School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK.
Curr Biol. 2022 Oct 24;32(20):4530-4537.e2. doi: 10.1016/j.cub.2022.08.052. Epub 2022 Sep 9.
The movements of animal appendages are determined by extrinsic and intrinsic forces. Extrinsic forces include gravity or friction, whereas intrinsic forces are generated by active muscle contraction or passive musculoskeletal elements. For lightweight appendages, such as insect limbs, movements depend more upon intrinsic than extrinsic forces. Indeed, passive movements of insect limbs can be large and oppose or aid joint flexion, extension, or both. Yet, how passive properties contribute to insects' goal-directed limb movements, such as targeted reaching and searching, remains unclear. Here, we show that mantids make targeted reaches and searches to objects by using their raptorial forelimbs, employing braking to slow passive flexion of the femoro-tibial (FTi) joint. In most reaches, tibial flexion ensures the forelimb contacts the object. Such tibial flexion is particularly clear when the forelimb misses the object and continues on a downward trajectory or during directed searching movements. We characterize the passive properties of the FTi joint by combining passive movements of excised limbs with apodeme ablations and muscle stimulation. These experiments show that passive properties of the flexor tibiae muscle-apodeme complex are the primary structural element producing tibial flexion in excised limbs. During reaching and searching, however, tibial flexion is slower and smaller than predicted. This is due to braking, which opposes passive flexion, thereby reducing the magnitude and velocity of tibial flexion. Braking retarding passive movements is a novel behaviorally relevant control strategy for the goal-directed movements of lightweight limbs, such as those of insects.
动物附肢的运动由外在和内在力量决定。外在力量包括重力或摩擦力，而内在力量则由主动肌肉收缩或被动肌肉骨骼元素产生。对于轻量级的附肢，如昆虫的肢体，运动更多地取决于内在力量而非外在力量。事实上，昆虫肢体的被动运动可以很大，并对抗或辅助关节的弯曲、伸展或两者兼有。然而，被动特性如何有助于昆虫有目标的肢体运动，如目标性的抓取和搜索，仍然不清楚。在这里，我们展示了螳螂利用其捕食性的前肢进行有目标的抓取和搜索，通过制动来减缓股骨-胫骨（FTi）关节的被动弯曲。在大多数抓取中，胫骨的弯曲确保了前肢接触到物体。当前肢错过物体并继续向下运动或进行定向搜索运动时，这种胫骨弯曲尤其明显。我们通过结合切除肢体的被动运动和附肢切除和肌肉刺激来描述 FTi 关节的被动特性。这些实验表明，弯曲胫骨肌-附肢复合体的被动特性是在切除肢体中产生胫骨弯曲的主要结构元素。然而，在抓取和搜索过程中，胫骨弯曲的速度和幅度都比预测的要慢和小。这是由于制动，它对抗被动弯曲，从而减少了胫骨弯曲的幅度和速度。制动减缓被动运动是一种针对轻量级肢体（如昆虫）的有目标运动的新型行为相关控制策略。
