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Lab 15 - continued Arthropods - grasshopper (Part 4 of 5)

By Kathy Egbert
Last updated about 5 years ago
45 Questions

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Of all the animal species known to exist today, 70-75% of them are insects. Without a doubt, insects, are the most successful of any animal group. They are found in all types of habitats. There are carnivorous insects, herbivorous insects, and parsitic insects. There are terrestrial walking insects, aerial flying insects, and aquatic swimming insects plus some that function in all three habitats. Some insects, such as termites and bees form complex societies. Insects are, by all means, a highly diverse and fascinating group of animals.
Insects belong to the large phylum. Arthropoda. This name comes from the Latin names: arthros (meaning joint) and poda (meaning foot). In addition to insects this phylum includes millipedes, centipedes, arachnids, and crustaceans. Arthropods all share the following characteristics:
  1. Body and limbs segmented externally
  2. Exoskeleton composed with chitin
  3. Without a true coelom but with a hemocoel a simple series of sinuses or spaces between tissues
  4. Jointed appendages modified for feeding
  5. Musculature distinctly striated
  6. Ventral nerve cord with dorsal brain
  7. Bilateral symmetry
Insects can be separated from the other classes of arthropods by having the following characteristics.
  1. Body separated into three regions
  2. Having three pairs of legs
  3. A single pair of antennae
A grasshopper is an excellent example of general insect anatomy. The grasshopper that is most commonly studied is the Plains Lubber grasshopper, Brachiostoma magna. The reasons for the popularity of this particular grasshopper are its large size, ready availability, and low cost. The Black Lubber grasshopper (genus: Romalea) is sometimes used as a substitute specimen for Brachiostoma.

Obtain a preserved grasshopper. Be sure to rinse it in water before using it.

Is your grasshopper specimen a Plains Lubber or a Black Lubber?

Look at the label on top of the specimen container
External Anatomy - Exoskeleton: Terrestrial animals must all have means of controlling, or at least minimizing, the loss of water from their bodies. Arthropods have evolved a hard multilayer, waterproof covering called an exoskeleton. In addition to controlling water loss the benefits of an exoskeleton are:
  1. Armor-like protection from physical and chemical damage
  2. An excellent platform for attachment of muscles that provides good leverage and results in efficient locomotion and greater speed
  3. Provides an effective barrier against disease
The major disadvantage of having an exoskeleton is that growth is limited until the exoskeleton is shed or molted. The new exoskeleton is soft and must expand to a larger size than the old exoskeleton in order to permit growth. At this point the exoskeleton is allowed to harden. Until this process has occurred the animal is vulnerable to damage and water loss. One of the major components of the arthropod exoskeleton is chitin. Chitin is a chemical similar to cellulose in that it is composed of many micro fibers. Insects further strengthens the chitin fibers during a process called sclerotization. This process converts the chitin into a hard plastic-like substance.

Look up sclerotization and describe how distinctively different this is in insects compared to aquatic organisms that use calcium carbonate to make and harden their exoskeleton.

Grasshopper Anatomy Extertior Part I - about 9 mins After you have studied the figures and watched the video and read the text about the external structures, you will demonstrate your new knowledge to your teacher.
Also locate the forewings and hind wings.
Exterior Body Regions:
Compare your grasshopper to Figure 1. Use this illustration to familiarize yourself with the various external features of the grasshopper. It is also important to identify the sex of your specimen by the secondary sexual characteristics shown in this illustration. Note the three distinct body regions: the head, thorax, and abdomen.
THE HEAD

The head (Figures 1 and 2) is a functional unit that performs various tasks; the major sensory area, the ingestion of food, and it is also the major area of information processing for coordinating body activities. Although the head appears to be composed of one single segment, it is in fact made up from six separate segments that have used into one. The six segments only appear during embryological development.
The grasshopper is a biting/chewing insect and as a result has mandibular mouthparts. The labrum, labium, and maxillae hold food between and against the mandibles which move from side to side to grind food for swallowing. The palpi help manipulate the food and also contain sense organs which serve as taste detectors. Food is further manipulated by the tonguelike hypopharynx (Figure 4). With a dissecting needle probe around the mouth area to identify the various mouth parts and their function. The use of a dissection microscope greatly aids in this.

