Reproduction in a cockroach, with the earthworm and the… | StemPy Lab
Course 32.3·Lesson 3 of 3
Reproduction in a cockroach, with the earthworm and the frog compared
A cockroach starts life as a tiny egg inside a capsule, and grows by shedding its skin. This lesson follows the reproduction of the cockroach from the reproductive organs to the egg capsule and the nymph. Then the earthworm and the frog are set beside it, in a table.
about 25 min1 simulation1 practice question
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Try the code and the simulations beside the lesson.
Male and female
The cockroach is dioecious, which means that the sexes are separate. The male and the female differ in the reproductive organs and also in the tip of the abdomen, where the male has anal styles.
The male.
A pair of testes lie in the fourth to sixth segments of the abdomen.
A thin vas deferens leaves each testis, and runs through a seminal vesicle into the ejaculatory duct. The duct opens at the male gonopore, below the anus.
A mushroom shaped gland in the sixth and seventh segments is an accessory gland.
The sperms are stored in the seminal vesicles. They are glued together into bundles called spermatophores, and these are passed on in mating.
The female.
A pair of large ovaries lie in the second to sixth segments. Each ovary has eight ovarioles, which are tubes with a chain of developing eggs.
The oviducts of the two sides join into one median oviduct, which opens into the genital chamber.
A pair of spermathecae in the sixth segment open into the genital chamber. They store the sperms.
From egg to adult
Fertilisation is internal. The fertilised eggs are enclosed in a hard capsule called an ootheca. The ootheca is dark reddish to blackish brown. A female produces about nine to ten oothecae, each with about fourteen to sixteen eggs. She drops or glues them in a safe place, such as a crack.
The young one that hatches is a nymph. It looks like a small adult but has no wings, and it cannot breed. It grows by moulting, which is the shedding of its exoskeleton. A nymph moults about thirteen times before it is an adult. The last nymph stage before the adult has wing pads, and the adult has wings. This kind of development, in which the young looks like the adult and there is no pupa, is called paurometabolous development. It is a gradual metamorphosis.
Notice that the eggs are many, but only a few reach the adult stage. This is the usual pattern for animals that leave their eggs without care.
The earthworm and the frog
NCERT teaches the earthworm (Pheretima posthuma) and the frog (Rana tigrina) in the same chapter. The NEET text for this unit lists the cockroach and not these two, so what follows is for the boards, A-Level and IB. The table puts the three animals side by side.
Group: the cockroach is an insect (Arthropoda). The earthworm is an annelid. The frog is an amphibian (Chordata).
Body: the cockroach has a head, thorax, abdomen and a chitinous exoskeleton. The earthworm is a long segmented tube with a thick ring, the clitellum. The frog has a head and trunk, moist skin and no neck.
Blood system: open with haemolymph in the cockroach. Closed with haemoglobin in the plasma in the earthworm. Closed with a three-chambered heart in the frog.
Breathing: tracheae and spiracles in the cockroach. The moist skin in the earthworm. The skin, the mouth lining and the lungs in the frog.
Waste: Malpighian tubules in the cockroach, nephridia in the earthworm and kidneys in the frog.
Nervous system: a brain and a ventral nerve cord with ganglia in the cockroach. A nerve ring round the pharynx and a ventral nerve cord in the earthworm. A brain, a spinal cord and ten pairs of cranial nerves in the frog.
Sexes: separate in the cockroach and the frog. The earthworm is hermaphrodite.
Fertilisation: internal in the cockroach. Internal, in a cocoon, in the earthworm. External, in water, in the frog.
Young: a nymph in the cockroach. Direct development with no larva in the earthworm. A tadpole larva in the frog, and then metamorphosis.
Three differences are worth remembering. The cockroach has an open blood system and the other two have a closed one. The cockroach breathes with tracheae, the earthworm through its skin and the frog with three surfaces. The frog has red blood cells with a nucleus.
Try it in Python
The program stores the table of three animals and finds which ones have a closed blood system.
Practise with new numbers
Common mistakes
Saying that the cockroach is hermaphrodite. The sexes are separate. The earthworm is hermaphrodite.
Mixing up the ovary and the ovariole. Each ovary has eight ovarioles.
Saying that the nymph has wings. The nymph has no wings. Wing pads show before the last moult.
Saying that the ootheca is the egg. It is a capsule that holds many eggs.
Saying that a cockroach has a larva and a pupa. The nymph grows into the adult with no pupa.
Giving the frog a four-chambered heart. It has three chambers, two atria and one ventricle.
Remember these
Dioecious. Male: testes, vas deferens, seminal vesicle, ejaculatory duct, gonopore, spermatophores. Female: two ovaries with eight ovarioles each, genital chamber, spermathecae.
Ootheca: about 9 to 10 per female, with about 14 to 16 eggs each. Nymph: about 13 moults. Paurometabolous.