Nervous System of Insects
Body plan and nervous system of insects
insect body consists of a number of consecutive segments and is differentiated
into three parts: the cephalon (head), the thorax, and the abdomen . The cephalic
part is fused from the anteriormost six segments. The compound eyes belong to
the first segment, the antennae to the second segment. The third segment bears
the labrum as an appendage. The other three segments are associated with the
mandibles, the maxillae, and the labium. The next three segments form the
thoracic part. A pair of limbs is attached to each of the segments. The first
thoracic segment is the prothorax. In many insect species, a pair of anterior
and posterior wings is attached to the second thoracic segment (mesothorax) and
third segment (metathorax) respectively. The abdominal part consists of eleven
or twelve segments but in many cases they are fused into eight or nine segments.
insect nervous system comprises the central nervous system and the peripheral
nervous system. The central nervous system consists of a chain of ganglia. The peripheral nervous
system includes a stomatogastric ganglion and sensory and motor nerves. In
addition, insect nervous system includes pars intercerebralis-corpus
cardiacum-corpus allatum and neuroendcrine system that consists of store and
structure of the insect central nervous system resembles that of a rope ladder,
therefore, it is also called ladder-like
nervous system. The stiles of a ladder correspond to the connectives, running
aligned to the body axis (except in the head, where the neuraxis is often bent).
A ladder rung corresponds to a ganglion
with commissures. (The CNS can be considered to be an ectodermal rudiment)
The most anterior ganglion is the cerebral ganglion (also supraoesophageal
ganglion or brain), a structure that evolved from the fusion of the three
preoral neuromeres (anterior with respect to the oesophagus). The suboesophageal
ganglion consists of the fusion of three postoral neuromeres. The oesophagus
is surrounded by the circumoesophageal connectives that link supra- and
suboesophaegeal ganglia. More posterior to them are thoracic ganglia and
abdominal ganglia. Ganglia posterior to the supraoesophagial ganglion lie
ventral with respect to the digestive tract and are, with the exlusion
of the suboesophageal ganglion, also called ventral nerve cord. In comparison,
the vertebrate spinal cord lies dorsal with respect to the digestive tract.
This difference is derived from their skeleton, each of which is external
skeleton and internal skeleton. Vertebrate has a body plan that vertebral
column puts up visceral organ whereas invertebrate has a body plan that
ventral plate puts up nerve cord and visceral organ.
variety of fusion patterns of ganglia reflects the phylogeny of taxonomic groups at the level of the CNS. For
example, honey bee, fly, and moth have a fused form of brain and suboesophageal
ganglion with an oesophageal foramen, through which the oesophagus extends
towards the mountparts. The locust embryo has eleven pairs of ganglion
rudiments. During ontogeny, the first to third abdominal ganglia fuse with the metathoracic
ganglion and the eighth to eleventh ganglia fuse to and form a large terminal
abdominal ganglion. In Diptera, fusion of thoracic and abdominal ganglia occurs
resulting in a large segmented agglomerate ganglion
Silkmoth brain (male). The silkmoth brain consists of protocerebrum,
deutocerebrum, and tritocerebrum. The brain has many identifiable regions
consisting of densely packed neurites of the constituent neurons: the
neuropils. Information processing occurs mostly in the neuropil. .
Representative neuropils are the optic lobes (in principle, this would have to
be divided), mushroom bodies, the lateral protocerebrum, and the central body
in the protocerebrum, and the antennal lobe in the deutocerebrum. Ventrally,
the brain is fused with the suboesophgeal
ganglion. Scale bar: 0.5 mm.
Basic structure of ganglia
are externally delimited by an insulating cell layer, the neurolemma, forming a
chemical and mechanical barrier. Outer layer of the neurolemma is called neural
lamella (or sheath) that is composed of extracellular matrix produced by an inner
layer, the perineurial sheath cell layer consisting of glia cells. The lamella protects the neuronal tissue
mostly mechanically while the sheath cell layer acts as a blood-brain barrier, being relatively impermeant
as demonstrated by lanthanum ion exposure.
tissue within a ganglion is grouped into two layers, an outer soma layer and an
inner neuropil core. Insect central neurons
are usually monopolar cells. A single neurite extends from the soma and branches
into several neurites, forming synapses with
adjacent neurites of other neurons. Generally, the majority of synapses are
localised in the neuropil.
