Lecture 22 announcements:
Course evaluation reminder, before Dec 7
Under Lecture Syllabus find Topics per lecture and keywords
Under Essays source papers find Membracid helmet function
Under Essays special topics find Tentorium
Morphological diversity in treehopper pronota/helmets
B Prud’homme et al. Nature 473, 83-86 (2011) doi:10.1038/nature09977
Sculpture by Alfred Keller
Berlin Nature Museum
Is this
structure
homologous
with the ‘lost’
prothoracic
wings of
Insecta?
Prud’home, B. et al. 2011. Body plan innovation in treehoppers through
the evolution of an extra wing-like appendage. Nature 473: 83-86
• From Wikki: membracids are “… known for their enlarged and
ornate pronotum, which most often resembles thorns, apparently to
aid camouflage… The specialised pronotum (or helmet) may not be
simply an expansion of the prothoracic sclerite, but a fused pair of
dorsal appendages of the first thoracic segment.
• These [fused appendages] may be serial homologues of insect
wings, which are dorsal appendages of the second and/or third
thoracic segments. Evidence for this theory includes the
development of the helmet, which arises as a pair of appendages
attached to each side of the dorsal prothorax by an articulation with
muscles and a flexible membrane that allow it to be mobile. Also, the
same [Hox] genes are involved in development of the helmet and
the wings.” Read my essay online, written before I read a paper by
Yoshizawa (2012) quarrelling with this hypothesis.
•
•
•
[Serial homology: structures are considered serial homologues if they represent
corresponding parts of organisms built according to the same body plan. For
example, the notopodia of Nereis are homologues.
The body plan of the insect thorax is three segments in series, each segment with
a pair of legs ventrally and a pair of wings dorsally. Treehoppers, alone among
modern insects, have evolved to ‘turn back on’ genes that made the beginnings of
wings, genes shut down in all other modern insects.
Information used long long ago in development of the prothoracic metamere of
insects and normally in modern insects turned off by Hox genes, is somehow
allowed to proceed. In membracids it begins to form a pair of articulated
structures [wing primordia] but these do not differentiate to wings, they just
begin. Other selection later acted on genes subsequently affecting these
primordia to give features of the helmet.
Yoshizawa, K. 2012. The treehopper’s helmet is not homologous with
wings (Hemiptera: Membracidae). Systematic Entomology 37: 2-6.
“Prud’homme et al. have misinterpreted a part of T2 as T1….all the nymphal
characteristics mentioned by Prud’homme et al. (2011) are interpreted
incorrectly.”
• Critically one looks for structures developing in the region
Prud’homme et al. identify as a wing hinge that are homologues of
the axillary sclerites, pleural wing process etc. “Evidence for the
‘joint between the helmet and T1 tergum is crucial…”
• A membranous region of cuticle that Prud’homme say serves as a
hinge is misinterpreted: it is actually “the intersegmental connection
between T1 and T2…” This region is not homologous with the wing
joint membranes (hinges) of T2 and T3.
Sensory processing: transduction of pressure into neuron
depolarizations
•
•
•
•
Organs called ears are mechanical transducers and their essence is a tympanum or
eardrum which tracks the pressure [or displacement] changes that associate with
sound travelling through [the fluid] air.
Insects have trachea, air-filled tubes coursing through their body to convey gases.
The tracheal system plays an important part in the workings of ears. To ‘collect’
pressure changes requires a thin membrane of cuticle and this must be free to
move readily – it cannot be damped by body fluids. So we have a tympanum
backed by a tracheal air sac.
The ear of insects is typically a pressure difference ear in that there are two routes
to the eardrum: internal and external. The movement of the eardrum is thus a
compounded effect of two path-lengths which may be different.
The moving membrane has special cells linked to it that perform the actual
transduction: converting pressure forces into depolarizing membranes (the
‘information currency’ of the nervous system.
What is an ear and how does a locust discriminate sound frequencies?
Ears are typically bilateral part of the bilateral symmetry of animals, a right and
a left. In the locust these are pressure difference ears; they are each situated
within a recess in the first abdominal segment and sound has access to the
rear of both ears (pressure difference).
The plane of the tympanum is angled to face backward slightly.
The auditory ganglion of each ear is visible through the transparent tympanum,
its nerve running anteromedially to join the metathoracic ganglion.
Also visible on and through the tympanum are dark brown chitinous structures
(e.g., pyriform vesicle, folded body) that lie on top of the tympanum.
At one time it was disputed
whether insects could
discriminate frequency (indeed
whether they even had hearing
capacity) [insect baloonists
experiments].
First the anatomy using an
anchient note but drawing in
chalk. Easier to follow when
one sees a diagram made in
front of you.
