The body of a cockroach and how it feeds · Biology… | StemPy Lab
Course 32.3·Lesson 1 of 3
The body of a cockroach and how it feeds
The cockroach is a small animal that is found in kitchens, and it is also a fine example of how a body is built. This lesson walks around its body from head to tail, then follows the food through its alimentary canal and names the parts of the foregut, the midgut and the hindgut.
about 30 min1 simulation1 practice question
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Try the code and the simulations beside the lesson.
Why we study a cockroach
Most of this chapter is about tissues. A tissue is only a part of the story. To see how tissues, organs and systems fit together, we study one whole animal. NCERT and NEET choose the cockroach, and A-Level and IB often use an insect as an example as well.
The common cockroach is Periplaneta americana. It belongs to the class Insecta of the phylum Arthropoda. Arthropods have jointed legs and a hard outer skeleton. Cockroaches are brown or black, with some species that are red, yellow or green. Sizes run from about half a centimetre to more than seven centimetres, depending on the species. They hide in the day, and they are active at night, so they are nocturnal.
The outside of the body
The body is segmented and has three regions: the head, the thorax and the abdomen. A hard chitinous exoskeleton covers all of it. In each segment, the hard plates are called sclerites. The plate on the back is a tergite and the plate on the belly is a sternite. A thin flexible articular membrane joins them, so the body can bend.
A cockroach drawn from the side, with the head on the left. The body has three regions marked by brackets: the head, the thorax with three segments, and the abdomen. The head carries a long thread-like antenna, a compound eye and the mouth parts. The thorax has three pairs of walking legs. A leathery forewing, the tegmen, lies over a membranous hind wing. At the rear of the abdomen a pair of anal cerci points backwards. Labels name each part.
The head. It is triangular. It lies at a right angle to the long axis of the body and is joined to the thorax by a flexible neck. NCERT says that it is formed by the fusion of six segments. It carries:
A pair of compound eyes. Each one has about 2000 hexagonal units called ommatidia. The cockroach sees many small images of an object. This is mosaic vision. It is sensitive in dim light but does not show fine detail.
A pair of long, thread-like antennae, which sense the surroundings.
The mouth parts: the labrum (upper lip), a pair of mandibles, a pair of first maxillae, the labium (lower lip, made of the fused second maxillae) and the hypopharynx, which acts as a tongue. They are of the chewing and biting type.
The thorax. It has three segments: the prothorax, the mesothorax and the metathorax. Each bears a pair of walking legs, so there are three pairs of legs. The mesothorax carries the first pair of wings and the metathorax carries the second pair. The forewings are called tegmina. They are dark and leathery, and they cover the hind wings at rest. The hind wings are transparent and membranous, and they are used for flight.
The abdomen. It has ten segments in both sexes. At the tip is a pair of jointed anal cerci. A male also has a pair of short anal styles, which a female does not have.
The alimentary canal
Follow a piece of bread. It is chewed by the mouth parts and mixed with saliva from a pair of salivary glands. Then it moves through a canal that has three regions.
The alimentary canal of a cockroach drawn as one tube from left to right with the mouth on the left. In order: mouth, pharynx, oesophagus, a balloon shaped crop, a rounded gizzard with six small teeth inside, a short midgut with a ring of finger-like hepatic caeca at its start, a fringe of many thin Malpighian tubules at the end of the midgut, then ileum, colon, rectum and anus. Brackets mark the foregut, the midgut and the hindgut. A pair of salivary glands lies beside the foregut.
Foregut. The mouth leads to a tubular pharynx and then a narrow oesophagus. The oesophagus opens into the crop, which stores food. Next is the gizzard, or gastric mill. It has thick circular muscles and a hard inner lining that forms six chitinous plates like teeth. The gizzard grinds the food. The whole foregut is lined with cuticle.
Midgut. A ring of six to eight blind tubes, the hepatic caeca (gastric caeca), lies at the junction of the foregut and the midgut. They secrete digestive juice. Digestion is completed in the midgut, and the food is absorbed.
Hindgut. It has the ileum, the colon and the rectum. The rectum opens to the outside through the anus. At the junction of the midgut and the hindgut there are 100 to 150 thin yellow Malpighian tubules. They are not part of digestion. They remove waste, as the next lesson shows.
Notice the name of the hepatic caeca. Hepatic means of the liver, but the cockroach has no liver. The caeca do some of the work that a liver and a pancreas do for us.
Notice that the gizzard is a short stop and the hindgut is a long one. The sim uses made up times. Real times depend on the food and on the temperature.
Try it in Python
The program counts the segments of the body, and the legs and wings.
Practise with new numbers
Common mistakes
Saying that the head has three segments. NCERT says six fused segments. The thorax has three.
Giving the cockroach eight legs. It has six, in three pairs, all on the thorax.
Saying that the wings are on the abdomen. They are on the mesothorax and the metathorax.
Mixing up the order of the foregut. Mouth, pharynx, oesophagus, crop, gizzard.
Putting the Malpighian tubules in the foregut. They lie at the junction of the midgut and the hindgut.
Thinking that the hepatic caeca are a liver. They are blind tubes that secrete digestive juice.
Remember these
Insecta, Arthropoda. Head (six segments fused), thorax (three), abdomen (ten).
Three pairs of legs, two pairs of wings (tegmina and membranous hind wings). Anal cerci in both sexes, anal styles in the male.