Structural Organisation in Animals – Detailed Study Notes
COCKROACH (Periplaneta americana)
Morphology
The American cockroach, Periplaneta americana, belongs to Class Insecta. Its coloration is typically reddish-brown to dark brown; tropical species can be yellow, red, or green based on environment and subtype.
The size of Periplaneta americana ranges from approximately (2 to 3 inches) in length. Males are long, with wings extending beyond the abdomen.
Body is covered by a hard chitinous exoskeleton (brown). In each segment, hardened plates called sclerites (tergites dorsally and sternites ventrally) are joined to each other by a thin and flexible articular membrane (arthrodial membrane).
Body divisions: The body is divided into three main regions: the head, thorax, and abdomen.
Head – This is a small, triangular structure optimally positioned at a right angle to the body axis. Formed by the fusion of six segments, it offers structural rigidity. A flexible neck enhances its mobility for exploration and food gathering.
Compound eyes: Each compound eye has approximately ommatidia. This multi-lensed structure provides mosaic vision, sensitive to movement and dim light (effective for nocturnal activity) but with low resolution compared to vertebrate vision.
Antennae – These are long, slender, multi-segmented filamentous structures on the head. They are primary sensory organs with chemoreceptors (smell) and tactile receptors (touch), crucial for navigation, food detection, and social interaction.
Mouth parts (biting & chewing): Adapted for biting and chewing, allowing consumption of various food sources. They include:
Labrum (upper lip): Covers the mandibles.
Mandibles: Strong, chitinous crushing jaws with serrated edges for grinding food.
Maxillae: Paired appendages (first and second) assisting in food manipulation, tasting, and antennae cleaning. They have maxillary palps.
Labium (lower lip): Formed by fused second maxillae, it helps hold food during chewing. It has labial palps.
Hypopharynx (tongue-like): A median, flexible structure within the mouth, similar to a tongue.
Thorax – Comprises three segments, each bearing a pair of walking legs: pro- (anterior), meso- (middle), and meta- (posterior) thorax. Each segment is robust for locomotion.
Wings – The cockroach has two pairs of wings:
Forewings (tegmina) – Opaque, leathery, dark, and protective. They originate from the mesothorax and cover hindwings at rest, providing physical protection but not used for flight.
Hindwings – Thin, transparent, and membranous. They originate from the metathorax and are primarily used for flight.
Abdomen – The abdomen is broad and has 10 segments in both sexes. It houses most digestive and reproductive organs.
Female: The 7th sternum (ventral plate) is boat-shaped; along with the 8th and 9th sterna, it forms a large brood or genital pouch. This pouch contains the female gonopore, spermathecal pores, and collateral glands (producing ootheca material).
Male: The male genital pouch is at the hind end of the abdomen, bounded dorsally by the 9th and 10th terga and ventrally by the 9th sternum. This pouch contains the anus, male gonopore, and gonapophyses (chitinous, asymmetrical structures, also called phallomeres, involved in copulation and spermatophore transfer). Anal styles, short, unjointed filamentous structures, are present only in males, originating from the 9th sternum.
Anal cerci: Both sexes have a pair of jointed, filamentous anal cerci on the 10th segment. These highly sensitive structures detect vibrations and air currents, aiding rapid escape responses.
Anatomy & Systems
Digestive System
The alimentary canal is a long tube from mouth to anus, divided into foregut, midgut, and hindgut.
Foregut: Begins at the mouth, leading into a short pharynx, then a narrow, coiled oesophagus. The oesophagus opens into a large, sac-like crop for temporary food storage. Food then passes into the gizzard (proventriculus), a muscular structure with six inner chitinous plates (denticles) for grinding. The foregut is cuticle-lined for protection.
Hepatic/Gastric caeca: At the foregut-midgut junction, 6-8 finger-like, blind tubules secrete digestive juices (enzymes).
Midgut (mesenteron): A relatively short, narrow tube where principal digestion and absorption occur. Lacks cuticle lining for efficient absorption.
