Download Biol 11 - Grasshopper Dissection Guide text (p. 334

Biol 11 - Grasshopper Dissection Guide
text (p. 334 - 339)
Animals w/o Backbones (p. 277-282)
Purpose: To observe the external and internal features of a typical insect (grasshopper)
Classification: This common species is Romalea guttata.
Kingdom
Phylum
Class
Order
Family
Animalia
Arthropoda
Insecta
Orthoptera
Acrididae
Genus
Romalea
Species
guttata
A.
General Information
Grasshoppers, locusts, crickets and katydids belong to a group of insects known as orthopterans
(meaning 'straight wings'). One of the most recognizable features of this group is their ability
to produce sounds by rubbing together certain parts of their body. This is known as stridulation.
Usually only the males sing to attract females but, in a few species, the female also sings.
Grasshoppers and locusts have a row of pegs like a comb on their back legs. They scrape these
pegs against the hard edges of the front wings to make sounds. Crickets and katydids produce
sounds by rubbing their wings together. To hear these sounds, orthopterans have a tympanum.
The Eastern lubber grasshopper, Romalea guttata is a large, slow-moving herbivore that has
reduced wings and can barely fly. They are found in the southeastern U.S. and their exceptional
size and ease of capture make them favorites for dissection.
The most noticeable feature of grasshoppers is their long, jumping hind legs, which enable them
to leap well over 20 times their body length (imagine a 6-foot tall person jumping 120 feet!).
However, while the powerful jumping muscles of the hind legs provide the force necessary for
leaping, they cannot propel the grasshopper in these impressive leaps unaided. Most of the
kinetic energy to do this comes not from the muscles, but from the semilunar crescent located in
the knee of the hind leg. This crescent-shaped organ is made of elastic fibers that store energy
in preparation for a jump; they release this energy explosively, propelling the grasshopper
forward many times its body length.
Grasshoppers live almost everywhere in the world, except for the colder regions near the North
and South poles. They live in fields, meadows, and anywhere they can find leaves to eat. Some
grasshoppers will only eat certain plants. Others will eat any plant they can find. They can eat
entire crops of alfalfa, clover, cotton, corn and other grains, which causes millions of dollars in
crop damages every year.
When a grasshopper is picked up, they "spit" a brown liquid which is known by many as
"tobacco juice". Scientists believe that this liquid may protect grasshoppers from attacks by
predators. Grasshoppers will also escape from their enemies by jumping and flying away, or by
hiding within leaves or the grass. If you have ever tried to catch grasshoppers in a field, you
know how quickly they disappear by dropping down into the grass.
The grasshopper's greatest enemies are assorted kinds of flies that lay eggs in or near
grasshopper's eggs. After the fly eggs hatch, the newborn flies eat the grasshopper eggs. Some
flies have been known to lay their eggs on the body of the grasshopper, even while it's flying.
The newborn flies eat the grasshopper after they hatch. Some other enemies of the grasshopper
include beetles, birds, mice, snakes, and spiders.
There are two main groups of grasshoppers: The first is long-horned grasshoppers, and the
other is short-horned grasshoppers. These groups are divided according to the length of the
grasshoppers' antennae, which are also called horns. You probably know short-horned
grasshoppers as locusts.
Scientists who study insects are called entomologists.
❖ On your chart, complete the example, phylum and general characteristics sections.
B. external structures and body plan
Get a lubber grasshopper and place it in a dissecting tray. The grasshoppers were fixed in
formalin, and there are gloves available to handle with them.
❖ Use your dissecting microscope (2X) to examine the external anatomy of the grasshopper.
First look at the wings. The front wings are thickened into a tegmen and function to protect
the membranous hind wings. Note how the wings are attached so that they can pivot and fold
over the back. This was a major evolutionary innovation of the neopterous (new wing)
insects. Remove the wings so you can continue dissection.
❖ Next examine the head, which is composed of several fused plates called sclerites that
compose the head capsule. Locate the ocelli (simple eyes), compound eyes, and antennae.
