The Cockroach, with the Earthworm and the Frog: Quick Revision

A brief note on The Cockroach, with the Earthworm and the Frog. Read it before a test. Try each question at the end before you open its answer.

The body and the gut

• Class and phylum: Insecta, Arthropoda. Head (six segments fused), thorax (three), abdomen (ten).
• Appendages: three pairs of legs, two pairs of wings (tegmina and membranous hind wings), anal cerci in both sexes, anal styles in the male.
• Eyes: two compound eyes, each with about 2000 ommatidia. Mosaic vision.
• Foregut: mouth, pharynx, oesophagus, crop, gizzard. Midgut with hepatic caeca. Hindgut: ileum, colon, rectum. Malpighian tubules at the junction of midgut and hindgut.

Blood, air, nerves and senses

• Open blood system: haemolymph (colourless plasma and haemocytes), a tubular heart with funnel shaped chambers and ostia.
• Air tubes: ten pairs of spiracles (two thoracic, eight abdominal), tracheae and tracheoles.
• Nerves: brain (supra-oesophageal ganglion), sub-oesophageal ganglion, ventral cord with nine ganglia (three thoracic, six abdominal).
• Senses: antennae, eyes, palps, anal cerci.

Reproduction

• Sexes: separate. Male: testes, vasa deferentia, seminal vesicle, ejaculatory duct, spermatophores. Female: two ovaries with eight ovarioles each, spermathecae, genital chamber.
• Eggs: in oothecae, about 9 to 10 of them, each with about 14 to 16 eggs.
• Young: a nymph, which moults about 13 times. Gradual development, no pupa.
• Beyond NEET: the earthworm has closed blood, skin breathing and nephridia. The frog has a three-chambered heart and external fertilisation.

A worked example

A female makes 6 oothecae, each with 14 eggs. How many eggs does she lay?

1. Each capsule holds 14 eggs.
2. The number of eggs is $6 \times 14 = 84$.

Practice with new numbers

A program to try

The program counts the spiracles of a cockroach.

Mistakes to avoid

• Saying that the haemolymph carries oxygen. The tracheae do.
• Giving the cockroach eight legs. It has six.
• Saying that the hepatic caeca are at the end of the midgut. They are at the start.
• Calling the nymph a larva with a pupa. There is no pupa.

Check yourself

1. How many ganglia are in the nerve cord of a cockroach?

2. What is an ootheca?

3. How many pairs of spiracles are in the thorax?

4. Which part grinds the food?

Sources

• OpenStax Biology 2e, chapter 15, Diversity of Animals.
• NCERT (2023). Biology, Textbook for Class XI, chapter 7, Structural Organisation in Animals.