Insects have compound eyes. This type of eye differs from the vertebrate eye by being made up of hundreds of separate lenses of facets each with its own separate photoreceptor. Together each lens and photoreceptor form a unit called an ommatidium. The exoskeleton is transparent over the eye and forms a protective cornea. Such an eye produces a mosaic type of image. The more ommatidia a compound eye has, the greater the resolution or detail the image forms. In addition to having compound eyes, grasshoppers have three simple eyes called ocelli. These eyes cannot form images but are for sensing changes in illumination. They may also play a role in coordinating the daily rhythms and activities according to the position of the sun.
The antenna are sensory structures for the sense of touch and smell. Insects are often seen moving about with their antenna waving. The antenna are well muscled and flexible structures containing many sensory bristles.
THE THORAX
The thorax is the middle section of the grasshopper and is composed of 3 segments: the large shield like prothorax, the mesothorax, and the metathorax. Each of these segments have a pair of legs and can be identified by this. Between each of these segments are a pair of respiratory openings called spiracles. The legs are composed of five segments. The coxa joins the leg to the body, followed by the small trochanter, the large femur, the thin tibia, and clasping tarsus.
The thorax also contains a pair of wings. On most grasshoppers these are well developed and used for flight. On Lubber grasshoppers, however, the wings are small and useless for flight. A secondary fucntion of the grasshopper wing is for producing mating calls. The male grasshopper vibrates his wings together advretising his presence to prospective mates.
THE ABDOMEN
Posterior to the thorax is the abdominal section that is composed of eleven segments. The first 8 segments each have a pair of spiracles. The last three segments form the external genitalia of both sexes. The cerci are also found emanating from the 11th segment. The cerci are sensory structures and have numerous touch sensors at their terminal end. Female grasshoppers have a muscular egg laying structure at the end of the abdomen called the ovipositor. The female lays her eggs in the ground with this organ.

The hind legs are well developed and used for jumping. How many pairs of legs are there?

Describe structures that make the grasshopper's locomotion adapted to land versus the water (crayfish).

SKILLS ASSESSMENT 20 pts: Demonstrate your knowledge of the external antamony of the grasshopper.

Remove the mouthparts by grasping them with forceps and pulling them out. Arrange them in order on a piece of toweling, and compare them with the figure provided at the beginning. These mouthparts are used for chewing and are quite different from those of a piercing and sucking insect.

Skills Assessment: 5 pts Show your mouth parts to your teacher.

Skills Assessment: 2 pts Show your teacher the tympana, one on each side of the first abdominal segment. The grasshopper detects sound vibrations with these membranes.

Skills Assessment: 1 pt Locate the spiracles along the sides of the abdominal segments. These openings allow air to enter the tracheae, which constitute the respiratory system.

Skills Assessment: 2 pts Find the ovipositors, four curved and pointed processes projecting from the hind end of the female. These are used to dig a hole in which eggs are laid. The male has claspers that are used during copulation. See figure at beginning. Identify whether you have a male or a female. Share with classmates to see if one of them has a specimen of the opposite sex so you can see its genitalia.

Before we can view the internal anatomy, you will need to remove all 6 legs. With a pair of fine tipped scissors cut through the coxa of each leg. Once this is done, put the legs back inside your plastic bag.

Pin the body down to a dissecting tray. Generally 4 pins, two at the rear of the abdomen and two go through the pronolum, are required. With a sharp pointed scalpel cut carefully through the exoskeleton at the top of the 8th abdominal segment. Take this cut all the way forward and around the abdominal segment. Take this cut all the way around returning to the starting point as illustrated in the figure below. Some students prefer to use a sharp pair of small pointed scissors to do the cutting. Cut crosswise behind the head so that you can remove a strip of the exoskeleton. If necessary, reach in with a probe to loosen the muscle attachments and membranes. Regardless, with tweezers remove the exoskeleton plates as they become free. Muscle tissue will come off with the exoskeleton which is what you want to happen in order to expose the internal organs. Do this carefully and your speciment will not be ruined. Cover your specimen with water to keep the tissues moist.
Video - Grasshopper Anatomy Part 2, Internal - abt 4 minutes
Locate the heart and aorta just behenth the portion of exoskeleton you removed. A grasshopper has an open circulatory system. Remove the heart and adjacent tissues.

Skills Assessment: 1 pt Show your teacher the heart you removed.

CIRCULATORY SYSTEM All insects have what is called an open circulatory system. It is so named because the insect blood or hemolymph is not restricted to system vessels as it is in animals with a closed system such as earthworms or humans. The insect circulatory system consists of the heart and aorta. The heart is a dorsal vessel divided into a series of pumping chambers restricted to the abdominal segment. The aorta extends forward into the thorax and head. Hemolymph enters the heart chambers through valve openings called ostia and is pumped forward through the aorta to the head. From the head the hemolymph travels rearward through the hemocoel. Hemolymph is directed in and out of various areas (such as the limbs) by a series of baffles. As the hemolymph bathes the digestive organs nutrients are picked up and distributed throughout the body. As the hemolymph travels rearward it is directed back to the heart for redistribution.

NEXT: Locate the fat body, a yellowish fatty tissue that covers the internal organs. Carefully remove it. Find the tracheae.

Skills assessment 1 pt: Show your teacher that you located the fat body and removed it successfully and identified the location of the tracheae.

