Locomotion

 

Insects are masters of movement:  roaches run, bees swarm, moths fly, mantids strike, diving beetles swim, caterpillars crawl, dragonflies dart, maggots squirm, water boatmen paddle, mole crickets burrow, mosquito larvae wriggle, fleas jump, whirligigs spin, collembola spring, water striders skate, army ants march, and backswimmers dive.  Indeed, the capacity for independent, goal-directed movement is one of the distinguishing characteristics that sets animals apart from most other forms of life on this planet.

Walking and Running

An exoskeleton can be awkward baggage, bulky and cumbersome for a small animal.  To compensate, most insects have three pairs of legs positioned laterally in a wide stance.  The body’s center of mass is low and well within the perimeter of support for optimal stability.  Each leg serves both as a strut to support the body’s weight and as a lever to facilitate movement.

At very slow walking speeds an insect moves only one leg at a time, keeping the other five in contact with the ground.  At intermediate speeds, two legs may be lifted simultaneously, but to maintain balance, at least one leg of each body segment always remains stationary.  This results in a wave-like pattern of leg movements known as the metachronal gait.  When running, an insect moves three legs simultaneously.  This is the tripod gait, so called because the insect always has three legs in contact with the ground:  front and hind legs on one side of the body and middle leg on the opposite side.  Clearly, it is no coincidence that insects have exactly six legs — the minimum needed for alternating tripods of support.

Coordination of leg movements is regulated by networks of neurons that can produce rhythmic output without needing any external timing signals.  Such networks are called central pattern generators (CPGs).  There is at least one CPG per leg.  Individual networks are linked together via interneurons and output from each CPG is modified as needed by sensory feedback from the legs.

Only animals with a rigid body frame can use the tripod gait for movement.  Soft-bodied insects, like caterpillars, have a hydrostatic skeleton.  They move with peristaltic contractions of the body, pulling the hind prolegs forward to grab the substrate, and then pushing the front of the body forward segment by segment.  This type of movement is exaggerated in larvae of Geometrid moths.  While grasping the substrate with their six thoracic legs, they hunch the abdomen up toward the thorax, grasp the substrate with their prolegs, and then extend the anterior end as far as possible.  This distinctive pattern of locomotion has earned them nicknames like “inchworms”, “spanworms”, and “measuringworms”.

Flight

In all flying insects, the base of each wing is embedded in an elastic membrane that surrounds two (or three) axillary sclerites.  One of these sclerites articulates with the pleural wing process, a finger-like sclerite that acts as a fulcrum or pivot point for the wing; a second sclerite articulates with the lateral margin of the mesonotum (or metanotum).  Together, these elements form a complex hinge joint that gives the wing freedom to move up and down through an arc of more than 120 degrees.  The hinge is a “bi-stable oscillator” — in other words, it stops moving only when the wing is completely up or completely down.  During flight, the wing literally “snaps” from one position to the other.

Power for the wing’s upstroke is generated by contraction of dorsal-ventral muscles (also called tergosternal muscles).  These are called “indirect flight muscles” because they have no direct contact with the wings.  They stretch from the notum to the sternum.  When they contract, they pull the notum downward relative to the fulcrum point and force the wing tips up.  Elasticity of the thoracic sclerites and hinge mechanism allows as much as 85% of the energy involved in the upstroke to be stored as potential energy and released during the downstroke.

In the more primitive insect orders (e.g. Odonata and Blattodea), the downstroke is initiated by basalar muscles that attach through ligaments directly to the wing’s axillary sclerites.  Contraction of these “direct flight muscles” literally pulls the wings into their “down” position.  Most other insects have dorsal-longitudinal muscles attached like bow strings to apodemes at the front and back of each thoracic segment.  These are “indirect flight muscles”.  When they contract, they cause the edges of the notum to flex upward (relative to the fulcrum point) causing the wings to snap down.

During flight, upstroke and downstroke muscles must contract in alternating sequence.  There are two different mechanisms for controlling this muscle action, synchronous (neurogenic) and asynchronous (myogenic):

Insects with synchronous control have neurogenic flight muscles, meaning that each contraction is triggered by a separate nerve impulse.  Central pattern generators in the thoracic ganglia coordinate the rate and timing of these contractions.  Since nerve cells have a refractory period that limits how often they can fire, insects with neurogenic flight muscles have relatively slow wing beat frequencies (typically 10-50 beats per second).

Insects with asynchronous control depend almost entirely on indirect flight muscles for upstroke (dorsal-ventrals) and downstroke (dorsal-longitudinals). These muscles have developed myogenic properties, that is, they contract spontaneously if stretched beyond a certain threshhold. When the nervous system sends a “start” signal, the dorsal-longitudinal and dorsal-ventral muscles begin contracting autonomously, each in response to stretching by the other. Contractions continue until the muscles receive a “stop” signal from the nervous system. Asynchronous control is not limited by the nerves’ refractory period, so wing beat frequency in some of these insects (notably flies and bees) may be as high as 500-1000 beats per second. Such high frequencies produce greater lift with smaller surface area and also improve maneuverability (e.g. hovering, flying backwards, and landing upside down on the ceiling!)

As an insect’s wing moves up and down during flight, it also twists about the vertical axis so that its tip follows an ellipse or a figure eight. This sculling motion maximizes lift on the downstroke and minimizes drag on the upstroke. Turning, hovering, and other acrobatic maneuvers are controlled by small muscles attached to the axillary sclerites. These muscles adjust the tilt and twist of the wing in response to feedback from the central nervous system and sensory receptors that monitor lift and thrust.

Swimming and Skating

Many aquatic beetles (Coleoptera) and bugs (Hemiptera) use their middle and/or hind legs as oars for swimming or diving.  These legs are usually flattened or equipped with a fringe of long, stiff hairs to improve their performance and efficiency in the water.  Legless larvae and pupae of mosquitoes, midges, and other flies (Diptera) manage to swim by twisting, contorting, or undulating their bodies.  Dragonfly naiads (Odonata) have a jet propulsion system:  they can propel themselves forward by contracting abdominal muscles and forcing a jet of water out of the rectal chamber that houses their respiratory gills.

A few aquatic insects, such as water striders, have a whorl of hydrophobic hairs on the tips of their feet.  These hairs prevent the insect’s legs from breaking the surface tension of the water and allow them to skate on the surface.