Unit2 - Subjective Questions

The insect cuticle is a non-cellular, multi-layered structure secreted by the single layer of epidermal cells. It provides protection and structural support. It consists of three main zones:

1. Epicuticle:

• The outermost, thinnest layer ().
• It is devoid of chitin and consists of four sub-layers:
  • Cement layer: Mucopolysaccharides (protection).
  • Wax layer: Lipids (prevents water loss).
  • Polyphenol layer: Non-cellular.
  • Cuticulin layer: Lipoprotein.

2. Procuticle:

• The thickest layer situated below the epicuticle.
• Composed of chitin and protein. It is differentiated into:
  • Exocuticle: The outer, dark, hard, and sclerotized layer. It contains the protein sclerotin. It is shed during molting.
  • Endocuticle: The inner, soft, light-colored, and flexible layer. It is essentially unsclerotized.

3. Epidermis (Hypodermis):

• A single layer of cells that secretes the cuticle.
• Resting on a Basement Membrane (non-cellular basal lamina).

Molting is the periodic shedding of the old cuticle to allow for growth. It involves a complex sequence of physiological and hormonal events:

1. Apolysis:

• Separation of the epidermis from the old cuticle.
• A sub-cuticular space is formed.

2. Secretion of Molting Gel:

• Inactive molting fluid (enzymes) is secreted into the sub-cuticular space.

3. Cuticle Formation:

• The epidermis secretes a new cuticulin layer (part of the new epicuticle) which protects the new epidermal cells.

4. Activation of Enzymes:

• The molting fluid becomes active. It digests the old endocuticle (chitin and protein) but leaves the exocuticle and epicuticle intact.
• The products of digestion are resorbed by the insect.

5. Ecdysis:

• The remnants of the old cuticle (exuviae) rupture along the ecdysial sutures (lines of weakness) due to internal pressure (swallowing air/water).
• The insect crawls out of the old skin.

6. Sclerotization (Tanning):

• The new cuticle is initially soft and pale. Bursicon hormone aids in the hardening (sclerotization) and darkening (melanization) of the new exocuticle.

The orientation of the head relative to the body axis varies based on feeding habits:

1. Hypognathous (Orthopteroid type):

• Orientation: The head is vertical at a right angle () to the body axis.
• Mouthparts: Directed downwards (ventrally).
• Examples: Grasshoppers, Cockroaches.

2. Prognathous (Coleopteroid type):

• Orientation: The head is horizontal and in line with the body axis.
• Mouthparts: Directed forwards (anteriorly).
• Examples: Ground beetles, Termites (Soldiers).

3. Opisthognathous (Hemipteroid type):

• Orientation: The head is deflexed backward.
• Mouthparts: Directed backwards between the front legs.
• Examples: Aphids, Bugs, Leafhoppers.

The antenna is a mobile, segmented, sensory appendage located on the head. A typical antenna consists of three main parts:

1. Scape:

• The basal segment attached to the head capsule within the antennal socket (antennifer).
• It is often the largest and strongest segment.

2. Pedicel:

• The second segment immediately following the scape.
• It contains the Johnston's organ (a sensory organ responsible for detecting vibrations/sound) in most insects.

3. Flagellum (Clavola):

• The remainder of the antenna, composed of many sub-segments called flagellomeres.
• Unlike the scape and pedicel, flagellomeres generally lack intrinsic muscles.
• This part varies significantly across species to form different modifications (e.g., plumose, serrate).

Modifications of Insect Antennae:

1. Filiform (Thread-like):

   • Segments are cylindrical and of uniform thickness.
   • Example: Grasshopper, Ground beetle.

2. Moniliform (Bead-like):

   • Segments are spherical or globular, resembling a string of beads.
   • Example: Termites.

3. Serrate (Saw-like):

   • Segments have triangular projections on one side, resembling the teeth of a saw.
   • Example: Pulse beetle.

4. Plumose (Feather-like):

   • Segments possess long whorls of hairs, giving a feathery appearance.
   • Example: Male Mosquito (used to detect female wing beat).

5. Geniculate (Elbowed):

   • The scape is long, and the remaining antenna bends abruptly at an angle to the scape (like a bent knee).
   • Example: Ants, Weevils, Honey bees.

