Comprehensive Study Notes on Earthworms and Cockroaches (Structural Organisation in Animals)

General Characteristics and Classification of Cockroaches

• Taxonomy: Cockroaches belong to the Phylum Arthropoda and the Class Insecta.

• Coloration: Typically, they are brown or black-bodied animals. However, in tropical regions, brightly colored varieties including yellow, red, and green have been documented.

• Dimensions: Their size varies significantly, ranging from $\frac{1}{4}\,\text{inches}$ to $3\,\text{inches}$ ($0.6-7.6\,cm$).

• External Features: They are characterized by long antennae, specialized legs, and a flat extension of the upper body wall that functions to conceal the head.

• Habitat and Behavior: They are nocturnal omnivores that prefer damp environments. Due to their tendency to inhabit human homes, they are classified as serious pests and vectors for numerous diseases.

Morphology of the Common Cockroach (Periplaneta americana)

• Body Size: Adults of the species Periplaneta americana typically measure between $34-53\,mm$ in length.

• Wing Characteristics in Males: In males, the wings possess a specific length that allows them to extend beyond the tip of the abdomen.

• Body Segmentation: The body is divided into three primary regions: the head, the thorax, and the abdomen.

• Exoskeleton: The entire organism is protected by a hard, brown, chitinous exoskeleton.

• Sclerites: Each segment of the exoskeleton contains hardened plates known as sclerites.

  • Tergites: These are the sclerites located dorsally (on the back).

  • Sternites: These are the sclerites located ventrally (on the belly).

• Arthrodial Membrane: The sclerites of each segment are interconnected by a thin, flexible articular membrane called the arthrodial membrane.

Detailed Head Anatomy and Mouthparts

• Shape and Orientation: The head is triangular and is positioned at a right angle to the longitudinal axis of the body.

• Formation: It is formed through the fusion of six embryonic segments.

• Mobility: The presence of a flexible neck allows for extensive mobility in all directions.

• Sensory Appendages:

  • Compound Eyes: A pair of compound eyes are located on the head capsule.

  • Antennae: Arising from membranous sockets in front of the eyes, these thread-like structures contain sensory receptors used to monitor the external environment.

• Mouthpart Structure: The mouthparts are of the biting and chewing type, located at the anterior end. Components include:

  • Labrum: The upper lip.

  • Mandibles: A pair of appendages used for grinding and incising food.

  • Maxillae: A pair of accessory jaws.

  • Labium: The lower lip.

  • Hypopharynx: A median flexible lobe located within the mouth cavity that functions as a tongue.

Thoracic Structure, Wings, and Locomotion

• Thoracic Segments: The thorax is subdivided into three distinct parts: the prothorax, the mesothorax, and the metathorax.

• Connection to Head: The head is attached to the thorax via the neck, which is technically a short extension of the prothorax.

• Walking Legs: Every thoracic segment bears exactly one pair of walking legs.

• Wing Distribution:

  • Forewings (Tegmina): These arise from the mesothorax. They are characterized by an opaque, dark, and leathery appearance. Their primary function at rest is to cover and protect the hind wings.

  • Hind Wings: These arise from the metathorax. They are transparent and membranous, serving as the primary organs for flight.

Abdominal Morphology and Sexual Dimorphism

• General Segmentation: In both males and females, the abdomen is composed of 10 segments.

• Female Reproductive Structures:

  • The $7^{th}$ sternum is boat-shaped.

  • The $7^{th}$, $8^{th}$, and $9^{th}$ sterna collectively form a brood or genital pouch.

  • The anterior portion of this pouch houses the female gonopore, spermathecal pores, and collateral glands.

• Male Reproductive Structures:

  • The genital pouch or chamber is located at the posterior end of the abdomen.

  • This chamber is bounded dorsally by the $9^{th}$ and $10^{th}$ terga and ventrally by the $9^{th}$ sternum.

  • It contains the dorsal anus, the ventral male genital pore, and the gonapophysis.

• Distinction via Anal Styles: Males possess a pair of short, thread-like anal styles which are completely absent in females.

• Anal Cerci: In both sexes, the $10^{th}$ segment features a pair of jointed, filamentous structures called anal cerci.

Internal Anatomy: The Alimentary Canal (Digestive System)

• Division of Regions: The alimentary canal is located within the body cavity and is divided into the foregut, midgut, and hindgut.

• Foregut Components:

  • Pharynx: A short tubular entrance from the mouth.

  • Oesophagus: A narrow tubular passage.

  • Crop: A sac-like structure used specifically for the storage of food.

  • Gizzard (Proventriculus): This structure has an outer layer of thick circular muscles and an inner cuticle featuring six highly chitinous plates called teeth. These teeth aid in the mechanical grinding of food particles.

