Cockroach: Morphology and Anatomy

NCERT grounding

The NCERT Class XI Biology chapter on Structural Organisation in Animals uses three representative animals — the earthworm, the cockroach and the frog — to show how the four basic tissue types organise into organs and organ systems across phyla. The cockroach section anchors the arthropod template: a metamerically segmented body covered by a chitinous exoskeleton of N-acetylglucosamine, a schizocoelous body cavity (haemocoel), and an open circulation. Periplaneta americana is the species treated in detail; males reach about 34–53 mm in length, females are slightly shorter and broader.

Because the chapter is anatomical rather than physiological, NEET questions on this subtopic test locations and counts — the segment that bears a structure, the number of pairs of spiracles, the number of heart chambers, the ring at the foregut–midgut junction versus the ring at the midgut–hindgut junction. The discussion below preserves those numbers exactly as the NCERT diagrams give them and as recent NEET answer keys confirm.

External morphology — head, thorax, abdomen

The cockroach body is divisible into three tagmata: a head, a thorax of three segments, and an abdomen of ten visible segments. The whole animal is encased in a hard, dark-brown exoskeleton made of chitinous plates called sclerites, joined by flexible arthrodial membranes. The dorsal sclerite of each segment is the tergum, the ventral one the sternum, and the two lateral plates are pleura. This segmental armouring is what allows the cockroach to be both rigid and mobile.

Head and mouthparts

The cockroach head is built from the fusion of six embryonic segments and hangs almost at right angles to the long axis of the body. It carries the principal sense organs — a pair of large kidney-shaped compound eyes, each made of about 2000 hexagonal ommatidia that together give mosaic vision — and a pair of long, many-jointed antennae that are densely studded with sensory hairs picking up touch and smell. Just behind the antennal sockets, two pale unpigmented spots called fenestrae mark the larval ocelli; though no longer functional, they are a routine NEET identification cue.

The mouthparts are of the typical biting and chewing type. Reading from the front, they are: an upper lip or labrum; a pair of hard, toothed mandibles for cutting and grinding; a pair of maxillae with sensory palps; a lower lip or labium; and a tongue-like median lobe, the hypopharynx, that lies within the pre-oral cavity formed by the other mouthparts. The salivary ducts open at the base of the hypopharynx, so saliva mixes with food before it enters the alimentary canal.

Thorax — legs, tegmina and hindwings

The thorax of three segments — prothorax, mesothorax and metathorax — is the locomotory tagma. Each segment carries one pair of jointed walking legs, giving the cockroach its six-legged gait. The prothorax carries no wings; the mesothorax bears the leathery, opaque forewings called tegmina that fold flat over the body and protect the second pair; the metathorax bears the membranous, transparent hindwings that unfold for flight. Confusing which pair of wings comes from which segment is one of the most repeated NEET traps in this chapter — see the trap box below.

Abdomen and sexual dimorphism

The abdomen has ten segments in both sexes. The 7th sternum in the female is boat-shaped; together with the 8th and 9th sterna it forms the genital pouch that holds the ootheca during egg-laying. The 10th abdominal segment, in both sexes, bears a pair of long jointed filamentous appendages called anal cerci — primarily sensory, picking up air vibrations. In males alone, the 9th sternum carries a pair of short, unjointed, chitinous appendages called anal styles; these are the diagnostic sexual-dimorphism mark and have appeared in NEET keys repeatedly (2018, 2025). A male can be told from a female by the presence of styles, not cerci.

Digestive system

The alimentary canal runs from the mouth on the head to the anus on the last abdominal segment and is divisible into three regions of distinct embryological origin: a chitin-lined foregut, an unlined midgut, and a chitin-lined hindgut. Each region is specialised for a different step of food handling, and NEET routinely asks students to place a structure in the correct region or at the correct boundary.

Two rings of appendages at successive gut boundaries are the most heavily examined detail in this chapter — they look superficially alike but sit at different junctions and have opposite functions. A ring of 6–8 finger-like gastric caeca at the foregut–midgut junction secretes digestive enzymes into the midgut. A ring of 100–150 thin yellow filaments at the midgut–hindgut junction is the Malpighian tubules, the excretory organ. Confusing the two — putting Malpighian tubules at the foregut–midgut junction, or saying gastric caeca are at the midgut–hindgut junction — is the classic NEET trap, asked verbatim in 2021 and again in 2024.