Malpighian tubules: At the midgut-hindgut junction, a ring of fine, yellow, filamentous Malpighian tubules are primary excretory organs, removing nitrogenous waste from haemolymph. Cockroaches are uricotelic, excreting uric acid.
Hindgut: Broader than midgut, differentiated into anterior ileum, middle colon, and posterior rectum (opening via anus). Crucial for water reabsorption and faeces elimination.
Circulatory System
The circulatory system is open, meaning haemolymph flows freely in the body cavity, the haemocoel, bathing organs directly. Haemolymph consists of clear plasma and haemocytes but lacks respiratory pigment, thus not transporting oxygen.
Heart: An elongated, muscular, dorsal tube along the mid-dorsal line of thorax and abdomen. Divided into funnel-shaped chambers, each with paired lateral ostia. Ostia are valvular, allowing haemolymph entry but preventing backflow. Rhythmic contractions, aided by pairs of triangular alary muscles, pump haemolymph forward into an anterior aorta, distributing it throughout the haemocoel.
Respiratory System
Respiration occurs via a network of chitinous tubes called tracheae. These open to the exterior through pairs of small, segmental spiracles laterally (2 pairs on thorax, 8 on abdomen). Tracheae branch into finer tracheoles, directly delivering oxygen to cells and removing carbon dioxide. Spiracle opening and closing are regulated by sphincters, controlling gas exchange and minimizing water loss.
Excretory System
Primary excretory organs are Malpighian tubules. These glandular, ciliated structures absorb nitrogenous waste from haemocoel, convert it to uric acid, and transport it to the hindgut for elimination. Uric acid excretion conserves water, essential for terrestrial insects.
Other assisting structures include the fat body (diffuse tissue with stored fat, glycogen, protein, acting as metabolic/excretory organ), nephrocytes (specialized haemocytes storing waste), and uricose glands (in males, associated with mushroom gland, involved in spermatophore formation and uric acid excretion).
Nervous System & Sense Organs
The nervous system consists of segmented ganglia joined by paired longitudinal connectives on the ventral side. There are three large thoracic ganglia (one per thoracic segment) and six smaller abdominal ganglia (one per first six abdominal segments), forming a ventral nerve cord.
The head houses a relatively small supra-oesophageal ganglion, often called the brain. This ganglion integrates sensory information from the antennae and eyes, and controls the mouthparts. It is connected to the rest of the ventral nerve cord by a pair of circum-oesophageal connectives.
Reproductive System
Cockroaches are dioecious and have well-developed reproductive organs.
Male reproductive system:
Consists of a pair of testes, one on each lateral side in the 4th-6th abdominal segments.
From each testis arises a thin vas deferens, which opens into the ejaculatory duct through a seminal vesicle.
The ejaculatory duct opens into the male gonopore, situated ventral to the anus.
A characteristic mushroom-shaped gland is present in the 6th-7th abdominal segments, functioning as an accessory reproductive gland.
External genitalia are represented by male gonapophysis or phallomere (chitinous, asymmetrical structures surrounding the male gonopore.
The sperms are stored in the seminal vesicles and are glued together in the form of bundles called spermatophores which are discharged during copulation.
Female reproductive system:
The female reproductive system comprises two large ovaries positioned laterally in the 2nd to 6th abdominal segments.
Each ovary contains eight ovarian tubules (ovarioles) with developing ova. The oviducts from each ovary unite to form a single median oviduct (vagina), which opens into the genital chamber.
Additionally, a pair of spermatheca is located in the 6th segment, also opening into the genital chamber.
Sperms are transferred via spermatophores, and fertilized eggs are encased in dark reddish-brown capsules called oothecae, about 8 mm long.
Females typically produce 9-10 oothecae, each containing 14-16 eggs, which are dropped or glued in humid crevices near food.
Periplaneta americana undergoes paurometabolous development, meaning it has nymphal stages that resemble adults. Nymphs molt about 13 times to reach adult form; the stage before adulthood has wing pads, but only adults possess full wings.
They are pests because they spoil food and contaminate it with their smelly excreta. They can transmit a variety of bacterial diseases by contaminating food material.