Fast and Powerful: Biomechanics and Bite Forces of the Mandibles in the American Cockroach Periplaneta americana

Knowing the functionality and capabilities of masticatory apparatuses is essential for the ecological classification of jawed organisms. Nevertheless insects, especially with their outstanding high species number providing an overwhelming morphological diversity, are notoriously underexplored with respect to maximum bite forces and their dependency on the mandible opening angles. Aiming for a general understanding of insect biting, we examined the generalist feeding cockroach Periplaneta americana, characterized by its primitive chewing mouth parts. We measured active isometric bite forces and passive forces caused by joint resistance over the entire mandibular range with a custom-built 2D force transducer. The opening angle of the mandibles was quantified by using a video system. With respect to the effective mechanical advantage of the mandibles and the cross-section areas, we calculated the forces exerted by the mandible closer muscles and the corresponding muscle stress values. Comparisons with the scarce data available revealed close similarities of the cockroaches’ mandible closer stress values (58 N/cm2) to that of smaller specialist carnivorous ground beetles, but strikingly higher values than in larger stag beetles. In contrast to available datasets our results imply the activity of faster and slower muscle fibres, with the latter becoming active only when the animals chew on tough material which requires repetitive, hard biting. Under such circumstances the coactivity of fast and slow fibres provides a force boost which is not available during short-term activities, since long latencies prevent a specific effective employment of the slow fibres in this case.明确咀嚼器官（masticatory apparatuses）的功能与性能，对有颌类生物（jawed organisms）的生态分类至关重要。然而，昆虫——尤其是物种数量极其庞大、形态多样性极高的类群——在最大咬合力及其与下颌张开角度的相关性方面，长期以来研究严重不足。为系统阐释昆虫的咬合机制，我们以具备原始咀嚼式口器的广食性蜚蠊——美洲大蠊（Periplaneta americana）——为研究对象开展实验。我们采用定制二维力传感器（custom-built 2D force transducer），在全下颌活动范围内测量了等长主动咬合力（isometric bite forces）以及由关节阻力产生的被动咬合力；通过视频系统量化下颌张开角度。结合下颌有效机械优势（effective mechanical advantage）与肌纤维横截面积，我们计算了下颌收肌（mandible closer muscles）产生的作用力及对应的肌肉应力数值。与现有稀缺研究数据对比后发现，美洲大蠊的下颌收肌应力（58 N/cm²）与体型更小的特化掠食性步甲（ground beetles）较为接近，但显著高于体型更大的锹甲（stag beetles）。与现有公开数据集不同，本研究结果表明昆虫的咬合活动同时涉及快肌纤维与慢肌纤维：慢肌纤维仅在昆虫取食韧性材料时被激活，此类场景需要反复进行高强度咬合。在此类工况下，快肌与慢肌纤维的共激活（coactivity）可产生力增益效应；但在短期咬合活动中无法实现该效果，因为慢肌纤维较长的潜伏期（latencies）使其无法在这类场景下有效发挥作用。