A pair of salivary glands with associated salivary reservoirs lies between the crop and the thoracic body wall. Their ducts unite and open at the base of the hypopharynx, so saliva reaches food before swallowing. Glandular hepatic (liver-like) cells line the midgut and secrete digestive enzymes; absorbed nutrients pass into the haemocoel, which bathes every organ.

Circulatory system

The cockroach has an open circulatory system — blood is not confined to vessels but freely bathes the organs in the body cavity, the haemocoel. The circulating fluid is called haemolymph; it is colourless (no respiratory pigment) and consists of plasma and free phagocytic cells called haemocytes. Because gas transport is handled by the tracheal system, the haemolymph carries no oxygen — it carries nutrients, hormones and wastes.

The body cavity is divided by two transverse muscular partitions, the dorsal diaphragm and the ventral diaphragm, into three sinuses: the dorsal pericardial sinus (containing the heart), the middle perivisceral sinus (containing the viscera), and the ventral perineural sinus (containing the nerve cord). Haemolymph flows from the pericardial sinus through ostia into the heart, is pumped forward through the aorta to bathe the head, then percolates back through the perivisceral and perineural sinuses to the pericardial sinus.

Respiratory system

Cockroaches breathe through a system of branching, air-filled tubes — the tracheae — that ramify from the body surface to every tissue, including muscle fibres and the brain. Atmospheric air enters through ten pairs of small lateral openings called spiracles: 2 pairs on the thorax and 8 pairs on the abdomen. Each spiracle can be closed by a valve to limit water loss. From the spiracles the tracheae give off finer branches called tracheoles, which deliver oxygen directly to the cells by diffusion; carbon dioxide leaves by the same route. Because the tracheal tree provides direct cellular gas exchange, the haemolymph plays no role in oxygen transport — a key contrast with vertebrates and with the earthworm.

Excretory system

The cockroach is a terrestrial uricotelic insect — it excretes nitrogenous waste mainly as uric acid, which conserves water. Excretion is carried out by a constellation of structures, of which the Malpighian tubules are the principal organ. A ring of 100–150 thin yellow Malpighian tubules at the midgut–hindgut junction projects freely into the haemocoel; their cells absorb nitrogenous waste from the surrounding haemolymph, convert it to uric acid and discharge it into the hindgut to leave with the faeces.

Nervous system & sense organs

The nervous system is the segmented ladder-like ventral plan typical of arthropods. A pair of cerebral ganglia, fused into the supra-oesophageal ganglion or "brain", lies above the oesophagus and innervates the eyes and antennae. Two circum-oesophageal connectives loop around the gut to a sub-oesophageal ganglion below it, which controls the mouthparts. From the sub-oesophageal ganglion runs a paired, fused ventral nerve cord that carries 3 thoracic ganglia (one per thoracic segment) and 6 abdominal ganglia. Each segmental ganglion locally controls its own segment — which is why a decapitated cockroach can still walk for some time.

Sense organs include the antennae (touch, smell), the compound eyes (mosaic vision), the anal cerci (vibration, escape reflex), and chemosensory hairs scattered over the cuticle. The fenestrae behind the antennae are remnants of larval ocelli and are non-functional in the adult.

Reproductive system & ootheca

Cockroaches are dioecious with internal fertilisation. The male reproductive system consists of a pair of pale ovoid testes lying on the 4th to 6th abdominal segments. From each testis a thin vas deferens arises and opens, together with the ducts of an accessory mushroom-shaped gland, into a common ejaculatory duct that runs through the median aedeagus (penis) and emerges between the anal cerci. The mushroom gland and the associated utricular glands secrete the proteinaceous outer coat of the spermatophore — the parcel of sperm transferred to the female. A short row of small chitinous hooks (gonapophyses) around the genital opening assists copulation.

The female reproductive system has a pair of ovaries lying laterally in the 2nd to 6th abdominal segments. Each ovary is made of eight ovarian tubules (ovarioles), each containing a linear row of developing ova. Short paired oviducts from the ovaries unite into a single median oviduct (the vagina) that opens into the genital pouch formed by the 7th, 8th and 9th sterna. A small sac-like spermatheca on the 6th segment receives and stores sperm from the male spermatophore. Surrounding the genital pouch are paired collaterial glands whose secretion hardens around each fertilised egg-mass to form the ootheca — the dark, capsule-like egg-case that is carried for some time protruding from the abdomen before being deposited in a crevice. A single ootheca contains 14–16 eggs; one female typically produces 9–10 oothecae in her life.

Worked examples

Common confusion & NEET traps