neuropil is classified into glomerular neuropil, non-glomerular neuropil, and neuronal
tracts. Glomerular neuropil is a region where finely branched neurites form
dense tangles and numerous synapses. The border of dense neuropil is clearly
discernable and often delineated by a glia
cell layer. non-glomerular neuropil has a low synapse density and poorly defined
borders. Its internal structure can not be revealed with general staining methods. A
neuronal tract is a bundle of axons that connects neuropil regions. Tracts
formed by a large number of axons, in particular large diameter ones, are
easily delineated, but this may not be so when the number of axons is small.
neuropils are further grouped into small modular structures of different shapes
such as spherical, columnnar, layered. The word, module reminds a column in
cerebral cortex of vertebrate. Although the vertebrate column can be
distinguished functionally they are not compartmented morphologically (not true
because there are the LAYERS! And for the horizontal coordinates, there is
barrel cortex and the like, and binocular dominance columns can also be
morphologically demonstrated!). On the other hand the insect module
is functionally and morphologically compartmented so that it can be used in the
in the insect nervous system are local microcircuits with similar properties supposedly arranged to process information in a parallel and
distributed manner. For example a single lamina cartridge in the peripheral
optic lobe of a fly processes information
received by 6 photoreceptors looking at the same point in space whereas a glomerulus in the antennal lobe generally
processes information from a population of olfactory receptor cells that express the
same odorant receptor protein. Neuropil
regions specifically concerned with a sensory modality (i.e. lamina of the
optic lobe or antennal lobe) are built from such modules that are concerned
with more fine-grained aspects within this modality (spatial distribution of
light for the lamina, specific odorants for the antennal lobe) In other word
one densed neuropile takes charge of one modality and each modules in the
neuropile takes charge of a certain quality of modality
Brain structure and function
brains consist of 105 to 106 neurons and brain neurons outnumber those composing the ventral nerve
cord . A house fly brain includes about 350,000 neurons, that of a honeybee
(worker) about 850,000. On the other hand each thoracic ganglion in any
insect consists of 3000 to 5000 neurons and each abdominal ganglion of
only 400 to 850 neurons .
The insect brain can be divided into three large regions, protocerebrum
deutocerebrum, and tritocerebrum. As mentioned above each of them is associated
with one of the three anteriormost ancestral body segments . [the use of
“frontal” is always a bit dubious, but especially in insects.
The protocerebrum bears to lateral protrusions, the optic lobes. . Depending
on the insect species, the optic lobe can be further subdivided into three
or four neuropils. Medially,inbetween the optic lobes are located another
conspicuous pair of bilaterally symmetrical neuropils, the mushroom bodies.
. The median protocerebrum also contains unpaired neuropils, in particular
the protocerebral bridge and the central body, which together form the
central complex. The shape, size, and disposition of these dense neuropils
differs somewhat between insect groups but the basic morphology is essentially
the same. The optic lobes process visual information, the mushroom bodies
are important for learning and memory related in particular to olfactory
stimuli, and the central complex is thought to play a crucial role in generating
behavioral output. The dense, clearly delineated neuropils are surrounded
by neuropil areas without conspicuous structural characteristics.
The deutocerebrum is a bilaterally symmetrical brain region that consists
of the antennal lobe and the dorsal lobe. The antennal lobe receives the
axons of olfactory receptor neurons from the antenna . In at least some
insects such as flies and moths, the antennal lobe also receives olfactory
receptor axons from the labium. The dorsal lobe receives mechanosensory
and gustatory receptor neurons from the antenna and also contains the motor
neurons controlling the antennal muscles. The dorsal lobe is called antennal
mechanosensory and motor center (AMMC) in some insects. For flies, this
name may be retained because they differ from almost all other insects
in not having an antennal gustatory system. The anntennal nerve is carrying
all sensory axons from the antenna to the deutocerebrum and also contains
efferent neurons, in particular the motor neurons of the pedicellus muscles.
Generally, a second nerve emanates from beneath the antennal lobe that
contains the axons of the motor neurons of the scapus muscles, which are
located in the head capsule.
