External Morphology

Shape and size. The cockroach has an elongated body that is bilaterally symmetrical (the left and right halves mirror each other) and dorso-ventrally flattened (flattened from top to bottom rather than side to side), which lets it slip into narrow cracks. Cockroaches are triploblastic (built from three embryonic germ layers), eucoelomate (they possess a true, lined body cavity) and truly segmented animals. Their body cavity is not a typical coelom filled only with coelomic fluid — because the circulatory system is of the open type, the cavity is filled with blood and is therefore specifically called the haemocoel.

Coloration. The body is glistening brown to reddish-brown in colour, which along with its flattened shape gives it excellent camouflage among wood, soil and household debris.

Exoskeleton. The entire body is covered by a hard, waxy, tough, non-living exoskeleton made of chitin, a nitrogenous polysaccharide. This exoskeleton gives the body strength and a degree of elasticity, and it also provides the surface area needed for muscles to attach to for movement. Each body segment is covered by four chitinous plates called sclerites: a dorsal tergum on top, a ventral sternum below, and two lateral pleura connecting them on either side.

Body division. The body is divided into three clear regions — head, thorax and abdomen. The head is set at right angles to the thorax and is joined to it by a thin, narrow, flexible neck or cervix, which is what allows the head to swivel and move independently of the rest of the body.

Head. The head is formed by the fusion of six embryonic segments into a single triangular or oval structure. Because of its flexible neck it is highly mobile. It carries a pair of long antennae, a pair of compound eyes, and mouthparts adapted for biting and chewing food. A pair of pale, ocellus-like fenestrae (ocellar spots) sits at the base of each antenna.

Antennae: also called feelers, these are long, thread-like (filamentous), many-segmented structures that can bend and move freely in almost any direction. Each is anchored in a membranous pit called an antennal socket. Antennae are dual-purpose sense organs — they are tactile (sensitive to touch) as well as olfactory (able to smell), which helps the cockroach locate food in its surroundings even in darkness.

Compound eyes: a pair of dark, kidney-shaped structures placed on either side of the head. Each is built from a very large number of individual visual units called ommatidia (singular: ommatidium). Every single ommatidium independently forms a tiny image of just a small part of the field of view; put together, all these partial images combine into one composite, mosaic-like picture for the cockroach — a form of vision more sensitive to movement than sharp in fine detail.

Mouthparts: in front of the true mouth is a small pre-oral cavity where food is first received, bounded on all sides by chewing-and-biting mouthparts. These are movable, jointed appendages that work together to take in food:

• Labrum (upper lip) — a single flap that covers the mouth from above, forming the front wall of the pre-oral cavity and helping to hold food in place while feeding.
• Mandibles (true jaws) — a pair of dark, hard, chitinous structures with serrated (toothed) inner margins, sitting just behind the labrum. Powered by adductor and abductor muscles, they move sideways against each other to cut and crush food.
• Maxillae (accessory jaws / first maxillae) — a pair lying behind the mandibles, each built from several small plates (cardo, stipes, galea, lacinia) plus a sensory maxillary palp. The maxillae hold food steady for the mandibles to chew, and are also used to groom and clean the antennae and front legs.
• Labium (lower lip / second maxilla) — a fused structure that closes off the pre-oral cavity from below, firmly attached to the back of the head. Its three-jointed, sensory labial palps help push chewed food further in and stop food from spilling out from between the mandibles while chewing.
• Hypopharynx (tongue/lingua) — a single, roughly cylindrical structure lying in front of the labium, between the first pair of maxillae. The salivary duct opens at its base, and its comb-like side plates (super-linguae) help mix saliva into the food as it is chewed.

Thorax. The thorax has three segments — the front prothorax, the middle mesothorax and the rear metathorax. Each segment carries one pair of walking legs on its underside, and the meso- and metathorax additionally each carry one pair of wings on top.

Legs: the three pairs of walking legs are attached ventrally, one pair per thoracic segment. Every leg is built from five podomeres (segments) in sequence — coxa (nearest the body), trochanter, femur, tibia, and finally the tarsus. The tarsus itself is subdivided into five small, movable tarsomeres, and its last tarsomere ends in a pair of claws plus a soft, cushion-like pad called the arolium that helps the cockroach cling to smooth or vertical surfaces.

Wings: two pairs sit on the dorsal side. The forewings (tegmina) are the first pair — dark, thick, opaque and leathery, and function mainly as a protective cover rather than for flight. The hindwings are thin, broad, membranous, delicate and transparent, attached to the tergum of the metathorax; because these are the pair actually used for flying, they are called the "true wings." …