Tension receptors on the apodemes of muscles in the walking legs of the crab,<i>Cancer magister</i>†

1. Mechanoreceptors monitoring tension in working muscles are described in the Decapoda Crustacea. 2. The receptors are associated with apodemes of muscles in the walking leg and are well‐developed in the extensor and flexor of the meropodite (Figures 1, 2). 3. The unbranched dendrites of the receptor neurones innervate the tissues surrounding the insertions of the muscle fibres (Figures 3, 4, 5(A)). 4. The receptors show spontaneous activity with the M‐C joint at resting position and this activity increases when the muscle is stretched by holding the joint at a different position (Figure 7). 5. Isometric tension increase in the muscle recruits sensory units (Figures 8, 10(A)) and increases the activity of units firing (Figure 9). 6. Apodeme receptors may be an entirely distinct input channel from chordotonal organs (Figure 10(B,C)). Joint movements produced by a standard muscle stimulus against increasing loads reveal very different responses (Figure 11). 7. Attempts to determine whether chordotonal organs (CP1, Figures 5(B), 6) monitor isometric muscle tension (Figure 12) suggest possible complexities in their dynamic responses. 8. Abbreviations used in this paper are FASN flexor apodeme sensory nerve, EASN extensor apodeme sensory nerve, BASN bender apodeme sensory nerve, and OASN opener apodeme sensory nerve.