Download TENTORIUM, CLAWS AND INSECT MOUTHPARTS

TENTORIUM, CLAWS AND INSECT MOUTHPARTS
Preamble
325 Biomechanics is a course in adaptive morphology: how and why organism bodies have evolved to work
the way they do. The lab is where we examine form, shape, stiffness, size, material etc. of body parts and how these
are arranged, how the parts might move relative to one another. For many organisms there are worthwhile pictures
on the web; of course you should search them out, especially videos that might show body parts behaving*
(*behaviour is ‘what something does’, how it moves), e.g., a wing during flight, fins during swimming etc. And the web
is also an effective encyclopedia: if you don’t know what a lampshell is or a trilobite, a rete, a nematocyst (etc.), you
can search online (Wikkipedia is good), keeping in mind that not all you will find is correct. Pictures alone aren’t good
enough: you need to experience real specimens which is why we have a lab. Best would be to see the animals and
plants alive in their habitats of course. Its too bad we can’t manage a pitcher plant, a swimming whale or a wandering
moose in the lab.
We use microscopes for study when the specimens are small. Always cover the material with 70% alcohol to
give clear visibility. Floating tissues show you their shape, but can collapse under their own weight when there is no
alcohol. Pin (00 insect pin) bits down in the trays so they don’t drive you mad with drifting about. Take cell-phone
pictures of dissections as you wish, to help you remember, but also remember such pictures do not replace drawings
or the benefits of studying body parts. Bring a jar with you with a sealable top and keep the results of your smaller
dissections preserved in alcohol, to study in later days for bellringers.
Our eyes filter out redundant detail in the images we process and we don’t really remember what we see
unless we make a special effort. The purpose of making the drawings is not just to record the structure, but to make
you study the structure. If you have to struggle to represent shape with a pencil you will come to understand the
shape better. A lot of people think of themselves as artistically-challenged. If you have ‘illustrophobia’ you’d best
get over it. Drawing is an important part of observation. You need to look at structures from different angles and
wiggle parts to understand their relationship to other parts.
Each lab begins, like this one, with a preamble: a discussion of some of the relevant background information.
Read this part ahead of time, but just before you do the lab. After the preamble is a section where various procedures
are described. The TA will clarify additions and exceptions. From time to time there are questions, usually with
rather open-ended answers; these are obvious attempts to make you think functionally about animal bodies. In each
lab there are demonstrations set upon side benches. These are also important and you need to take time to visit
them during the afternoon.
Function and effect
Function is defined as adaptive consequence. A male cardinal’s plumage (Cardinalis cardinalis,
Cardinalidae) is scarlet red, a colour that functions as a signal to other individuals of his species. His feathers’ red
pigmentation is the product of selection for signalling to his mate, and to conspecific males when defending his feeding
territory. The colour makes him conspicuous to same-species competitors (other males) and his plumage influences
the female’s choice of him as a partner. The reflection of red-light wavelengths by this bird's feathers has an adaptive
consequence (the transfer of information that increases his reproductive success) a consequence that was the basis
of the pigmentation coming into existence historically and of its ongoing selection.
Think of functions as competing hypotheses, possibities that could explain an organ’s form. Multiple
functions may be argued for a single feature, and not all features are necessarily adaptive. For example there are
deepwater marine sponges whose body like the cardinal reflects scarlet-red wavelengths. When photographed
these sponges appear a uniform bright red. Could this red colour be a signal like the cardinal’s? For example could
it announce to would-be marine feeders on sponge that this sponge sequesters chemicals that make it toxic? This is
a hypothesis: sponge red integument is warning coloration.
But the sponge’s red colour must be seen as an effect not a function. We know this because red light cannot
penetrate to the ocean depths where these sponges live. There is no natural red light at depth to be reflected back
into a viewer's (potential sponge-feeder’s) eye; the only red is what a human diver brings down in a photoflash.
Building red pigment into the sponge’s body cannot have been effected historically (evolutionarily) by the material’s
capacity to reflect red light. But this is not to say that the material of the sponge's body surface that happens to reflect
red isn’t adaptive in some other way*. It is just that the physics of light in the sea negates the hypothesis that the
sponge colour was selected as a warning colour. (*The red pigment could exist because of pleiotropy: genes may
have multiple phenotypic effects, so the presence of a particular phenotype/morphological feature, might be the result
of selection acting on one of the other phenotypic effects.) Effect as an explanation for a feature is always a
possibility.
Aphorism: a precept or principle expressed shortly and pithily: as for example, ‘cleanliness is next to
godliness’. {Not sure how pithy it is, but here is the primary course aphorism: ‘Imagine it as it isn’t’; this helps to
stimulate hypotheses of possible function. Another aphorism might be ‘compare for insight’: organisms have such
diversity and the differences between them, products of their (evolutionary) history, help you see how different
selection pressures can bring contrasting results. The contrast lets you guess at the history.