6. Capitate (Knobbed):

   • Terminal segments become abruptly enlarged to form a knob.
   • Example: Butterflies.

Mandibulate mouthparts are the primitive and most common type, adapted for cutting, crushing, and chewing solid food. They consist of:

1. Labrum (Upper Lip):

• A broad, flap-like structure covering the mouth cavity from the front.
• Inner surface (epipharynx) contains taste buds.

2. Mandibles (Primary Jaws):

• Paired, heavily sclerotized, unsegmented structures.
• Possess teeth: Incisor region (cutting) and Molar region (grinding).

3. Maxillae (Secondary Jaws):

• Paired structures behind the mandibles.
• Parts: Cardo, Stipes, Lacinia (cutting), Galea (sensory/hood), and Maxillary Palp (sensory).

4. Labium (Lower Lip):

• Fused structure forming the floor of the mouth.
• Parts: Postmentum (Submentum + Mentum) and Prementum (bearing Labial Palps, Glossa, and Paraglossa).

5. Hypopharynx (Tongue):

• A median, tongue-like structure situated between the maxillae.
• The salivary duct opens at its base.

Piercing and Sucking Mouthparts (Hemipterous/Dipterous type):

Adapted for piercing plant or animal tissue and sucking fluids (sap or blood).

Key Structural Features:

1. Labium (Proboscis):

   • Elongated, grooved, and forms a protective sheath.
   • It does not pierce the tissue but bends back during feeding (in bugs) or guides the stylets.

2. Stylets:

   • Needle-like modifications of the Mandibles and Maxillae.
   • Usually 4 stylets total (2 Mandibular + 2 Maxillary).
   • Maxillary Stylets: Interlock to form two channels:
     • Food Channel: For sucking up fluid.
     • Salivary Channel: For injecting saliva (anticoagulant or digestive enzymes).

3. Labrum:

   • Usually a small flap at the base of the beak (in bugs) or covers the groove (in mosquitoes).

4. Action:

   • The mandibles pierce the skin, and the maxillae slide in to suck the fluid. The labium stabilizes the bundle.

1. Siphoning Type:

• Found in: Butterflies and Moths (Lepidoptera).
• Adaptation: Sucking nectar from flowers.
• Key Feature: The proboscis is formed only by the greatly elongated Galeae of the maxillae, which are grooved and locked together to form a tube.
• Coiling: When not in use, the proboscis coils like a watch spring due to elasticity.
• Reductions: Mandibles are absent; Labium is reduced to labial palps.

2. Sponging Type:

• Found in: Houseflies (Diptera).
• Adaptation: Lapping up liquid or semi-liquid food.
• Key Feature: The proboscis (modified Labium) ends in two large, sponge-like lobes called Labella.
• Pseudotracheae: The labella have transverse channels called pseudotracheae that filter and conduct liquid food to the mouth by capillary action.
• Reductions: Mandibles are absent.

The thorax is the middle body tagma, specialized for locomotion. It consists of three distinct segments:

1. Segments:

• Prothorax: Anterior segment (bears 1st pair of legs, usually no wings).
• Mesothorax: Middle segment (bears 2nd pair of legs + 1st pair of wings).
• Metathorax: Posterior segment (bears 3rd pair of legs + 2nd pair of wings).
• Note: The Meso- and Metathorax are collectively called the Pterothorax (wing-bearing).

2. Sclerites (Plates) per Segment:
Each segment is a box-like structure composed of:

• Notum (Tergum): Dorsal plate (e.g., Pronotum).
  • In the pterothorax, the notum is divided into Prescutum, Scutum, and Scutellum.
• Sternum: Ventral plate.
• Pleura (Singular: Pleuron): Lateral plates (sclerotized membranous regions). Divided by a pleural suture into the anterior Episternum and posterior Epimeron.

An insect leg typically consists of five segments (podites) articulated with one another:

1. Coxa:

   • The basal segment that articulates with the thorax (pleuron/sternum).
   • Generally stout and functional as a pivot.

