MEZOFF, S.; ISSBERNER, J.P.; TRIMMER, B.A.*: The control of Manduca proleg movements during crawling and grasping

Despite being slow and energetically inefficient, crawling by soft-bodied larval arthropods is evolutionarily successful because it allows them to exploit specialized environmental niches. In particular, the ?swinging, discrete, big-footed? gait characteristic of caterpillar crawling is stable, does not need widely spaced articulated legs and provides a small ratio of frontal area to body size for climbing in 3-dimensional structures such as branches. Central to this climbing ability are gripping cuticular hooks (crochets) on the non-articulated prolegs of the abdomen. We have previously shown that grip is released during crawling at the start of the proleg swing phase by activating the principle planta retractor muscle (PPRM). We now show that grasping (adduction) can be initiated by stimulating mechanosensory hairs (MHs) on the medial surface of the proleg. These hairs are necessary and sufficient for reflexive adduction of prolegs in a single segment but in the absence of normal hydrostatic pressure MH stimulation evokes proleg retraction instead. In reduced preparations MH stimulation weakly activates PPRM but strongly activates a single motor neuron supplying the ventral internal lateral muscle (VIL). This lateral muscle extends from the anterior ventral appodeme to the posterior margin of the segment. MH stimulation also activates VIL contralaterally and in adjacent segments. We propose that contraction of VIL creates pressure on hemolymph and tissues in the ventral cavity between the gut and the body wall to extend the proleg. Coincident with this activity, weak excitation of PPRM stiffens the medial plane of the proleg causing the proleg to move towards the midline and engage the crochets. Funded by NSF/IBN grant #0117135 to BT.