Lecture 10

Locust jumpJoints: coxal monocondylic joint: many axes of rotation

Pinnate muscle, apodemes: adaptation

Heitler W.J. 1974. The locust jump. Specializations of the metathoracic femoral-tibial joint. Journal of comparative Physiology 89: 93-104

• Announcements:• Snow cancellation adjustment: we will get back into phase by taking 2 wks

to complete lab 4 [jump]; those that missed this week will do lab 4 next week and for everybody else: there will be no labs Feb 8,10,11

• Content for term test: questions drawn from material presented in lecture and lab preambles

• Four labs completed by term test time (Feb 17): skeletons, tentoria, fluids, jump

• See Locomotion Augmented on course website for: squid locomotion, polychaete parapodia body waves, slug locomotion, flea jump, locust jump, levers [ignore non-click flight for the moment and white muscle fibres fishes]

• First class lever: insect flight: good answer

Tettigoniidae (katydid) from Papua New Guinea: particularly elongate femur and tibiafeature shared with grasshoppers

Saltatorial adaptation locust jump

Some insects are adapted to run: cursorial adaptation: e.g., tiger beetle,Cicindellidae [photo.net]

When grasshoppersevolved to use their metathoracic legs for leaping; they compromised their ability to run: an adaptive tradeoff.

Igor Siwanowicz

Anatomy Tibial levator, Tibial depressor

Levator muscle (extensor): raises an appendage

Depressor muscle (flexor: lowers an appendage

tibia levator takes up much of femur volume

depressor of tibia and its apodeme much smaller

pinnate: muscle fibres angled to insert on apodeme

Apodeme: adaptation

• Increases surface area available for muscle insertion• Improves mechanical advantage: farther from axis of rotation• Relocates muscle to regions remote from appendage: leg extremities tarsi,

lighter (roach); bulk where there is room for it, adductor muscle in head; rope apodeme

• Stabilizes: resists stresses and strains: tentorium acts as tie and as a strut

Pinnate ‘feather-like’ muscle arrangement: adaptation

•Parallel-fibred muscles, individual fibres longer so insertion point moved through greater distance; pinnate improves force at expense of distance an appendage is displaced•Muscle fibres dilate toward the middle when they contract; angling fibres minimizes problems of expansion within confines of exoskeleton

Rope apodeme and its elastic antagonistResilin? Muscles a,b,ccontract to pull rope tendon and depress claw (of pretarsus)first class leverResilin antagonist isthird class lever

• A grasshopper gets extra force into its leap by simultaneous contraction of the antagonists: tibial depressor and tibial levator. It prepares to jump by contracting both muscles simultaneously and isometrically (no movement at the joint). This distorts the exoskeleton in the neighbourhood of the joint and so stores elastic ‘deformation’ energy that can be released later during the jump and so contribute to the forces the leg exerts against the ground.

How is it that during this isometric contraction stage the smaller muscle is able to match the effort of the much larger muscle? Ans. Two things are involved: 1) mechanical advantage [‘pulley’ effect of lump changes force direction] which favours the flexor at maximum flexion and 2) a bifurcated apodeme ‘cap’ that catches behind the lump.

This involves a pulley-like arrangement that changes the direction of forces. A bifurcated ‘pocket’ or ‘cap’ is part of the tibial flexor apodeme; at maximal flexion of tibia on the femur the bifurcation is pulled over the lump and helps to hold the weaker flexor muscle during the isometric preparatory (to the jump) contraction.

The force advantage of the flexor muscle is different at different angles of flexion of

the femorotibial joint. Part of the answer is that when the angle of flexion is less than 5 degrees, the force advantage

of the flexor muscle is superior to that of the extensor.

***As mentioned in class: the lump inthis diagram should be coloured to matchthe purple of the femur not the blue of the tibia.

•1) The apodeme of the flexor makes an angle with the effort arm of the lever that is almost 90 degrees; this is because it rides up over the 'lump' located between the dicondylic joints. The lump functions as a pulley in that it changes the line of action of the force of the flexor muscle; it changes the direction in which the apodeme pulls on the tibia, making it nearly 90 degrees; by contrast the stronger extensor pulls at a very poor angle of 5 degrees.2) The effort arm (the distance between the point of insertion of the apodeme on the tibia and the axis of rotation) is quite short for the extensor; much longer for the flexor. So the moments of force can be balanced.

***As mentioned in class: the lump inthis diagram should be coloured to matchthe purple of the femur not the blue of the tibia: thelump is part of the ventral wall of the femur.