Furthermore, insects have six legs (and/or one or two
pairs of wings). How on earth are ants, for example, able
to coordinate such rapid movement between six different
appendages while climbing trees, scooting through grass,
and tip-toeing along a twig?
The answer lies inside a bug’s head — its brain. Or,
more specifically, brains! Original neural/motor research in
entomology was focused on the observable: fixed, repetitive
patterns of leg movements and coordination in insects. Fast
forward 80 years and research showed that walking
movements were not fixed but highly adaptable.
Add another 30 years of research: Individual legs can
function as independent units with walking patterns
developing via coordinating neural pathways within the
nervous system, sensory inputs, and mechanical attachment
through the ground.
Each of these research bits can be collectively
combined into a much better picture of insect movement.
Therefore, taken as a whole, the explanation for the
locomotion in bugs ranges from readily observable
biomechanical factors through central and peripheral
neurobiological controls, culminating in muscular force
development. Whew! That’s a lot of stuff going on inside
that ant’s head!
Watch the movement of some hexapod robots and you
will notice that, typically, the leg movement pattern begins
with the hind legs; moves onto the middle legs; and, then,
concludes with the front legs on either side. This is called a
simple walking gait. Regardless of the insect leg shape or
function, they all operate pretty much the same. There are
two excitatory motor neurons: one fast and one slow. As
you might guess, the operation of these neurons follows
along with their name. During a slow gait, for example, the
slow neuron fires in bursts which contribute to muscle
electrical potentials resulting in joint extension. Then, at
faster speeds, the fast motor neurons kick in and generate
some electrical spikes, which suddenly produce rapid
extension of the joint, greatly increasing the joint velocity.
How are these neurons controlled? A local control
system called the thoracic ganglion controls the
basic insect leg/joint gait and stance patterns. In
turn, the ganglion is able to adjust muscle timing
and force on-the-fly (no pun intended).
An overview of the ganglion at work would
consist of monitoring each joint and ensuring that
they alternate between extension and flexion. This
basic timing function is often performed by central
pattern generation (CPG) circuits.
Insects have evolved local control circuits that
work between CPGs for each leg joint and sensory
reflexes. Sensory reflexes are used to adjust the
strength of motor activity, thereby creating
effective “burst” movement for environmental
hazards (e.g., climbing over a branch). On a more
general electrical level, currents through the thoracic
ganglia make a leg on the “negative” side flex and a leg on
the “positive” side extend.
The discovery of neural/muscular/electrical leg/joint
movement potential is a rather common laboratory exercise
that involves a decapitated cockroach dissected from the
dorsal surface with the alimentary canal removed. The
insect is suspended by hooks from its anterior and posterior
ends, so that the legs are completely free. Recordings are
made of the nerve and muscle action potentials. The
tracheae are left intact during this dissection — to improve
the condition of the ganglion.
HA! Take “that” you dirty cockroach ... oh, and, “thank
you” for helping us understand insect leg movement.
In a Bug’s Brain
The basic fundamental unit of the insect’s central
nervous system is the neuromere. This neuromere is a
collection of neurons that are responsible for receiving
sensory input and thereby controlling the movement of that
segment. In the case of a thoracic segment, this movement
would be a leg movement. Remember our poor dissected
cockroach? These segment neuromeres can be combined
and fused into a clump or ganglia. These ganglia, in turn,
are formed into the pre-oral, dorsal brain, the
subesophageal ganglion, and the thoracic and abdominal
ganglia of the ventral nerve cord as shown in Figure 2.
The thoracic ganglia are contained in body segments
which house the bug’s legs. Inside the head and dorsal
brain, there are three fused ganglia:
• Protocerebrum — the first ganglia; associated with
processing visual sensory information from the optical
• Deutocerebrum — the second ganglia; associated
with processing tactile sensory information from the
• Tritocerebrum — the third ganglia; associated with
integrating the information from the first two ganglia, as
40 SERVO 11.2015
Figure 2. The location of a
grasshopper's ventral nerve cord.