The tympanum (ear drum)
is a very-much thinned
region of the cuticle with a
ganglion sitting more or
less in the middle. Behind
the tympanum, applied
overtop of ganglion and
acoustic nerve is a tracheal
sac.
Backing the membrane
with air is an important
adaptation: if the
tympanum were backed by
haemolymph of the
circulatory system the
tympanum’s movements
would be significantly
damped by the blood and
it would not respond with
sensitivity to the ariborne
sound.
Chordotonal sensilla occur in
the ganglion, 60 to 80 in four
groupings; each sensillum
involves several cell types. The
sensillum transduces the
mechanical movements of the
pyriform vesicle or other
cuticular eardrum parts. When
the modified dendritic region
surrounded by the scolopale
cell is mechanically stimulated
(by sound and resulting
eardrum movement) the axon
develops an action potential
and the neurosensory cell, by
firings, sends information to the
CNS. The cell’s position on the
eardrum, its mechanical
linkage, and the behaviour of
the eardrum itself, codes for
particular frequencies.
Fusiform body (fb) and
pyriform vesicle (pv)
and frequency
discrimination.
At 3-kHz sound input
the whole ganglion
follows the motion of
the pyriform vesicle
(pv); so both ends
move in phase. But at
the higher frequency
of 10 kHz the relative
motions of ganglion
(K1) vs pv are quite
different and the
strand of nervous
tissue (fb) is shaken
and jolted, leading to
many firings of the
chordotonal
neurosensory cells
within.
fusiform body
pyriform
vesicle
Stephen R.O., Bennet-Clark H.C. 1982. The anatomical and mechanical basis of stimulation
and frequency analysis in the locust ear. J. exp. Biol. 99: 279-314.
James F.C. Windmill, Martin C. Gopfert and Daniel
Robert 2004. Tympanal travelling waves in migratory
locusts Journal of experimental Biology 208: 157168.
Scanning laser vibrometry used to investigate
the movements of the eardrum when
stimulated by different frequencies.
Frequency analysis in the locust involves a “travelling
wave that funnels mechanical energy to specific
tympanal locations, where distinct mechanoreceptor
neurones project”.
“For each frequency the tympanal deflections
do not stay in position, but travel across the
tympanum from posterior to anterior... At 3.3
kHz the wave travels across the thin
membrane, moving towards a focus point
located at the folded body...”
Travelling waves vs standing waves
eardrum movement when subjected to four
different frequencies;
scanning laser videos show the complex movement
of different regions; profiles: red is outward
movement of the tympanum and green is inward
movement
What can one tell about function from the morphology of a neuron?
Giant neurons occur in many animals, e.g., Annelids,
Arthropods, Molluscs. They are adaptive in
emergency escape: sabellid worms use giant
neurons to retract their ‘lophophore’ into the
safety of their tubes when attacked. There is no
‘consulting’ with the brain: speed is essential.
• These cells were used by early physiologists
trying to understand depolarization of the nerve
cell membrane. In 1936 J.Z. Young discovered
that certain long structures present in squids
(previously thought to be blood vessels) were
actually single unusually large neurons. They had
axons of unusually large diameter: up to 1 mm.
• A typical axon is about 40 microns in diameter:
these giants are about 700 microns. They run
through the length of the body.
• Kenneth Roeder and cockroach experiments.
Stellate ganglion in squid mantle. The stellate nerve
contains a giant axon. This large-diametered motor nerve
depolarizes very rapidly, ensuring nearly synchronous
activation of mantle muscles in a jet-propelled escape.
Varying thickness promotes different speeds of
conductivity: longer are thicker and go faster so everything
happens at once.
Pictures taken from a website
illustrating the dissecting out of
a nervous preparation of the
squid giant synapse (USCRC
IBRO)
Interneurons and
shape specificity
(From J. Insect Physiology, George
Boyan, 1984)
Interneurons that receive auditory
input:
note topographical similarity of
omegas of cricket Gryllus and
katydid Tettigonia, reflecting binaural
comparison.
Neurons can be identified across
taxa based upon their morphology,
e.g., making crossbody comparisons
or as in the case of AN1 conveying
activity from the prothoracic ganglion
to the brain.
efferent, afferent
Analogous to wiring, a nerve or a neuron says
something about its function by where it starts
and where it winds up: a motor message going
from the CNS to a certain muscle (efferent), a
sensory message returning from an eardrum to
the CNS (afferent).
Interneurons are shaped adaptively: the story of the omega
neuron of orthopteran insects
•
The adaptiveness of this neuron’s
shape relates to bilateral
perception of sound by right and
left ears, located on the front legs
of crickets and katydids. sound
localization mechanism and
contralateral inhibition.
There are two overlapped mirror-image interneurons; each feeding back upon the other.
