Arthropods.

 
 

The animal Phylum Arthropoda includes many, many species, who share a few characteristics: their body / muscle support is on the outside, in the form of an exoskeleton which covers their whole bodies, and is jointed at flexing points; they generally have many pairs of legs; they circulate blood through their hollow interior, through what is called an open circulatory system; they, like many other invertebrate groups, have a nervous system whose central cord is paired and solid and runs along their ventral (belly) surface. They include arachnids, like spiders, scorpions, ticks, and mites, and two groups we'll look at in the lab - the crustaceans, mostly aquatic, and the insects, which are mostly terrestrial and usually winged.


Part One. Crayfish.

Crayfish are common freshwater crustaceans, related to lobsters, crabs, and shrimp. They are bottom dwellers and live as predators and scavengers. They live in small hollows or burrows that they back into, with their chelipeds, or claws, facing out.


External Anatomy. Get your dissection equipment, including a dissection pan, and a preserved crayfish. There should be a sheet with labeled drawings to help you identify the structures. Crayfish, like all crustaceans, are also characterized by doubled, or biramous appendages - note that, although the main antennae are single, the shorter antennules below them are doubled; we'll also later see doubled structures at the bases of the legs.


1. The front legs, the chelipeds, are supposed to be a specialized adaptation of one of the sets of regular walking legs - this means that the chelipeds and the walking legs are homologous, built from the same basic structure. Look closely at them - what similarities support this idea?





2. The cheliped, of course, is not a walking leg - it's adapted for defense and for grabbing prey. How is it different from a walking leg?





3. Arthropods are considered to be segmented - their basic body plan, at least as embryos, consists of repeating units. Some of this feature remains visible on the outside. What parts of the crayfish body show signs of segmentation?






4. What parts of the crayfish do not seem to be segmented?





Inspect the swimmerets - they are the fine, leglike (they are also modified legs) structures under the abdomen, behind the walking legs. In females, they all look roughly the same, but males have much larger anterior ones which are used to transfer sperm packets from the male's genital opening (a small pipelike structure just in front of the swimmerets, usually difficult to see) to the female's. The female receives sperm in the autumn and stores it in a seminal receptacle until the following spring - she then lays her eggs and deposits the sperm on them. The fertilized eggs (and the larvae that hatch from them) are then carried around attached to the mother's swimmerets for as long as three months.

5. Which gender is your crayfish specimen?



6. Is fertilization in crayfish external or internal? Explain briefly.





Carefully pry up the carapace, the shield-like covering, and cut it away from the crayfish. This should expose the gills. When out of the water, a crayfish can continue to breathe as long as it can keep some water trapped in there around the gills.

7. How many sets of gills are there on one side?



Take a walking leg by the base and twist it so that both it and a gill are detached. In a petri dish, float the gill. Note how, supported by the water, the gill spreads and reveals its complexity.

8. Why is the gill so intricate?



Crayfish - Internal Anatomy.

Take off the rest of the carapace, but be careful not to remove internal organs that are attached to the underside. The heart lies just below the carapace and often is damaged by this procedure; when intact, it would have several arteries that would carry nutrient-rich blood to areas that particularly need it and "squirt" the blood into spaces there. There are no veins for incoming blood, although some arthropods have them; ostia, openings on the heart itself, gather blood from the body cavity.

The gonads, testes or ovaries, lie ventral to the heart. These are glands - note that gland tissue has a distinctive "look" to it. Anterior to the heart, apparently in the head, is the stomach. Remove the stomach from the esophagus, intestine, and digestive gland (kind of a combination of liver and pancreas) and cut the stomach open. You may need to wash material out of it.

9. Inspect the dark hard structures attached to the inner wall. You may need a dissecting 'scope.  Describe them.




Near the eyes you may be able to see the brain, a large swelling of the forward nerve cord. The antennal glands, sometimes called green glands, are also in the head - they extract wastes from the blood, and form urine, which is stored in bladders and expelled from excretory pores in the face. The urine is slightly toxic, but not enough to give it any use as a defensive poison.

10. If it's not defensive, what advantage does a crayfish get from expelling urine from its face? There is a clue from earlier information in the lab.





Remove the remaining internal organs so that you can see the nerve cords that run along the ventral body wall. In the abdomen, you'll need to remove the swimming muscles, which surround the hindgut. Ganglia, processing centers, should be present in each segment. Trace the nerve cord through the abdomen.

11. How far back does the actual nerve cord run?

 

 

Arthropods - Part Two. Grasshoppers.


Grasshoppers are insects, the largest arthropod subgroup. There are more named insect species than in any other group of animals; whether there are more actual species is much more difficult to determine. Insects are considered a terrestrial, or land, group. Grasshoppers are a type of herbivorous (plant-eating) insect from an order that includes crickets, mantises, and cockroaches. Our specimens are from a particularly large species of grasshoppers, commonly called "lubbers."

External Anatomy. Get a preserved grasshopper specimen. The bodies of insects are divided into three sections: the head, with many of the sensory organs, the main processors / brain, and the mouth; the thorax, with its six legs (insects characteristically have six and only six true legs) and, in most adults, four wings (although wings have often been adapted as protective covers); and the abdomen, which usually carries the sex organs.

