Cockroaches traverse crevices, crawl rapidly in confined spaces, and inspire a soft, legged robot

Abstract

Jointed exoskeletons permit rapid appendage-driven locomotion but retain the soft-bodied, shape-changing ability to explore confined environments. We challenged cockroaches with horizontal crevices smaller than a quarter of their standing body height. Cockroaches rapidly traversed crevices in 300-800 ms by compressing their body 40-60%. High-speed videography revealed crevice negotiation to be a complex, discontinuous maneuver. After traversing horizontal crevices to enter a vertically confined space, cockroaches crawled at velocities approaching 60 cm⋅s(-1), despite body compression and postural changes. Running velocity, stride length, and stride period only decreased at the smallest crevice height (4 mm), whereas slipping and the probability of zigzag paths increased. To explain confined-space running performance limits, we altered ceiling and ground friction. Increased ceiling friction decreased velocity by decreasing stride length and increasing slipping. Increased ground friction resulted in velocity and stride length attaining a maximum at intermediate friction levels. These data support a model of an unexplored mode of locomotion--"body-friction legged crawling" with body drag, friction-dominated leg thrust, but no media flow as in air, water, or sand. To define the limits of body compression in confined spaces, we conducted dynamic compressive cycle tests on living animals. Exoskeletal strength allowed cockroaches to withstand forces 300 times body weight when traversing the smallest crevices and up to nearly 900 times body weight without injury. Cockroach exoskeletons provided biological inspiration for the manufacture of an origami-style, soft, legged robot that can locomote rapidly in both open and confined spaces.

Keywords: confined; crawling; exoskeleton; locomotion; soft robotics.

Fig. 1.

Performance of cockroaches traversing horizontal crevices. (A) Comparison of freestanding height of American cockroach, Periplaneta americana, relative to the near-minimum crevice height traversed equal to 3 mm, two stacked pennies. (B) Crevice traversal apparatus with cockroach about to enter (Fig. S1A). (C) Body compression (white vertical bars) resulting from a 100-g load across segment. Percent body compression shown below segment (Fig. S1C and Table S1). (D) Crevice traversal stages extracted from high-speed movie frames with corresponding time stamp for 3-mm height (Movie S1). (E) Crevice traversal time at three crevice heights. Each behavioral stage duration is stacked onto the next from left to right to also show total time. Points and error bars represent mean ± 1 SD. (F) Probability of crevice traversal for three crevice heights (represented by three colors). Cockroaches successfully traversed the greatest heights more frequently and failed to traverse the lowest crevice heights by turning back or getting stuck within the crevice. Number of trials is shown above bar.

Fig. S1.

Experimental setup for collecting data and robot schematics. (A) Crevice traversal setup featuring a clear acrylic tube with adjustable opening. (B) Confined-space crawling setup featuring acrylic track with an adjustable ceiling plate and fixed ground plate for determining the effect of gap size. To measure the effect of friction, the ceiling and ground plates were fixed 4 mm apart and surfaces were varied with different grades of sandpaper. (C) Exoskeletal compression process with an anesthetized cockroach and 100-g weight in unloaded (Left) and loaded (Right) configurations.

Fig. 2.

Confined-space crawling performance of cockroaches. (A) Crevice crawling apparatus with cockroach about to enter (Fig. S1). Ceiling heights used represent freestanding (12 mm), crouched (9 mm), just beginning to compress body (6 mm), and minimum ceiling height within which animals crawled (4 mm). (B) Side (from movie) and front view of cockroach crawling within chamber at two ceiling heights. Front view shows the increase in sprawl angle, but not foot-to-body midline distance (tarsus midline distance) as ceiling height was reduced. (C) Performance metrics, velocity (gray), stride length (red), stride period (magenta), sprawl angle (green), tarsus midline distance (dark blue), stride success ratio (ratio of successful strides with no foot slipping relative to the total number of strides; light blue), and tortuosity index (forward displacement of cockroach relative to the length of the actual path taken; orange) as a function of ceiling height with their respective units are indicated in parentheses after label. Points and error bars show mean ± 1 SD. Red stars represent a significant difference at 4 mm relative to larger ceiling heights.