Gerry Pollock
Scientific American
Huber & Thorson
Pressure difference ear
with 4 different inputs
phase shifter
omega
interneuron is in
the prothoracic
ganglion
Adaptiveness of omega shape of the
neuron of a cricket
Gerry Pollack, Montreal
Contralateral inhibition
Omega interneuron: its name
suggested by the shape, lies
within the prothoracic ganglion
of a cricket or katydid. It
receives input from the
acoustic nerve running back
up the foreleg from the ears.
Parts of a neuron: cell body
(soma), dendritic arborization,
axon.
Omega neuron is here filled
with green dye via the
electrode that once monitored
its firing activity; omega
neurons occur in the
prothoracic ganglion of a
cricket as an overlain mirrorimage pair; the firing of each
feeds back upon and inhibits
the activity of the other.
Thus a slight difference in
perception to one side is
enhanced, supporting better
localization.
SOUND LOCALIZATION: DETERMINING DIRECTION AND DISTANCE TO A
SOUND SOURCE. How does a too-small female field cricket find her mate by
phonotaxis using his too-long sound wave?
We determine direction to sound sources by comparing IIDs: interaural intensity
differences. These differences arise in a right and left ear because of different path
lengths (one ear is closer to the sound than the other) and they arise because of
sound diffraction (bending) about the body. Small bodies relative to sound
wavelength have minimal diffraction. Large bodies make effective obstacles and
create a ‘sound shadow’. Human ears are separated by ~21 cm. This distance is
about the wavelength of a 800 Hz sound. For this wavelength the (diffraction) cast by
the human head gives an intensity difference of about 8 dB. For higher frequencies,
e.g., a tone of 10 kHz, this effect becomes more pronounced, e.g., a 20 dB drop in
intensity at the farther ear for 10 kHz. Humans make use of IIDs, turning their head to
equalize sound levels and by doing so facing in the direction of the source of a
sound.
Assigned reading: Michelsen, A. 1998. The tuned cricket. New Physiol. Sci. 13: 32-38.
Michelsen, A. & Lohe, G. 1995. Tuned directionality in cricket ears. Nature 375: 639Michelsen A. et al. 1994. Physics of directional hearing in the cricket Gryllus bimaculatus
J. comp. Physiol. A 175: 153-164.
*
Explain phase.
Cricket directionality is achieved because the ears are pressure gradient ears, i.e.,
sound has access to both front and rear of the eardrum, and also because in crickets
this back access involves cross-body transfer of sound in a large prothoracic
transverse trachea. The activity of the eardrum is a resultant of pressure changes both
outside and inside. And at any given orientation of the female cricket, the net pressure
at each ear is governed by phase produced by path-length differences. The
physiologists refer to IIDs: interaural intensity differences. The different path lengths
to the rear of the eardrum create phase differences which activate right and left
eardrums to differing extent. Phase changes with orientation.
Because a cricket’s body is
small relative to the song
sound wavelengths it
broadcasts, it cannot create
useful interaural intensity
differences IIDs by
diffraction. Cricket call
wavelength is about 70 mm;
the distance between the
leg-situated ears with the
legs in walking position, is
no more than 10 mm. The
body as an obstacle causes
no significant drop in
intensity to a farther ear.
The female cricket can turn 360º in relation to a distant calling male and IIDs
remain nearly the same. Yet in fact female crickets localize male calls well, at
night, at a distance and can readily walk toward that sound source. How do
they do it?
*
*ipsilateral same side as source;
contralateral opposite side
Imagine sound from a male’s call reaching the front of the near eardrum. It also
arrives at the inner surface of this same eardrum via 3 other routes : two prothoracic
spiracles on the thorax (IS & CS: ipsilateral* spiracle and contralateral spiracle) and the
contralateral tympanum. The path lengths of the three routes change as the female
turns. Thus the sound pressures on the back of the eardrum will change with changing
phase relative to those on the outside. So right and left eardrums show different activity
as a function of body direction relative to the source. Sound reaching the back of the
eardrum later than sound reaching the front is shifted in time, i.e., its phase has
changed.
Michelsen broadcast 4.5 kHz to
female crickets in an arena,
moving a speaker to 12 different
surrounding positions and
monitoring eardrum activity with a
laser vibrometer. The dotted line P
is a vector (magnitude and
direction) for the driving force
(pressure) at the eardrum for these
12 sound-source directions. The
resulting directional pattern for
the right ear is asymmetrical, i.e.,
as the phase angle changes there
are side-to-side differences in
eardrum activity which can be
transduced and compared by the
omega neurons of the prothoracic
ganglion. Now the cricket has
IIDs and a way of localizing the
sound source in the absence of
effective diffraction.