Body systems
Being an animal you already know a great deal about the main body systems: the things an animal must do to live:
1. defend itself from other animals and the elements: with special body coverings
2. locomote: position itself in its environment
3. ingest: find and take in food; then digest: break it down into usable chemicals
4. egest: excrete waste products and maintain body solutions
5. exchange respiratory gases: take in oxygen, give off carbon dioxide
6. transport chemicals internally
7. sense its environment
8. control its body
9. reproduce.
These fundamental life activities are reflected in the major systems of the body:
1. Integument
2. Muscular and skeletal
3. Digestive
4. Excretory
5. Respiratory
6. Circulatory
7. Sensory
8. Nervous
9. Reproductive
Locust study animal
This one insect is much employed in 325 so it deserves a special introduction. Locusta migratoria
migratoriodes (Reiche & Fairmaire, 1849) [Family Acrididae, Order Orthoptera, Phylum Arthropoda] is a grasshopper
associated with a large literature and much scientific effort. Its common name is the African Migratory Locust. It is a
polyphenic species, meaning it appears in two phases or morphs, gregarious and solitary, these phases developing
according to local population density. Development with many individuals nearby shifts toward production of the
gregarious form. Soon bands of winged adults migrate, having eaten all the food where they grew up. Adult locusts
are good fliers, capable of 15-20 km/h and their swarms can travel >100 km in a day. They are a fearsome pest of
crops in many parts of the world including Australia, South America and Africa. Turns out physiologists like to study
them and they keep colonies, which is useful for this course.
LAB ONE
Tentorium, claws and insect mouthparts
Comparison between taxa can give insight and stimulate hypotheses of function, because the same structure
in one taxon may be subject to different selective pressures in another taxon. Today in two species of insects (beetle
and grasshopper (both Phylum Arthropoda, both Order Insecta) we compare an organ – the tentorium -- comprised of
inflected insect cuticle. It is one kind of apodeme, [defined as a ridge or ingrowth of the exoskeleton serving for the
attachment of muscles and/or resistance to forces]. Insect integument must be both skin and skeleton and is termed
exoskeleton. The cuticle of the tentorium arises during embryonic development; its arms grow inward, meet
internally and fuse into the ‘body’ of the tentorium. The resulting structure functions [stated generally] to give stiffness
to the lower region of the head. This interesting apodeme would be called a truss by an engineer. The tentorium
serves to keep the head capsule, which is open below and behind (like a box with two sides removed) from distorting.
In the course of their evolutionary history grasshoppers and beetles evolved with different orientations of their
mouthparts. This orientation resulted in very different tentorial shapes. In the lab the idea is to see how the shapes
differ and to ask whether one can make sense of the shape difference in relation to how the forces they resist have
changed. These forces must come in part from the actions of the mandibles and maxillae, the labium and labrum.
i.e., the mouthparts that attach to the lower margins of the cranium.
Throughout Orthoptera mouthparts are oriented downward, i.e., they point ventrally, toward the ground. This
arrangement is termed hypognathous. With its hypognathous mouthparts a grasshopper (typically a herbivore)
perched on a leaf blade has its mouth pointing at what it eats. Grasshoppers move about more often in
3-dimensional plant space, on leaves and stems, not on the ground; its good to have your food at your feet. In the
order Coleoptera by contrast, mouthparts are commonly prognathous, oriented anteriorly. (What are beetles doing
differently in addressing their food?)
Sclerites are (usually) demarcated regions of the exoskeleton. Along with their prognathous mouthpart
orientation, beetles have evolved a throat sclerite called a gula, closing off the head beneath and behind the
mouthparts between their ventral bases and the neck membrane. Orthoptera do not have a gula. As beetles
evolved from some hypognathous ancestor, selection favoured mutations which resulted in a changed head angle,
with more anteriorly directed mandibles and maxillae. (What sort of selective forces might have been involved in
favouring such a change of orientation? Same question as above really.) If the head evolves to be prognathous in
the course of evolution, then its lower boundary is becoming more open and unprotected unless a new sclerite is
produced: hence evolution of the gula. In the evolving beetle ancestor, the region where the two posterior arms of the
tentorium join the cranium (posterior tentorial pits occur on the surface here marking the internal projection of the
posterior tentorium apodemes) was involved in gular formation: thus in beetles the posterior arms of the tentorium
have a greatly altered shape: they are drawn-out into a long ploughshare shape that engages the edges of the gula.
