Effect of temperature on leg kinematics in sprinting tarantulas (Aphonopelma hentzi): high speed may limit hydraulic joint actuation

Abstract

Tarantulas extend the femur-patella (proximal) and tibia-metatarsal (distal) joints of their legs hydraulically. Because these two hydraulically actuated joints are positioned in series, hemolymph flow within each leg is expected to mechanically couple the movement of the joints. In the current study, we tested two hypotheses: (1) at lower temperatures, movement of the two in-series hydraulic joints within a leg will be less coupled because of increased hemolymph viscosity slowing hemolymph flow; and (2) at higher temperatures, movement of the two in-series hydraulic joints will be less coupled because the higher stride frequencies limit the time available for hemolymph flow. We elicited maximal running speeds at four ecologically relevant temperatures (15, 24, 31 and 40°C) in Texas Brown tarantulas (Aphonopelma hentzi). The spiders increased sprint speed 2.5-fold over the temperature range by changing their stride frequency but not stride length. The coefficient of determination for linear regression (R(2)) of the proximal and distal joint angles was used as the measure of the degree of coupling between the two joints. This coupling coefficient between the proximal and distal joint angles, for both forelegs and hind-legs, was significantly lowest at the highest temperature at which the animals ran the fastest with the highest stride frequencies. The coordination of multiple, in-series hydraulically actuated joints may be limited by operating speed.

Keywords: Arachnids; Locomotion; Running.

Fig. 1.

Marker placement and leg anatomy of the tarantula Aphonopelma hentzi. (A) Lateral view of the tarantula as it walks forward (to the right). The first (fore) and fourth (hind) leg on the animal's right side were examined. (B) A representative spider leg with the seven segments colored alternately in black and gray. Arrows point to the hydraulically extended joints examined: blue arrows indicate the femur–patella (or proximal) joint, red arrows indicate the tibia–metatarsus (or distal) joint.

Fig. 2.

Selected frames from a complete stride of an individual sprinting from left to right at 24°C. The sequential frames show the side view and the simultaneous top view of the animal provided by a mirror positioned at 45 deg to the camera. Four points on each of the first and fourth legs on the right side were digitized to provide the joint angles for the two hydraulically extended joints (blue arrow, proximal femur–patella; red arrow, distal tibia–metatarsus).

Fig. 3.

Sprinting parameters change with temperature. (A) Speed increased with a Q₁₀ of 1.56 across the temperature range (BL, body length). Numbers indicate the Q₁₀ from 18 to 24°C, 24 to 31°C, and 31 to 38°C. (B) Stride frequency increased with temperature with a mean Q₁₀ of 1.51. Numbers indicate the Q₁₀ from 18 to 24°C, 24 to 31°C, and 31 to 38°C. (C) Stride length remained constant with temperature. (D) Extension fractions of the foreleg (dashed line, open squares) and hindleg (solid line, filled circles) did not change with temperature. All symbols represent means±s.e.m. across individuals.

Fig. 4.

Representative joint angle changes during sprinting for the proximal and distal joints at 19 and 39°C. Proximal joints are shown in blue; distal joints are shown in red. Dashed lines represent unfiltered joint angles, while solid lines represent the filtered joint angles. (A,B) Kinematics of the foreleg at 19 and 39°C, respectively. (C,D) Kinematics of the hindleg at 19 and 39°C, respectively. The vertical double-headed arrow indicates the counterphase relationship between the foreleg and hindleg.

Fig. 5.

Coupling of the proximal–distal joint angles at 19 and 39°C for the same trials shown in Fig. 4. Lines represent linear regressions of the proximal and distal joint angles for each trial. R² was used as a measure of the coupling coefficient between the proximal and distal joints within each leg. (A,B) Coupling of the forelimb joints at 19 and 39°C, respectively. (C,D) Coupling of the hindlimb joints at 19 and 39°C, respectively. Proximal–distal coupling coefficients were (A) 0.87, (B) 0.27, (C) 0.73 and (D) 0.17.

Fig. 6.

Coupling coefficient decreases with temperature for both legs. The degree of proximal–distal joint coupling, represented by the coupling coefficient or R², decreased for both legs as temperature increased, but less for the foreleg than for the hindleg.