RESPIRATORY SYSTEM
Animals with lungs utilize their blood to distribute dissolved oxygen to cells along with disposing of carbon dioxide produced by metabolism. Insects, however, utilize a different respiratory system called a tracheal system. A series of tubes or trachea are used to distribute gases directly to and from body tissues. Hemolymph play no role in gaseous exchanges. The tracheal system begins with the spiracle which are the openings of the system. These are valve-like and can open and close under control of the nervous system. Spiracles open into air-sacs.
These air sacs are elastic. By compressing and relaxing the abdomen, in conjunction with the opening and closing of the spiracles, grasshoppers can pump air into the tracheal system. Not all insect have this ability. The grasshopper, however, requires this due to its large size.
From the air sacs, the trachea branch into the body tissues. They become smaller with each division until the trachea reach the cellular level. If your grasshopper specimen came color injected - it has its tracheal system filled with colored latex. Without this latex, the tracheal system is extremely difficult to identify.

REPRODUCTIVE SYSTEM

Identify the reproductive organs that lie on either side of the digestive tract in the abdomen.
Malte: If your specimen is a male, look for the testis, a coiled, elongated cord containing many tubules.
FenakeL If your specimen is a female, look for the ovary, essentially a collection of parallel, tapering tubules containing cigar-shaped eggs.

Skills Assessment 1 pt: Show your teacher whether you were able to locate the reproductive organs in your specimen.

DIGESTIVE SYSTEM

Locate the digestive tract and in sequence, the crop, a large pouch for storing food (a grasshopper eats grasses); the gastric ceca, digestive glands attached to the stomach; the stomach and the intestine, which continues to the anus; and Malpighian tubules, excretory organs attached to the intestine. Insects secrete a dry solid nitrogenous waste that looks like a little pellet. Work the digestive tract free, and move it to one side. Now identify the salivary glands that extend into the thoracic cavity.

DIGESTIVE SYSTEM and EXCRETORY SYSTEM Food is chewed into small pieces and moistened with secretions from the salivary glands. The muscular hypopharynx manipulates the food for swallowing. Food travels through the esophagus to the crop. The muscular crop grinds the food into fine particles for final digestion. When released from the crop by a muscular sphincter the food enters the stomach. Here digestive enzymes released by the gastric ceca mix with the food and digestion begins. The broken down nutrients are absorbed by the stomach and the gastric ceca. When the absorption process is finished the food is released and moved into the intestine. The intestine is where the waste material becomes progressively more dehydrated as water is extracted by the intestine walls. This process continues until the fecal material is reduced to a dry pellet in the rectum ready for expulsion. The malpighian tubules are the grasshopper’s organs for removing urea (protein metabolic wastes) from the hemolymph. These tubules radiate out into the hemocoel and empty into the rear of the hind gut. The tubules utilize a series of ph changes to extract uric acid from the hemolymph and deposit it in the hindgut with very little water loss.

Skills Assessment 1 pt: Show your teacher what you were able to find of the digestive tract. (crop, gastric ceca, stomach, intestine, Malpighian tubules, anus, salivary glands)

Describe how the grasshopper's excretory system is an adaptation to life on land.

Explain where the grasshopper gets the water it needs to live.

Remove the internal organs. Now identify the ventral nerve cord, thickened at intervals by ganglia. Remove one side of the exoskeleton covering the head. Identify the brain, anterior in the esophagus.

Skills Assessment 1 pt: Show your teacher the ventral nerve cord, ganglia, and brain.

THE NERVOUS SYSTEM The grasshopper brain consists ofo 3 ganglions (ganglions are masses of nerve cells that serve as processing centers). There receive and process impulses from the eyes, antenna, and mouth. The brain is connected to the ventral nerve cord by the subesophageal ganglia. The ventral nerve chord is a chain-like structure of paired ganglions and nerve chord that travel back to the 7th abdominal segment. Each thoracic segment has its own ganglia for controlling the movement of its limbs. There are 5 ganglions located in the abdominal segments controlling digestive, respiratory, and reproductive functions. This decentralization of the nervous system allows some insects to maintain some functions even though the head has been removed. For example, female preying mantis often eat the heads off of their male partners that are able to move and complete the mating process.
CONCLUSIOS:

Compare the adaptations of a crayfish with those of a grasshopper by completing Table 15.5. Write in the physical characteristics in the table. After you have entered the characteristics, mark the ones that indicates an adaption to life in the water (crayfish) and to life on land (grasshopper) with a star/asterick.

How many "stars" did you place in the chart?

Given a specimen of a beetle

does it have...

Given a butterfly as an specimen

does it have

Given a grasshopper as a specimen

does it go through

Use your present knowledge of insects or look up another insect and give its name here.

Does your specimen you gave in Question #37 have