Levers and dicondylic joints
Forces may apply leverage (developing moments). An input force (effort) is affected in its magnitude and
direction by a structure that pivots (that turns on a fulcrum) and so force movement [translocation is the proper term]
results in a different output force (acting upon a load), different in magnitude and direction and moved/translocated to
a different location. One can recognize three kinds, or classes of lever: first, second and third class. They differ on
the basis of where the forces are located relative to the fulcrum. First class lever: a teeter totter or a person rowing
a boat are examples: input force on one side of the fulcrum, output force on the other side: sequence is force in,
fulcrum, force out. Second and third-class levers have both force in and force out on the same side of the fulcrum: in
the case of a second class lever, a person pushing a wheelbarrow, or opening a car door are examples, the sequence
is force in (effort), force out (load), fulcrum; in the case of a third class lever the sequence is force out, force in, fulcrum.
An example of third-class lever is a pair of forceps.
Cleared grasshopper head dissection
Obtain a cleared* grasshopper head. [*Cleared just means the specimen has been placed in KOH solution for
some time at some temperature to remove soft tissues.] Begin by examining surface and mouthparts: with forceps
holding the specimen in a dissecting tray under alcohol, look through the microscope. The microscope light must be
adjusted to concentrate intensity where you need it. Then pin the head down using one fine insect pin or a minuten
pin. Forceps may be able to wiggle appendages, which helps to see how the parts go together and move relative to
each other: you want to view specimens from different angles if you can. It’s difficult to wiggle the grasshopper
mandibles though.
Identify vertex (top), face (frons), segmented antennae, compound eyes and ocelli. (Further guidance is
provided by supplied handouts.) Exoskeletal regions in an arthropod are referred to as sclerites. The
grasshopper’s face is mostly a single sclerite called the frons, that blends without any clear border (suture) into right
and left cheeks called genae. Above, again without a clear suture, it becomes the vertex. There are 4 pairs of
mouthparts slung below the head (hypognathous): clypeus and labrum are anteriormost (making a kind of front lip);
then mandibles, maxillae, labium hang down, progressively farther back (the labium is the lower lip). In the midline,
surrounded by the mouthparts, there is a protruding hypopharynx (tongue) heavily invested with tasting sensory
hairs. The segments of the maxilla are: cardo, stipes, lacinia, galea, maxillary palp. The segments of the labium
are the: submentum, mentum, prementum, paraglossa, glossa, and labial palp (see diagrams provided). You
should know the names of these parts and be able to recognize them.
The mandibles are blocky and massive (relative to the other mouthparts) with two regions, one proximal
termed molar and one distal termed incisor, in analogy with our own teeth. Recognize where these two regions are
and notice how they are strongly darkened and made very hard by sclerotization. These surfaces are adapted to
resist wear, their shape designed to penetrate (incisor) and to grind (molar).
You can determine the plane of movement of the mandibles by examining where they attach to the head. At
the base of each mandible there is a dicondylic joint, so-called because it is comprised of two ball-and-socket pivots.
Imagine a line joining these two pivots; this line marks the fulcrum (see levers above), an axis of rotation to be
followed by the mandible. By muscle contraction the mandible can be drawn away from the midline (abduction) or
toward the midline (adduction). Observe the dicondylic nature of the joint by which each mandible is attached below
the head.
Re the maxillae, lower and upper lips: look at the parts and try to figure out what the different parts might do.
The function of a mouthpart like the maxilla is obviously ‘to manipulate food’. But this answer can become much
more specific, much more detailed – much better. Some of the parts that make up the maxilla may function to impale
(you need to pin material firmly down in order to bring forces to bear upon it with other tools); perhaps some tear apart.
They may function to handle some foods differently than others. Realize that among the thousands of species of
arthropods these mouthparts are varied in structure in accordance with adaptiveness for different foods: the
mandibles, maxillae etc. of a tiger beetle are very different from those of a weevil etc. Mouthparts are exceedingly
diverse. Adaptation involves nested detail.
Two alternative planes
1) Use a single-edged razor blade or (communal) microscissors and cut the grasshopper head exactly in half
along a median (middle) sagittal plane. A sagittal plane includes body length and body depth but not width;
taken in the the middle of the body it divides an animal into equal right and left halves). Immerse and pin
down near the middle of a dissecting tray, one of the halves [keep the other for a second try], gena-down in
70% alcohol: examine the cut face under a dissecting microsope. Pin to avoid meniscus distortions (by
which I mean angle your pins away so you can see the parts without highlights and distortions). Consult the
drawings provided: identify body, anterior and posterior arm of the tentorium and also the apodemes of the
mandible.
