External Morphology

Head

The head is small, triangular, and held bent downward and backward beneath the body so that the mouthparts point downward and backward rather than forward — this orientation is called a hypognathous head. The head capsule is a hardened cuticular box carrying:

• A pair of compound eyes — large, reniform (kidney-shaped), placed dorsolaterally, giving a wide field of mosaic vision.
• Ocelli — true ocelli are absent in the cockroach; in their place lie two pale, translucent, ocellus-like spots called fenestrae, considered vestigial light-sensitive areas.
• A pair of long, filamentous, many-jointed antennae, arising from membranous antennal sockets in front of the eyes — these are the chief organs of touch and smell, constantly waved about to explore the surroundings.
• The mouthparts, slung from the underside of the head.

Mouthparts

The cockroach has biting-and-chewing type mouthparts, arranged from outermost to innermost as:

1. Labrum — an unpaired, flap-like upper lip hinged to the clypeus; it covers the other mouthparts from above and helps hold and taste food.
2. A pair of mandibles — hard, dark, tooth-edged jaws that cut, crush and grind solid food.
3. A pair of maxillae, each bearing a segmented maxillary palp — these push food toward the mouth and bear sensory palps for tasting.
4. Labium — the fused second maxillae forming a lower lip, bearing a pair of labial palps, which supports the food mass from below.
5. Hypopharynx — a median tongue-like lobe on the floor of the pre-oral cavity, at the base of which the common salivary duct opens; it helps mix saliva with food and aids swallowing.

Thorax and tripod locomotion

The thorax has three segments — prothorax, mesothorax and metathorax — each bearing one pair of jointed walking legs, so the cockroach has three pairs of legs (six legs in all) arising from the ventral thoracic sterna. Each leg is built of the same segments — coxa, trochanter, femur, tibia, and a five-jointed tarsus ending in a pretarsus with claws — giving a strong, spiny grip for fast running.

• The prothorax bears a broad shield-like pronotum but no wings.
• The mesothorax bears the second pair of legs and the front wings (tegmina).
• The metathorax bears the third pair of legs and the hind wings.

While walking or running, the cockroach uses tripod locomotion: the fore- and hind legs of one side move together with the middle leg of the opposite side, forming an alternating triangle (tripod) of support, while the other three legs swing forward. This alternating-triangle gait keeps the body statically stable at every instant and allows very rapid, scuttling movement — the hallmark of the cockroach's escape response.

Wings

Two pairs of wings arise from the meso- and metathorax:

• Tegmina — the front pair, arising from the mesothorax; they are thick, leathery, opaque and vein-marked, and function mainly as protective covers for the delicate hind wings and the abdomen rather than for active flight.
• Hind wings — the rear pair, arising from the metathorax; they are thin, membranous and fan-folded beneath the tegmina at rest, and are the true flight organs, though the cockroach flies only weakly and rarely.

Abdomen

The abdomen is broad, dorsoventrally flattened, and made up of ten segments, each with a dorsal tergum and a ventral sternum joined by soft pleural membrane bearing the abdominal spiracles. At the hind end: …