2. Trochanter:

   • A small, pivot-like segment between the coxa and femur.
   • Sometimes fused with the femur.

3. Femur:

   • Usually the largest, longest, and strongest segment.
   • Contains the major muscles for running or jumping.

4. Tibia:

   • A long, slender segment usually equal in length to the femur.
   • Often armed with downward-pointing spines or spurs for traction.

5. Tarsus:

   • The distal part, often subdivided into 2–5 sub-segments called tarsomeres.
   • Pretarsus: The terminal tip bearing a pair of claws (ungues) and pads (arolium/pulvillus) for adhesion.

Insect legs are modified according to the habit and habitat of the insect:

1. Cursorial (Running/Walking):

• Legs are long and unmodified for fast running.
• Example: Cockroach, Tiger beetle.

2. Saltatorial (Jumping):

• Hind femur is greatly enlarged and muscular.
• Example: Grasshopper, Cricket.

3. Raptorial (Grasping/Hunting):

• Forelegs are modified. The tibia folds back against the femur (like a jackknife) to grasp prey.
• Example: Praying Mantis.

4. Fossorial (Digging):

• Forelegs are broad, strong, and shovel-like with teeth on the tibia.
• Example: Mole Cricket.

5. Natatorial (Swimming):

• Hind legs are flattened and fringed with dense hairs (acting as oars).
• Example: Water Beetle (Dytiscus), Water Boatman.

6. Scansorial (Clinging):

• Tarsus bears a single claw to hold onto hair/fur.
• Example: Head louse.

The Comstock-Needham system is a uniform nomenclature for naming insect wing veins. The major longitudinal veins, from the anterior (leading) edge to the posterior, are:

1. Costa (C): Unbranched, forms the thickened leading margin (anterior margin) of the wing.
2. Subcosta (Sc): Located behind the Costa, usually unbranched or forked (). concave.
3. Radius (R): A stout vein. Branches into and Radial Sector (), which further branches ( to ). convex.
4. Media (M): Situated in the middle of the wing. Often branches into to .
5. Cubitus (Cu): Primary vein of the posterior part. Branches into and .
6. Anal Veins (A): Unbranched veins behind the cubitus ().

Cross Veins: Connect longitudinal veins (e.g., h (humeral), r-m (radio-medial), m-cu (medio-cubital)).

Wing Coupling Apparatus:
A mechanism that joins the forewing and hindwing together during flight so they beat as a single unit, increasing aerodynamic efficiency.

Types:

1. Jugate Type:

   • A lobe called the jugum at the base of the forewing overlaps with the hindwing.
   • Example: Primitive moths (Hepialidae).

2. Frenate Type:

   • Frenulum: A spine (or group of bristles) arises from the base of the hindwing.
   • Retinaculum: A hook or catch on the underside of the forewing.
   • The frenulum locks into the retinaculum.
   • Example: Most Moths (Lepidoptera).

3. Hamulate Type:

   • A row of small hooks called Hamuli is present on the anterior margin of the hindwing.
   • These hooks catch a fold on the posterior margin of the forewing.
   • Example: Honey bees, Wasps (Hymenoptera).

4. Amplexiform: No specific apparatus; wings overlap broadly (Butterflies).

1. Elytra (Singular: Elytron):

• The forewings become hard, thick, opaque, and heavily sclerotized.
• Function: They are wing-covers that protect the membranous hindwings and the abdomen. They are not used for flight.
• Order: Coleoptera (Beetles) and Earwigs.

2. Halteres:

• The hindwings are reduced to small, knobbed structures consisting of a scabellum, pedicel, and capitellum.
• Function: They act as gyroscopic organs to maintain balance and equilibrium during flight.
• Order: Diptera (Houseflies, Mosquitoes).

3. Tegmina:

• The forewings are leathery, parchment-like, and semi-transparent.
• Function: Protection of hindwings.
• Order: Orthoptera (Grasshoppers, Cockroaches) and Mantodea.

The abdomen is the third tagma, generally concerned with metabolism and reproduction. It typically consists of 11 segments (though often reduced in adults).

Structure:

• Sclerites: Each segment has a dorsal Tergum and a ventral Sternum, connected by a lateral Pleural Membrane (facilitating expansion during breathing or egg production).

