The following writeup is based on two cockroach climbing papers:

1. Tarsal Structure and Climbing Ability of Cockroaches (L. Roth and E. Willis, J Exp Zoo 119: 438-517)
2. Adaptive Features on the Tarsi of Cockroaches (J.W. Arnold, Int J Insect Morphology and Embryol 3(34):317-334,1974)

The distal tarsus (claws and arolium) engages the substrate during the stance phase of walking by action of a single muscle, the retractor unguis. Tarsal disengagement occurs at the end of stance via the action of elastic elements in the penultimate tarsal joint. This joint exhibits very rapid recoil independent of the retractor unguis.

• Specific elastic structures at joints are used as partial or complete antagonists to the active contractions of skeletal muscles

• Elastic vs. muscle antagonist – minimize mass, conserve metabolic energy (springs), generate very rapid movements, computationally efficient, passive forces independent of nervous system

• Tarsus contains no muscles, functions merely by the retractor unguis; returns to default position because of elasticity of cuticular condyle at pretarsal joint (elastic protein resilin)

• Elasticity functions to 1) help disengage the contact of the claws with the substrate when the retractor muscle relaxes, 2) substantially lift the fifth segment (pretarsus) as a whole away from the walking surface. This biomechanical design can facilitate the rapid and efficient use of the tarsus in walking while under active control by a single muscle.

Conclusions

• Walking over wire mesh surface – joint extended i.e. disengages claws before swing phase; does this happen when walking over ‘non-dangerous’ surfaces?

• Elasticity is alone sufficient to lift the pretarsus from the substrate . . . it is not a catch mechanism?

• During swing phase, distal tarsal segment and pretarsus are held above the substrate by joint ligaments. At the start of stance, the tarsus as a whole is first depressed, presumably through the action of the tarsal depressor muscles and by extension of the leg as a whole.

• In walking over rough terrain, the claws and arolium are then actively engaged through the action of the retractor unguis muscle. At the end of stance, tension in the retractor declines and the grip of the claws and arolium relaxes. The distal tarsus is then rapidly elevated by the action of the stretched elastic ligaments.

• In walking over smooth terrain, in which the friction and adhesion of the pulvilli is sufficient and the pre-tarsus is not engaged, the joint elasticity can hold the distal tarsus elevated throughout the stance phase

Functions of the default position in posture and locomotion: elasticity confers flexibility of use – allows claws and arolium to be selectively engaged in walking over diverse terrains: (Roth and Willis)

1. Claws are essential for vertical climbs and walking on inverted surfaces (Larsen)
2. Claws / arolium regularly engaged only in traversing rough terrain or climbing
3. Claws only effective on surfaces that can be grasped or penetrated / impediment on hard smooth surfaces but displaced laterally by flexibility of tarso-pretarsal joint (2 degrees of freedom) exposing arolium (?)
4. Arolia provide extra adhesion to smooth surfaces (Snodgrass) by surface tension of cuticular secretions, gripping action of minute setae on ventral arolium . . . can hold twenty times its own weight! Peel body away from substrate by tilting arolium with claws.

Common mechanisms in claws of cockroaches and cats: Advantages of elastic ligaments as antagonists in distal joints of appendages – substantial decrease in inertial eliminate need of separate antagonist muscles . . . rapid and light and simple to control!