❖ Look at the mouth parts. Use a probe and scalpel to examine the mouth parts called the
palps (the labrum, mandibles, maxillae, and labium.)
❖ Cut open the tough head capsule and look for the brain and examine the internal support
structures.
❖ Remove a leg and identify the segments with jointed limbs and claws.
❖ Locate the tympanum (eardrum) on the thorax.
❖ Locate the thoracic spiracles, which are the openings to the tracheal system, which allows
direct gas exchange with the tissue.
❖ Look at the abdomen; the dorsal surface is called the tergum and the ventral surface the
sternum. Locate the abdominal spiracles.
❖ On your chart, make an accurate drawing of the external features of your specimen and
❖
C.
label: (animals w/o backbones p. 277)
exoskeleton, abdomen, thorax, head, antennae
spiracles, wings and legs, tympanum
simple eye, compound eye, palps
Below your diagram, describe the major adaptations of grasshoppers to a dry terrestrial
environment.
Internal Structures:
❖ Place the grasshopper right side up and carefully cut through the exoskeleton with your
scissors. Place some water in your dissecting tray, which will help float the organs,
particularly the trachea which will look like a mass of branched silvery tubes. If you are
careful you will see the heart, which is a dorsal tube, often it gets removed with the
exoskeleton. Notice the many muscles that are attached to the exoskeleton, try to follow
some and discern their purpose.
i. Skeletal system
❖ on your chart, describe the advantages and disadvantages of an exoskeleton.
ii. Reproductive System
Grasshoppers develop through incomplete metamorphosis. The nymphs appear similar to the
adults except that they lack wings and have incomplete reproductive organs. The number of
instars (larval stages between molts) through which a grasshopper develops before reaching
adulthood is fixed in some species (typically 4 to 6). In others, it depends on growing
conditions: the better the conditions, the fewer the immature stages.
Male grasshoppers attract females both visually and acoustically. Males of banded-winged
grasshoppers (subfamily Oedipodinae) can typically be seen in the summer taking short flights,
flashing their brightly coloured wings, snapping them together, or both, producing a distinct
sound. These short flight noises are called crepitation; the sounds are usually species-specific.
Males also attract females by stridulation (scraping the hind femora against the forewing); this
too produces species-specific mating sounds. Not all sounds produced by grasshoppers function
solely to attract females, some species spend much of their life on a single bush claimed as a
territory, stridulating to warn away other males as well as to attract females.
Females deposit eggs which hatch into nymphs and develop into adults if conditions are favorable
❖ on your chart, describe how grasshoppers reproduce. What is molting and why is it
necessary? What is the function of ovipositors? What is the difference between incomplete
and complete metamorphosis and describe why these types of life cycles are adaptations.
❖ determine the sex of your specimen and identify as many reproductive structures. Find
another team who has a grasshopper of the opposite sex and observe the differences.
iii. Digestive system
❖ If your grasshopper is female everything may be covered with a large yellow mass of
ovaries, follow them back to the ovipositor. Then push the ovaries aside and look for the
gut. The esophagus leads from the mouth to the crop where functions to store food. Under
the esophagus you may be able to see very small grayish grape-like clusters of the salivary
glands. This leads to the midgut that is covered with gastric caeca, which function for
storage. These are grainy triangle-shaped objects on top of the gut. Follow the gut back to
the hindgut where the very slender malpighian tubules, are attached. These act as a kind of
kidney for removing nitrogenous wastes in insects.
❖ Identify the parts of the digestive system on your specimen.
❖ On your chart, describe the major adaptations of the grasshopper’s digestive system.
iv. circulatory system
❖ On your chart, describe the open circulatory system present in grasshoppers.
v. respiratory system
❖ if you have not already located the spiracles, do so now.
❖ describe the function of spiracles, trachea and air sacs.
vi. nervous system
❖ Remove the digestive system and look for the ventral nerve chord and the ganglia this will
appear as two white lines along the inside of the sternum which are periodically connected
with larger white masses.
❖ on your chart, describe the function of the cerebral ganglia (brain)