  • Cuticle Lining: The entire foregut is lined with a cuticle.

• Midgut and Junctions:

  • Hepatic (Gastric) Caeca: A ring of $6-8$ blind tubules located at the junction of the foregut and midgut. These tubules secrete digestive juices.

  • Mesenteron: The midgut itself.

• Malpighian Tubules: Located at the junction of the midgut and hindgut is a ring of $100-150$ yellow-colored, thin filamentous tubules. These are responsible for removing excretory products from the haemolymph.

• Hindgut Components:

  • The hindgut is broader than the midgut and is differentiated into the ileum, colon, and rectum.

  • The rectum terminates at the anus.

Circulatory and Respiratory Systems

• Blood Vascular System (Open Type):

  • Blood vessels are poorly developed and discharge into an open space called the haemocoel.

  • Visceral organs are submerged in blood, referred to as haemolymph.

  • Haemolymph Composition: It consists of colorless plasma and specialized cells called haemocytes.

  • Heart: An elongated muscular tube situated along the mid-dorsal line of the thorax and abdomen. It is divided into funnel-shaped chambers with openings called ostia. Blood enters through the ostia and is pumped anteriorly back into the sinuses.

• Respiratory System:

  • Tracheal Network: A system of trachea opens to the exterior through 10 pairs of small holes called spiracles, located on the lateral sides of the body.

  • Tracheoles: Tracheal tubes branch into smaller tracheoles that carry oxygen directly to all body parts.

  • Regulation: The opening of spiracles is controlled by sphincters. Gas exchange occurs at the tracheoles via diffusion.

Excretion and the Nervous System

• Primary Excretion: Malpighian tubules, lined with glandular and ciliated cells, absorb nitrogenous wastes.

• Chemical Conversion: Wastes are converted into uric acid and excreted via the hindgut. Due to this, cockroaches are classified as uricotelic.

• Auxiliary Excretory Organs: The fat body, nephrocytes, and urecose glands also assist in the excretory process.

• Nervous System Organization:

  • It consists of segmentally arranged, fused ganglia connected by paired longitudinal connectives on the ventral side.

  • Distribution: Three ganglia are located in the thorax and six in the abdomen.

  • Regional Survival: Because the nervous system is ventral and distributed, a cockroach can survive for up to one week even if its head is removed.

• Brain: Represented by the supra-oesophageal ganglion in the head, which provides nerves to the antennae and compound eyes.

Sensory Organs and Vision

• Sensory Repositories: Includes the antennae, compound eyes, maxillary palps, labial palps, and anal cerci.

• Compound Eyes: Positioned on the dorsal surface of the head. Each eye consists of approximately $2000$ hexagonal units called ommatidia.

• Mosaic Vision: The use of multiple ommatidia allows the cockroach to receive several images of a single object. Features of this vision include:

  • High sensitivity.

  • Low resolution.

  • Optimization for night (nocturnal vision).

Reproductive Systems and Development

• Dioecious Nature: Cockroaches have separate sexes with well-developed reproductive organs.

• Male Reproductive System:

  • Testes: One pair located laterally in the $4^{th}-6^{th}$ abdominal segments.

  • Duct Path: Vas deferens leads to the seminal vesicle and then the ejaculatory duct, which opens at the male gonopore (ventral to the anus).

  • Mushroom Gland: An accessory gland in the $6^{th}-7^{th}$ abdominal segments.

  • External Genitalia: Represented by phallomeres (chitinous asymmetrical structures).

  • Spermatophores: Bundles of sperms glued together for discharge during copulation.

• Female Reproductive System:

  • Ovaries: Two large ovaries located laterally in the $2^{nd}-6^{th}$ abdominal segments.

  • Ovarioles: Each ovary contains eight ovarian tubules (ovarioles) housing chains of developing ova.

  • Oviducts: Two oviducts join into a single median oviduct (vagina) opening into the genital chamber.

  • Spermatheca: A pair located in the $6^{th}$ segment opening into the genital chamber.

• Fertilization and Oothecae:

  • Fertilized eggs are encased in dark reddish to blackish brown capsules called oothecae, measuring approximately $\frac{3}{8}"\,(8\,mm)$ in length.

  • Placement: Oothecae are placed in cracks or crevices with high humidity near food.

  • Productivity: Females produce an average of $9-10$ oothecae, with each containing $14-16$ eggs.

• Development (Paurometabolous):

  • Growth occurs through a nymphal stage. Nymphs resemble adults.

  • Moulting: Nymphs undergo approximately $13$ moults to reach maturity.

  • Wing Development: Wing pads appear in the penultimate (next to last) nymphal stage, but only adults possess functional wings.

• Economic and Medical Impact: While many species are wild, those associated with humans are pests that contaminate food with excreta and transmit bacterial diseases.