Dissection of the following is required in A-level.
- Toad frog (Amphibian)
- Cockroach (Insects)
Aims for dissection
- To strengthen one’s understanding of general anatomy of these organisms, how their organs function and interrelationship between position (location), shape, etc
- Train student to be able to follow instructions, handle apparatus or instruments, draw and label.
Mastering skills of dissection
Dissect the actual specimens and display the systems draw and label as many things as possible.
Precautions to good dissections
- Always use clean sharp cutting instruments
- Stick to the instructions and learn to memorize them incase they are not provided.
- Avoid bleeding incase of rats and frogs by being careful and use right instruments.
- Incase of bleeding, mop up blood immediately with cotton wool. Do not panic but be speedy and accurate.
- Do not cut anything until you are sure that what it is.
Always dissect along and not across. After dissection, draw.
- Use a sharp Hb pencil always, this gives drawing, of uniform thickness.
- The drawing must not be artistic but should be biological, i.e. do not shade or colour your diagrams.
- Pay attention to position, number, sizes, proportions, and shapes of each component part of the specimen you are drawing. Marks will be lost without this; e.g. where joint exist, pay attention to their shapes as of the insect legs, antennae etc.
- Parts attached to major specimen e.g. legs, hairs; spines should be drawn touching without any gap between them and the specimen. The gaps left appear to be hanging.
- Make sure you are drawing the right view of system i.e. dorsal, ventral, anterior, posterior, lateral, left or right of the specimen etc.
- Draw directly without first making a sketch.
- Always produce an outline of the whole animal or part of it in the region dissected, this shows the location of the area dissected.
A smooth outline no gaps/blockages, weary outlines earn you no marks
Use of single lines
These are used for drawing nerves.
Size of drawing
Use three quarters of the space provided.
The title and labeling
Every drawing must have a title obtained from the instructions. It must be specific, relevant and state the specimen e.g. if the specimen is called specimen X, stick to using X. Use label lines i.e. but not arrowss i.e.
Technical terms must be correctly spelt or interpreted e.g. nucleus vs mitochondrion vs mitochondria, ceacum vs ceaca.
Vessels that suck blood to (arteries), vessels that drain blood from.
- Do not erase labels.
- Label in singular if the label is touching one part and in plural if touching more than one.
- Do not label in the drawing i.e. like in maps, if this is done all the others are erased.
- Do not label what is not part of the question e.g. if required to draw the arterial system of the left limb; if you label that of the right, all will b crossed and you earn a mark for the drawing only.
- Going beyond what is wanted earns you zero. If requested to label any 5, do not exceed.
The magnification must be stated which must have a bearing on the space given.
Use three-quarters of the space for drawing which must be X 1 or X 2 for a rat, and X1, X2, X3 for a toad, and X4-5 for a cockroach.
– Take into consideration of everything in the drawing e.g. lines, labels, etc.
Maximum time spent on drawing = 15 minutes. The rest 45 minutes labeling correctly.
Adventurism should be encouraged.
In summary, marks are awarded in the following for any drawing represented at the dissection;
- Drawing = Correct view, proportion, Number, position, correct size, sharp pencil, outline, continuous lines.
- Labeling = Any labeled organ not part of the required system will make you lose all the marks for other labels i.e. you earn zero.
Transport. Unless you are comparing 2 or more statements, do not write negative statements if given only specimen eg lacks tail.
The following are important.
- Posture and relative numbers} Topographical relative position.
- Size (volume, shape, length, breath, surface area).
- Give exact measurements.
- Appendgages (numbers, pattern, etc)
- Nature of surfaces and borders (ie smoothness/roughness/stiffness stickiness
- Burnt/sharp border
- Warty (with secreral swellings etc.
- Always differentiate between structural and functional differences.
- A lot of practice at drawing and labeling.
Posterior dorsal view of a male cockroach
- Head features
- Oval, flat heat.
- A pair of large black compound eyes.
- A pair ofd very long slender jointed antennae (sensitive to touch, smell and vibrations)
- White oval structures near bases of antennae known as tenestrae.
- It is composed of 6 segements, fused together, ie the 2 epicranial plates on top and back, the frons, clypeus, covering the front and 2 genae besides.
- Has 5 mouth parts, maxilla with long jointed maxillary palps, for holding food.
- Mandibles with toothed edges for chewing.
- Labrum (upper lip)
Anterior view of the head of a cockroach.
S.5 Biology practicals
Dissection of cockroach
- Classify as far as possible, with the cockroach given.
2a. Spread out one pair of the wings, make an accurate size
Drawing of all the external features and label.
- State the structural features that adapt the specimen to its
mode of life.
3a. State the features that can be used to differentiate between the
sexes of the specimen.
- Draw the posterior end of a male and female cockroach to show
those structures used to identify the sexes in 3a) above.
- State the structural difference between the styles and anal cera.
5a. How many segments in each leg made of? Name of segments.
- Detouch the left hind leg of the cockroach. Draw and label.
- Measure the length of the fore and hind leg. State the ratio of
length of hind leg to fore leg.
- What is the significance of the ratio?
7a. How many segments is the thorax made up of? Name them.
A drawing of the external features of one pair of the wings of the cockroach.
Outer wing (Fore) Inner wing (Hind)
Smaller surface Large surface
Oval shaped Triangular shaped.
Functions of parts of the alimentary canal and zoology.
During chewing; the food is mixed with saliva, brought into the pre-oral cavity by the common salivary duct.
Salivary amylase converts starch receptacle.
Enzymes from the gut also come into the crop through the gizzard for digestion of some fats and proteins. The gizzard helps to masticate the food further by chitinours teeth and is filtered by the bottles.
The food, here mixes with secretions from the mid-gut lining and from the hepatic/digestive caroa which contains many enzymes e.g.
- Protedytic enzymes which convert proteins to amino acids.
- Lipase which breaks fats into fatty acids and glycerd.
- Amylase which completes digestion of starch into finally product glucose (reducing sugar).
Ileum. The end of the mid gut whose lining absorbs digested food.
…..gut (color). The undigested food passes into the hind gut tat absorbs H20.
- The last part of the hind gut helps to absorb further more water changing it almost into solid faeces.
- The faeces are temporarily stored in the rectum from where it is passed out at intervals through the anus.
NB Malphigian tubules are excretory organs attached to the ileum responsible for uric acid excretion but is not part of the alimentary canal.
The water in the uric acid can further be absorbed by the rectal glands in rectum to produce solid uric acid.
Mouth parts of cockroach under light microscope/hand lens after separation.
Functions of major mouth parts and adaptations.