1. Compare. How are the grasshopper's external features similar to those of the crayfish?





2. Contrast. How are the grasshopper's external features different from those of the crayfish?





The Head. Locate the large compound eyes. Arthropod eyes typically don't have one big lens in front of a screen of detectors, like our eyes - they have a layer of small lenses, each with a small set of detectors. This takes up much less space. Grasshoppers do have three single-lens eyes, above and between the bases of the antennae. The antennae are used for other senses, such as touch and some parts of the sense of smell. Take a razor blade and carefully make a small shaving from the surface of the eye. Mount the slice on a microscope slide (dry) and look at it at low and/or medium power, or at high magnification under the dissecting ‘scope.

3. Describe the appearance of your slice of compound eye.





Around the mouth opening are a number of different types of mouthparts. Like a crayfish's chelipeds (and some crayfish mouthparts), several of these are modified legs. Note the somewhat leglike palps that help move food around, the plates that provide a kind of "cheek" cavity, and the tongue-like hypoglottis and grabbing and chewing mandibles in the actual mouth opening.

The Thorax. The thorax is derived from three original segments, each with a pair of legs; the last two, in many forms (but virtually always only in the adult stage) also support wings. The thorax is usually very solidly built, to support the power of the walking and flying muscles. In many insects, the grasshopper included, the front set of wings may be very different from the back set. Compare and contrast the wings in your grasshopper - be sure to spread the back wings. If your specimen is not an adult, the wings are not developed and may be small and difficult to do this with.

4. The front wings:





5. The back wings:





The Abdomen. On the first abdominal segment, just behind and below each wing, is a structure called the tympanum. Humans have a similar structure with the same name and function (but it's not on our abdomens).

6. Describe the grasshopper's tympanum.





7. The tympanum contains many nerves and processors. It picks up vibrations carried in the air. Why?





Grasshoppers, as most insects do, breathe through a tracheal system, an organization of tubes that connect the cells of the body directly with the outside air. The blood carries almost no oxygen. The openings for the system, spiracles, can be seen along the sides of the thorax and abdomen. Also, the reproductive structures are on (and in) the abdomen. Female insects can often be recognized by their ovipositors - structures specialized for laying eggs. Since grasshoppers dig a shallow hole for a packet of eggs, the ovipositor is a large spade / pusher, and only seen on the female.

8. Is your grasshopper male or female?



Grasshoppers - Internal Anatomy.

Clip the wings and, if you wish, the legs from your specimen. Starting from the anus, cut with scissors shallowly along the sides of the body, all the way up each side to the neck. Clip across the top of the neck and gently lift the top half of the body wall off the specimen, making sure that it doesn't take internal organs with it. Inside the thorax of the removed section is the heart, almost impossible to make out on these specimens. The heart gathers blood from the body cavity and squirts it toward the head.

Inside the abdomen are the gonads. If your specimen is an adult, they will be more developed, but they will be present (but not functional) in a subadult specimen. The ovaries look like rows of oval eggs, and testes look like simple glands, often yellowish.

1. Describe your specimen's gonads.





Remove the gonads. Most of what is left is digestive system. From the mouth, a short esophagus leads to a thin-walled storage sac, the crop; the crop leads into a thick-walled but narrow gizzard, where food is ground up; beyond the gizzard is the stomach (midgut), with attached gastric cecae where enzymes and bacteria help break cellulose-rich food down; this leads to an intestine (hindgut) leading to the rectum. Attached to the stomach-intestine junction are tubes that gather wastes from the blood - these malpighian tubules then dump the wastes into the hindgut. Remove the digestive system to answer the next series of questions.

2. Describe the crop.





3. Open up the gizzard and describe the lining. Use a probe to "feel" it.





4. What is the function of the ridges on the gizzard?




Most of what's left in the body cavity are muscles and nerves. Muscles in arthropods are not huge - attached as they are to the inside of the skeleton, they can generate more power with less mass than endoskeletal muscles. As in the crayfish, the nerve cord of the grasshopper runs along the inside of the ventral body cavity. It often looks like a doubled track of white or yellowish strands with flat, round ganglia from which several nerves radiate. Ganglia are processing centers, dealing with complex input (senses) or complex output (coordination of actions).

5. Describe the appearance of the ventral nerve cord in the thorax.






6. Note how big the thorax ganglia are. Why would a grasshopper need such big processors there?

 

 

Arthropods - Part Three. Live Crickets.

The inside of a living insect looks quite a bit different from a preserved specimen. For the live crickets, you'll need a small dissecting pan, a dropper bottle of Insect Ringer's Solution, a pair of fine-tipped scissors, and several pins. You'll also need a dissecting microscope eventually.

Take a live cricket and pin it down through the neck. You may need to clip its legs off to be able to work with it. If the cricket is an adult with full wings, clip them off. The pinning or leg clipping will usually cause some leakage of blood, called hemolymph.

1. Describe the hemolymph.




Starting at the anus, cut very shallowly up the center of the back, all the way up to the neck. Slip pins under the upper body wall, then pivot them so as to open up the body cavity without trapping the organs. As soon as the organs are exposed, get a lot of Insect Ringer's Solution on them. This will keep them from drying out without flooding the cells, which pure water would do. Several structures should be visible - the digestive system tends to be brown, the malpighian tubules lemon yellow, tracheal system silvery and reproductive organs gray or white. If free to move, many of the organs will show some kind of activity.

2. Describe the appearance and activity, if any, of the digestive system.





3. Describe the appearance and activity, if any, of the malpighian tubules.





4. Describe the appearance and activity, if any, of the tracheal system.

 

 

 
     

 

First Written 1990;  Last Update 2008;  Web Version 2001 - 2008,  M. McDarby

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