Regions:

1. Pregenital Segments (1-7): Visceral segments containing internal organs. Usually lack appendages (except in primitive forms like silverfish which have styli).
2. Genital Segments (8-9): Bear the external genitalia.
   • Female: Ovipositor (Segments 8 & 9).
   • Male: Aedeagus/Claspers (Segment 9).
3. Postgenital Segments (10-11):
   • Segment 10 is often visible.
   • Segment 11 bears the anus and sensory appendages called Cerci.
   • Telson: The terminal non-segmental region.

Functions of the Insect Exoskeleton (Cuticle):

1. Protection: Acts as an armor against physical injury, abrasion, and predation.
2. Water Conservation: The wax layer of the epicuticle is impermeable to water, preventing desiccation (crucial for terrestrial life).
3. Structural Support: Provides the rigid framework for the body shape.
4. Muscle Attachment: Internal ridges (apodemes) provide attachment sites for skeletal muscles (acting like an internal skeleton).
5. Sensory Interface: Forms sensory organs (sensilla) like hairs and bristles to detect environment.
6. Coloration: Structural colors (interference) or pigments in the cuticle aid in camouflage or mating display.
7. Lining: Lines the foregut, hindgut, and tracheae.

Structure:
The ovipositor is an egg-laying organ located at the end of the abdomen (segments 8 and 9). It consists of three pairs of valves (valvulae) attached to basal sclerites (valvifers).

Modifications:

1. Standard Ovipositor:

   • Used to deposit eggs in soil or plant tissue.
   • Example: Grasshoppers (Short), Ichneumon fly (Very long for drilling into wood).

2. Sting (Modified Ovipositor):

   • The ovipositor is modified into a sting for defense or paralyzing prey. The eggs are ejected from the base rather than through the channel.
   • Example: Worker Honey bees and Wasps.

3. Pseudo-ovipositor:

   • The true ovipositor is absent. The terminal abdominal segments act like a telescopic tube to lay eggs.
   • Example: Houseflies, Fruit flies.

Definition:
The Tentorium is the internal endoskeleton of the insect head, formed by the ingrowth (invagination) of the body wall (exoskeleton).

Structure:
It consists of pairs of cuticular arms that fuse to form a framework.

1. Anterior Arms: Arise from the subgenal suture.
2. Posterior Arms: Arise from the occipital region.
3. Dorsal Arms: Arise from the anterior arms and extend to the vertex.
4. Corpus Tentorium: The central body where arms may fuse.

Functions:

• Provides rigidity to the head capsule, preventing collapse.
• Provides attachment points for muscles controlling the mouthparts and antennae.
• Supports the brain and foregut.

1. Cerci (Singular: Cercus):

• Description: Paired appendages arising from the 11th abdominal segment (though often appearing on the 10th). They are primarily sensory (tactile/olfactory).
• Modifications:
  • Long & Segmented: Silverfish, Mayflies.
  • Short & Unsegmented: Grasshoppers.
  • Forceps-like: Earwigs (Defense/Offense).

2. Styli (Singular: Stylus):

• Description: Small, unsegmented, finger-like processes found on the ventral side of the abdominal segments.
• Function: They are vestigial legs believed to assist in locomotion in primitive insects.
• Occurrence: Found in Apterygotes (wingless insects) like Silverfish (Thysanura) and Bristletails.

The insect cuticle is chemically complex, primarily consisting of:

1. Chitin ():

   • A nitrogenous polysaccharide (polymer of N-acetylglucosamine).
   • It is chemically stable, insoluble in water, dilute acids, and alkalis.
   • Provides tensile strength and flexibility.

2. Proteins ():

   • Arthropodin: Water-soluble, found in soft cuticle.
   • Sclerotin: Tanned, water-insoluble protein responsible for the hardness and dark color of the exocuticle.
   • Resilin: Elastic, rubber-like protein found in wing hinges and leg joints (stores energy).

3. Others:

   • Lipids: Waxes in the epicuticle.
   • Pigments: Melanin (dark), Pterines, Carotenoids.
   • Inorganic salts (calcium) in some larvae.