Fig. 3.

Effects of varying ground and ceiling friction for confined-space crawling performance. (A) Confined-space body-friction legged crawling characterized by drag on the dorsal and ventral surface of the body and friction-dominated thrust by legs in a nonflowing medium. (B) Performance metrics, velocity (grays), stride length (red-brown), and stride success ratio (blues) at 4-mm ceiling height (respective units indicated in parentheses below label) as a function of ceiling kinetic friction varied at three levels (low, medium, and high) with ground kinetic friction constant. (C) Performance metrics, velocity (grays), stride length (red-brown), and stride success ratio (blues) at 4-mm ceiling height (respective units indicated in parentheses) as a function of ground kinetic friction varied at three levels (low, medium, and high) with ceiling kinetic friction constant. Bars show mean ± 1 SD. Red stars represent a significant difference from other kinetic friction levels.

Fig. 4.

Model of body-friction legged crawling. (A, Left) Model simplified representation of a cockroach in a confined space depicted as a compressible body (light gray solid oval) with a single leg (wavy tan line) ending in a foot (dark gray box) confined within two parallel plates (hashed boxes). (Center) Free body diagram of foot and body. Leg force (F_L) is indicated in tan, thrust (T) in blue, drag—ceiling (f_c) and ground (f_g)—in red, body weight (W), and all normal forces—ceiling–body (N_c), ground–body (N_g), and ground–foot (N_f)—in black. (Right) Foot positions, where forward body displacement occurs with (slip zone) and without (stick zone) foot slippage, are marked in green, whereas positions where no body motion is possible are indicated in red. Leg orientations at the above-foot positions—slip angle (transition from stick to slip, θ_slip) and the maximum (θ_max) and minimum (θ_min) angle to overcome body drag are indicated. (B) Performance metrics, velocity (gray) and stride length (red) at three ceiling heights (4.4, 4.7, and 5 mm) as a function of ceiling kinetic friction with ground kinetic friction constant. (C) Performance metrics, velocity (gray), and stride length (red) at three ceiling heights (4.4, 4.7, and 5 mm) as a function of ground kinetic friction with ceiling kinetic friction constant.

Fig. 5.

Material properties of cockroaches during compression and bioinspired robot. (A) Materials testing apparatus with custom-built chamber positioned atop load cell to measure force during cyclic compression (Movie S3). (B) Normalized body compressive force (measured force/body weight) as a function of crevice size (red) and abdomen strain (change in abdomen compression/maximum abdomen thickness). Abdominal strain increases from left to right corresponding to a decrease in crevice size. Blue lines show a compression rate of 0.5 mm⋅s⁻¹. Tan lines show rate of 4 mm⋅s⁻¹. Shaded bands represent 95% confidence limits. (Inset) Compressive force cycles as a function of body compression distance for two rates of compression. The corresponding crevice or ceiling height is shown for comparison. Areas within the loop represent the energy lost per cycle. (C) Prototype robot with adjustable sprawl and abdominal compression-inspired exoskeletal plate-like shell (Movie S4) Top row photos, Side view of freestanding and confined-space posture for robot between two surfaces with ceiling labeled. Bottom row photos, Front view of freestanding and confined-space posture for robot between two surfaces. Top black small arrow shows direction of compression, and bottom black arrows show leg sprawl direction when compressed. (D) Schematic of the robot with a bioinspired compressible shell to visualize the degrees of freedom in unconfined standing (Left) and confined sprawled (Right) postures. The robot uses inspiration from the cockroach, not only with respect to body compression but also by changes in leg posture, allowing effective contact by the foot (tarsus) when standing and the leg (tibia) when running in a confined space.