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The femur–tibia control system in a proscopiid (Caelifera, Orthoptera): a test for assumptions on the functional basis and evolution of twig mimesis in stick insects

Harald Wolf^1,*, Ulrich Bässler², Roland Spieß¹ and Rolf Kittmann³

¹ Neurobiology Department, University of Ulm, D-89069 Ulm, Germany,
² Chamissostrasse 16, D-70193 Stuttgart, Germany and
³ Neunlindenstrasse 28a, D-79106 Freiburg, Germany

View larger version (41K): [in a new window]   Fig. 1. cf. Prosarthria teretrirostris, female (A) and male (B) individuals. Note the twig-like shape of the body and legs, elongation of the pro- (p) and mesothoracic (m) segments and of the head (the arrow indicates the mouth of the male) and the brown-to-green coloration. Average body lengths were 10–11 cm for females and 6–7 cm for males. A Cuniculina impigra male (C) shown for comparison; labelling as in B. Scale bars, 2 cm.

View larger version (37K): [in a new window]   Fig. 2. Anatomy and innervation of the femoral chordotonal organ (fCO). (A) A Ni2+-filled preparation of the middle leg nerves and fCO. The anterior dorsal cuticle of the trochanter and femur has been removed to provide a view of the internal structures. (B) Drawing of the preparation in A, identifying the main structures, named according to Snodgrass (1929) for the locust. Note the proximal (pr) and distal (d) portions of the fCO, the leg nerves 5B1 and 5B2 and the receptor apodeme. Shading indicates the cut surface of the leg cuticle, exposed by removing the anterior-dorsal half of the trochanter and femur (see above). (C) Drawing of a female hind leg and fCO; the arrow indicates the location, and the bracket the size, of the fCO in the leg. Labelling as in B. The inset shows an expanded view of the fCO. Scale bars, 800 µm for A and B, 10 mm for C.

View larger version (25K): [in a new window]   Fig. 3. Catalepsy in Prosarthria teretrirostris. (A) Original recording from a female hind leg. Starting from a resting position, the joint was (almost fully) flexed to 40°, kept there for approximately 25 s and released (light arrow). When the leg had reached a constant position, it was (almost fully) extended to 170 °, kept there for 25 s and released (heavy arrow). (B) Consecutive return movements of two hind legs recorded after passive flexion to 40°. The movement velocities in animal a (female) were the fastest recorded. The animal was briefly activated by touching its abdomen between return movements 2 and 3. Movement velocities in animal b (male) were the slowest recorded. Between return movements 4 and 5, the animal was briefly activated by touching its abdomen. Other individuals yielded curves located between the sample recordings shown. The shaded area indicates the range of return movements reported for Carausius morosus (Bässler, 1972b). (C,D) Time constants () for the return movements (means ± S.D.) of seven middle (C) and six hind (D) legs of males, before and after cutting the femoral chordotonal organ receptor apodeme. Differences between the intact and operated situations are statistically significant for both hind and middle legs. The leg position was monitored with the attached-flag method in A and B and with the optical method in C and D.

View larger version (28K): [in a new window]   Fig. 4. Sinusoidal stimulation of the femoral chordotonal organ (fCO) (amplitude 100 µm); movement response. (A) Middle legs. (Ai) Relationship between stimulus frequency and response amplitude (tibia movement). Solid line, mean values (± S.D.) from seven male middle legs (n=41–162 per data point) (optical recording method). Dotted line, mean values from a strongly responding female (attached-flag method). In both cases, values from the first 10 cycles were used. The shaded lines are the curves for Carausius morosus (upper curve, data for the first stimulus cycle) (Kittmann, 1991) and Cuniculina impigra (lower curve, habituated situation) (Bässler and Foth, 1982). Horizontal broken lines indicate a gain of 1 (see text). (Aii) The phase shift between the stimulus and the response plotted versus stimulus frequency; data from the same animals as in Ai. Values are plotted separately for the relationship maximum joint flexion minus maximum fCO release (open circles) and maximum joint extension minus maximum fCO elongation (filled circles). The shaded line shows the corresponding curve for Carausius morosus, as above. (Bi,ii) Corresponding graphs for hind legs (seven males; one female).

View larger version (18K): [in a new window]   Fig. 5. Sinusoidal stimulation of the femoral chordotonal organ (fCO) (amplitude 350 µm); SETi activity (female middle legs). (A) Circular phase histogram (bin width 5°) of SETi spike activity (spikes s-1) recorded during three stimulus cycles at 0.0065 Hz. The mean vector (arrow) is at 280°. (B) Modulation (Mod., filled circles) and mean activity (open circles) of SETi with respect to stimulus frequency (means ± S.D., N=6; except at 5 Hz, where N=1). The shaded lines are the corresponding curves for Carausius morosus (Kittmann, 1997). (C) Phase shift between maximum fCO elongation and the mean vector (see A; means ± S.D., N=6; except at 5 Hz, where N=1). The shaded line shows the corresponding curve for Carausius morosus, as in B.

View larger version (19K): [in a new window]   Fig. 6. Ramp-and-hold stimulation of the femoral chordotonal organ (fCO); movement response (attached-flag method). (A) Original recording from a male middle leg. Top trace, femur–tibia angle (the dotted reference line indicates the steady-state position reached after apodeme extension); bottom trace, fCO stimulus (amplitude 100 µm). For a definition of the position-sensitive and velocity-sensitive portions of the response, see inset in C. (B) The amplitude of the velocity-sensitive response component (for a definition, see inset in C and text) plotted versus stimulus velocity; pooled data from eight animals (means ± S.D.; n=1–4 per animal). With the exception of two animals, only flexion movements were evaluated. The shaded line shows the corresponding curve for Carausius morosus (Bässler, 1993). The standard deviation was not calculated for the first (left-hand) two data points because the data were not normally distributed. (C) The half-life of the decrease in the velocity-sensitive portion of the response plotted versus stimulus velocity. Same data set as in B.

View larger version (38K): [in a new window]   Fig. 7. Ramp-and-hold stimulation of the femoral chordotonal organ (fCO); motoneuron discharge. (A) Sample recordings from the three relevant species: Prosarthria teretrirostris (top; stimulus amplitude 350 µm, corresponding to approximately 40°), Carausius morosus (middle, 200 µm, approximately 40°) and Schistocerca gregaria (bottom, 240 µm, approximately 40°). CI, common inhibitor neuron. (B) Position-sensitive portion of the motoneuron response, P (for a definition of P, see in the inset in C and the text) plotted versus femur–tibia angle. Prosarthria teretrirostris, circles (N=6); Schistocerca gregaria, triangles (N=5); means ± S.D. The data do not differ significantly between species. (C) Velocity-sensitive portion of the response, V (for a definition of V, see the inset and the text) plotted versus stimulus velocity. Prosarthria teretrirostris, circles (N=6, n=3 stimuli per animal); Carausius morosus, squares (N=5, n=3); Schistocerca gregaria, triangles (N=7, n=3); means ± S.D. A regression line (not shown) through data points for Schistocerca gregaria has a slope not significantly different from zero.