2) Single-edged razor blade cutting head in coronal plane, i.e., you remove the vertex of the head so that you can
look down from above and see the structures within. (This is the approach of grasshopper brain surgeons.)
The coronal plane should come at about mid-compound eye height. If you do the sagittal midline section you
will of course cut right through the central body of the tentorium. The coronal approach leaves the tentorium
body intact. On first looking down from above you will not see the tentorium clearly because of the upward
projecting large adductor apodemes of the mandibles.
The tentorium of grasshoppers (Orthoptera) is roughly X shaped, but with the central axis of the X shifted
rearward. There is a central body. Two anterior arms run forward from this to the ventro-anterior corners of the
cranium; two posterior arms run rearward to the ventro-posterior corners. There are also two dorsal arms, reduced in
this insect to mere strands, that angle upward from near the junction of the central body with the anterior arms. In
other species these dorsal arms give support to the frons. Make a drawing of the tentorium within the head, labelling its
parts as well as the frons
Try to detach one of the mandibles, together with its 2 attached mandibular apodemes: adductor and
abductor apodemes. Both these structures are thin curving sheets of exoskeleton that project upward into the head
from a narrow attachment to the mandible base. In life, muscles, cleared away here by KOH, originate on the inner
surface of the cranium (insect skull) and insert onto these apodemes, which in turn attach to the mandible bases.
Increased apodeme surface area is a way of raising the number of muscle fibres that can be involved in mandible
biting, a way of making the bite more powerful. The adductor apodeme is much larger than the abductor apodeme.
This is because the abductor has only to open the mandibles in preparation for biting; the adductor has to do the heavy
work of pinching, crushing and grinding. Try to see how the mandibles must move as this antagonistic pair of
muscles (adductor muscle, abductor muscle) pulls on them. Classes of levers were explained above. Can you
decide what class of lever is involved in muscle movement?
Beetle head dissection
Following the same general technique as for the grasshopper head, obtain and examine under a microscope
a cleared beetle head – a prognathous insect. Notice what it means to say the beetle’s mouthparts are
prognathous: its mouthparts are directed forward. The two insects, beetle and grasshopper, have all the same
homologous head parts (except for the gula). (Homology: existence of shared ancestry between structures; the
labrum of a beetle has a shared ancestry with the labrum of a grasshopper and did not evolve independently.)
The tentorium and its arms and central body are also present in the beetle. But because of the change in
mouthpart orientation and the creation of the gula, the tentorium is very differently shaped. Try to find the beetle’s
tentorium by dissection and understand and describe in words and drawings which parts of the beetle’s tentorial arms
correspond to those of the grasshopper. Could you now address the question: how has evolution of prognathous
mouthparts changed the shape of the ancestral hypognathous tentorium? Note that we wind up making use of
comparison to gain insight into function.
Dicondylic joints in crayfish claws; useful in fine restaurants
Compare for insight: one of those aphorisms (see above). So obtain and examine the cheliped of a crayfish.
This limb/appendage is used like forceps to manipulate food and make aggressive display. It consists of a series of
segments each jointed to the other by a dicondylic joint (joints like that of the grasshopper mandible). Take insect
pins and insert them so that each pin follows the fulcrum of the dicondylic joint of that segment. Look at this ‘pinned’
cheliped end on; how do the planes change as one progresses distad along the limb (out from the body)? Imagine
this as it isn’t: i.e., imagine that all the joint axes’ angles are the same: what would be the effect upon the range of
limb movement? Dissect the claw and describe its muscles and apodemes as a lever system. (You will never eat
lobster the same way again.).
Demonstrations
Specimens are set out at a side bench.
1) See insects with different mouthparts: tiger beetle, weevil, butterfly etc. The point here is that mouthparts
vary widely between insect species, modified by different selection pressures in the context of capturing and
processing different foods. Every set of mouthparts evokes a different set of functional hypotheses.
2) There is another type of arthropod joint shown: a monocondylic joint where the insect leg joins to the
thorax via a segment called a coxa. There is only one point of attachment of the coxa and this allows it movement in
multiple planes, not the single plane of the dicondylic joint. Where is the ball situated, the socket? Is one
arrangement better than the other, i.e., imagine it as it isn’t.
3) The cheliped of a pistol shrimp has evolved with a tuberculate process on the moveable finger which can be
inserted suddenly into a socket on the immovable finger. This is the water pistol of Alpheus a genus of crustaceans in
the family Aphaeidae. Go stick your head in the sea (nothing personal) just offshore and you will perhaps hear a
‘frying bacon noise’ of multiple pistol shots from these crustaceans. Needless to say I’ve done this. Pistol shrimp live
in rock crevices and quarrel with their pistols which displace a stream of water along a groove as the plunger engages.
They actually do squirt each other.