searching movement. Black, elements and transitions that occur in both the 30 mm and 50 mm trials; red, elements and transitions that occur only in the 30 mm trials; green, elements and transitions that occur only in the 50 mm trials; FL, front legs; ML, middle legs; HL, hind legs; Ant, antennal. For further abbreviations see Fig. 1 (sections of gap crossing) and Fig. 5 (ethogram; elements of behaviour used in the flow chart diagram are printed red).