Tritocerebrum is the smallest region and little studied. In the primitive
insect ground plan, the tritocerebrum is connected to the suboesophageal
ganglian by a pair of circumoesophageal connectives (for example in locusts,
crickets, cockroaches), in more derived groups, the tritocerebrum, the
posterior part of the deutocerebrum, and the suboesophageal ganglion are
fused such that it can be difficult to determine the borders between them.
The tritocerebrum contains at least two commissures, one of which is running
above the oesophagus in the primitive ground plan. The tritocerebrum receives
sensory inputs from the labrum through the labral nerve, from the tegument
through the tegumentary nerve, and at least in some insects also direct
sensory input from the mouthparts (mandible, maxilla, and labium). The
frontal nerves of the tritocerebra on each side run to the frontal ganglion
that connects the brain and the stomatogastic system, lying dorsal with
respect to the oesophagus and innervating cardiac and gastrointestinal
muscles. The frontal ganglion is connected to the hypocerebral ganglion
thourgh the recurrent nerve.In some insects (honeybee), the frontal and
labral nerves are fused at the root to form a labro-frontal nerve. Tritocerebrum
is connected to suboesophageal ganglion through a pair of connective. Frontal
ganglion is center of stomatogastric ganglion innervating cardiac and gastrointestinal
muscle and is connected to suboesophageal ganglion via recurrent nerve.
The suboesophageal ganglion is largely concerned with the processing of
sensory information from and the motor control of the mouth parts. The
mouth parts consist of three appendages, the mandible, the maxilla, and
the labium. These appendages can be modified in various ways in different
insect groups. For instance, many insects have paired maxillary and labial
palps. In the bee, a part of the labium forms the unpaired glossae Axons
of the receptor cells of the sensilla distributed on the mouth-parts project
to the suboesophageal ganglion and in at least some insects also to higher
areas. The suboesophageal ganglion also receives direct sensory input from
the labrum and through non-olfactory sensory cells of the antenna. Generally,
there are separate mandibular, maxillary, and labial nerves innervating
the appendages (for example in the bee), but they can also be fused (such
as in Dipera, in which there is a fused maxillary-labial nerve) At least
in the bee, a little-known fourth nerve emanates from the labial neuromere:
the labial gland nerve.
The thoracic ganglia possess motor centers that control important behaviors,
such as flight, walking, and vocalization.
The abdominal ganglia possess motor centers related to posture control,
rhythmic behavior, such as respiration, and circulation and the control
of copulation and egg laying.
motor centers residing in thoracic or abdominal ganglia autonomously express
and correct motor patterns in response to their own local sensory inputs
(trigger inputs) largely without depending on inputs from the brain. However, the initiation and
maintenance of their motor patterns are generally controlled by descending
interneurons from the brain.
higher order neuropils in protocerebrum, such as mushroom body, central
complex, and lateral protocerebrum, receive sensory information from optic
lobe, deutocerebrum, tritocerebrum, suboesophageal ganglion, and ventral nerve
cord through several layers of filters in a preprocessed form. The higher neuropils integrate information from various modalities
and relay outputs that can eventually
results in activating output neurons such as motor neurons of the brain or
descending interneurons transmitting command information to the ventral nerve cord,
which is important in controlling a wide range of behaviors. Thus, overall
control mechanisms such as for triggering particular behaviors are localized in
the brain and are separated from local control mechanisms that are responsible for aspects
like the maintenance of motor patters for wing flapping or walking are
localized in the thoracic ganglia. This
is not to say that the ventral nerve cord is a fully hardwired system: In
seminal experiments, Horridge could demonstate that cockroaches and locusts are
capable of leg position learning, which is even improved by removal of the
head. Thus, plasticity is possible at short time scales even in thoracic
circuits controlling very basic behaviors.
Neuronal composition of the insect brain
brain consists of neurons and glia cells. Both neuron and glia cell are
differentiated from the same stem cells. Glia cells have traditionally been
mostly seen as structural components, but there is no doubt that they also have
important functional roles, which are still poorly understood in the insect
(nerve cells) play the major role in information processing in the brain. Ethymologically,
“neuron” is Greek for “string”, “tendon”, “nerve”, i. e. a term for elongated objects.
the mammalian brain, glia cells are ten times more numerous than neurons
whereas in the insect brain, the ratio is approximatedly reversed (Reference 1).
basic structure and function of neurons is conserved across animal species.
