Fig. 1. Preparation for measurement of muscle length changes during tethered flight and length–tension measurements in the intact thorax. (A) Lateral view of Manduca showing placement of length transducer probes (p) and optical sensors (os) used to measure length changes of the dl₁ muscles during tethered flight. (B) The five sub-units of each dl₁ muscle (a–e) attach to two invaginations of the exoskeleton, the 1st phragma anteriorly and the 2nd phragma posteriorly. Acting indirectly through a complex wing articulation, contraction of the dl₁ muscles depresses the wings. The probes were inserted through incisions in membranous areas of the abdomen and neck and hooked onto the 1st and 2nd phragmata. The moth was held on a brass rod (br) glued between the bases of the mesothoracic legs. We define the anatomical rest length, L_r, as the length of dl_1a along its ventral surface in the intact thorax of a quiescent moth. In practice, we measured L_r as the distance separating the hooks of the displacement transducer probes with the moth at rest. For isometric twitch length–tension measurements in the intact thorax, the probe hooks were bilaterally paired rather than single, but the method of insertion and placement was the same. The anterior probe was rigidly fixed in place, and the posterior probe was connected to an isometric force transducer. s, silhouette of body scales.