Simple nervous systems first emerged in Cnidaria, such as sea anemones and
jellyfish, and evolved from loose nerve nets to compact centers, ganglia, which
by further fusion and increase of complexity led to strctures one calls brains.
From the evolutionary point of view
insects represent a branch different from that to which humans belong. Nevertheless,
their neurons have many functional properties in common. Therefore, the differences
in the performance and capabilities between insect and human brain mainly arise
from a difference in size and complexity. Insect brain contains 105 to 106 neurons whereas
human brain harbours some 1011 neurons.
neurons may be categorized into five types:
Sensory neuron: innervating sensory organs mostly in the cuticle, project from
the periphery into the central nervous system.
neuron: connects two or more separate regions in the central nervous system.
interneuron: confined to a certain region in the central nervous system.
neuron: projects from central nervous system to muscle targets.
cell: innervates an endocrine organ.
these neuron types, only sensory neurons
have somata localised outside the rind cortex (outer soma layer) of the central
nervous system. This placement of somata is a prominent feature of the insect
brain (and other arthopod brains). In the mammalian cortex for example, the
gray matter contains somata, neurites, and synaptic contacts. In insects, the
neuropil contains neurites and synapses and the somata are almost completely
located in the rind, where synaptic contacts are exceptional.
mammalian neurites are myelinated, i.e. ensheathed by glia cells whereas many
insect neurites are generally not myelinated. Insect glia cells often envelop
bundles of neurites (such as tracts) or neuropil compartments, effectively
neurons in insect brains are uniquely
characterized by their morphology and function and are thus called identified neurons (Fig. 3). The
fact that the insect brain possesses a comparatively small number of neurons
most of which are identifiable allows us to study the insect brain at the
cellular level. An extreme in terms of identified neurons is the nematode
Caenorhabditis elegans, in which all 302 neurons have already been
identified and even their lineage has been elucidated.
are neurites that represent the predominant input regions of a neuron,
receiving information from other neurons, mostly through synaptic contacts. In mammals, dendrites directly originate from
the soma whereas in insect central neurons, only a primary neurite is sent off
into the neuropil by the soma. The site of synapse can be inferred from the
morphology of dendrite of a neuron. In many case dendritic spine (Insect neuron
possesses spine?) is an input site whereas dendritic bouton and varicose are
output sites. In addition dendrite near soma often resides input synapses
whereas dendrite far from soma does output synapses. I find it a little
Figure 9. The structure of silkmoth brain (reduced silver stained horizontal
section). AL, antennal lobe; Ca. calyx of the mushroom body; CB, central
body; LAL, lateral accessory lobe; LALC, lateral accessory lobe commissure,
LP, lateral protocerebrum; P, peduncle of the mushroom body. The LAL is
an important premotor center generating command information.
Figure 3. Identified neurons from
the silkmoth brain. (A) Group-1 descending neuron (DN). (B)Group-2 DN. Group-1,
and -2 DNs are both identified neurons that transmit a premotor command, the flip-flop
signal, to the thoracic ganglia. The schematic illustration indicates the location of the somata of Group-1 and-2
Morphology of neurons in the insect brain
Figure 8 shows examples of neurons in the insect brain. The spherical structure
containing the nucleus is called soma (cell body). The size of somata is
about 10 μm in diameter in many neurons (but varies considerably depending
on the cell type within a species and also accross species). From the soma
protrusions extend: the neurites. Neurons with a single neurite are called
monopolar cells (A). Those with two neurites are bipolar cells (B). Those
with more than two neurites are multipolar cells. Most insect neurons are
monopolar cells. Their primary neurite runs into the neuropil and forms
complex branching patterns. In general, the longest neurite within a neuron
is the axon and other neurites are dendrites. The axon is representing
the output channel of a neuron. On the axon, the output is relayed to other
neurons through synapses, the axon being the output element. On the other
side of a synapse, a dendrite, the input region of another neuron, receives
this output. The patterns of connectivity of synapses between neurons results
in a complex network for information processing.
Insect Mimetics (2009)
Shimozawa and Hariyama eds. NTS Japan
With contributions from S.S.
Haupt and A. Takashima
updating of the site