Unit 2: Insect Morphology

1. The Insect Cuticle and Molting

The integument (body wall) of an insect is an exoskeleton that serves as the interface between the internal environment and the outside world. It is composed of a cellular layer (epidermis) and an extracellular layer (cuticle).

Structure of the Cuticle

The cuticle is a non-cellular secretion of the epidermis. From the outside moving inward, the layers are:

1. Epicuticle (1–4 µm):
   • The outermost, thinnest layer.
   • Composition: Contains no chitin. Composed of waxes, cement, and cuticulin (lipoproteins).
   • Function: Waterproofing (prevents desiccation) and protection against microorganisms.
2. Procuticle (Up to 200 µm): The primary structural layer, composed of chitin (a polysaccharide) and proteins (arthropodin, resilin, sclerotin). It is differentiated into:
   • Exocuticle: The outer, rigid, sclerotized (hardened) layer. Pigments (melanin) are deposited here. Not present in flexible areas (joints).
   • Endocuticle: The inner, flexible, unsclerotized layer. It allows for movement and is reabsorbed during molting.
3. Epidermis: A single layer of living cells that secretes the cuticle.
4. Basement Membrane: A non-cellular connective tissue sheet separating the epidermis from the hemocoel (body cavity).

Functions of the Cuticle

• Protection: Physical barrier against abrasion, pathogens, and predators.
• Support: Provides the skeletal framework for internal organ attachment.
• Muscle Attachment: Internal ridges (apodemes) allow muscles to attach for locomotion.
• Water Regulation: The waxy epicuticle prevents lethal water loss.
• Sensory Interface: Modified cuticular structures (setae) act as sensory receptors.

Molting (Ecdysis)

Since the exoskeleton is rigid, insects must shed it to grow. This process is called ecdysis or molting.

The Process of Molting:

1. Apolysis: The epidermis separates from the old cuticle.
2. Secretion: Epidermal cells secrete molting fluid (inactive enzymes) and form a new cuticulin layer.
3. Activation: The molting fluid activates, digesting the old endocuticle. The products are reabsorbed by the insect.
4. Ecdysis: The insect swallows air or water to expand, splitting the old exocuticle along lines of weakness (ecdysial sutures). The insect crawls out.
5. Sclerotization (Tanning): The new cuticle is soft (teneral). Proteins cross-link (sclerotize) to harden the exoskeleton and darken its color.

Hormonal Control:

• PTTH (Prothoracicotropic Hormone): Stimulates the prothoracic glands.
• Ecdysone: The molting hormone; initiates apolysis.
• Juvenile Hormone (JH): Determines what the insect molts into (Larva Larva, or Larva Pupa).

2. Body Segmentation (Tagmosis)

The insect body is divided into three distinct functional regions called tagmata (singular: tagma).

1. Head (Cephalon): The anterior tagma focused on sensory perception and ingestion.
2. Thorax: The middle tagma focused on locomotion (legs and wings).
3. Abdomen: The posterior tagma focused on digestion, excretion, and reproduction.

3. Structure of the Head

The insect head is a hard, sclerotized capsule formed by the fusion of six embryonic segments.

Major Sclerites and Sutures

• Vertex: The dorsal surface (top) of the head.
• Frons: The front facial area.
• Clypeus: The lip-like area just below the frons.
• Gena: The lateral area (cheeks) below the compound eyes.
• Occiput: The back of the head connecting to the neck (cervix).
• Tentorium: An internal bony framework (endoskeleton) that supports the brain and provides muscle attachment points.

Types of Head Orientation

Based on the position of the mouthparts relative to the body axis:

1. Hypognathous: Mouthparts project downwards (at a right angle to the body).
   • Example: Grasshoppers, Cockroaches.
2. Prognathous: Mouthparts project forwards (in line with the body). Used for burrowing or predation.
   • Example: Ground beetles, Termites.
3. Opisthognathous: Mouthparts project backwards between the front legs. Common in sucking insects.
   • Example: Aphids, Leafhoppers.

4. Insect Antennae

Antennae are paired, movable, segmented appendages located on the head, originating from the second embryonic segment.

General Structure

1. Scape: The basal segment attached to the head. It is muscularized.
2. Pedicel: The second segment. Contains the Johnston’s Organ (detects vibration/flight speed).
3. Flagellum: The remaining segments (flagellomeres). Lacks intrinsic muscles.

Modifications of Antennae

5. Mouthparts

Insect mouthparts are modified appendages of head segments 3 through 6.

Generalized Components (Chewing Type)

1. Labrum: Upper lip; covers the preoral cavity.
2. Mandibles: Paired, heavily sclerotized jaws for cutting/grinding. Unsegmented.
3. Maxillae: Paired auxiliary jaws for food manipulation and sensing. Possess maxillary palps.
4. Labium: Lower lip; formed by the fusion of the second maxillae. Possesses labial palps.
5. Hypopharynx: Tongue-like structure; salivary ducts usually open here.

Modifications of Mouthparts

1. Biting and Chewing (Mandibulate): The primitive type. Strong mandibles.
   • Examples: Cockroaches, Grasshoppers, Beetles.
2. Piercing and Sucking: Mandibles and maxillae are modified into distinct stylets (needles) to pierce tissue and suck fluids. Enclosed in a beak (labium).
   • Examples: Mosquitoes (6 stylets), Bugs/Aphids (4 stylets).
3. Chewing and Lapping: Mandibles are used for molding wax (chewing); maxillae and labium form a proboscis for sucking nectar (lapping).
   • Examples: Honey bees.
4. Sponging: Mandibles and maxillae are lost or reduced. Labium ends in a sponge-like labella to sop up liquids.
   • Examples: House flies.
5. Siphoning: Maxillae form a long, coiled tube (proboscis). Mandibles are absent. Used for drinking nectar.
   • Examples: Butterflies, Moths.
6. Rasping and Sucking: Asymmetrical mouthparts. One mandible scrapes the plant surface; fluids are sucked up.
   • Examples: Thrips.

6. The Thorax

The thorax is the locomotion center, consisting of three segments:

1. Prothorax: Anterior segment. Bears the first pair of legs. Never bears wings.
2. Mesothorax: Middle segment. Bears the second pair of legs and the first pair of wings (forewings).
3. Metathorax: Posterior segment. Bears the third pair of legs and the second pair of wings (hindwings).

Note: The Mesothorax and Metathorax are collectively called the Pterothorax (wing-bearing thorax).

Sclerites of a Thoracic Segment:

• Notum: Dorsal plate (e.g., Pronotum).
• Pleuron: Lateral plate.
• Sternum: Ventral plate.

7. Insect Legs

Insect legs are hexapodous (3 pairs) and jointed.

Structure of a Typical Leg

1. Coxa: The basal segment connecting the leg to the thorax.
2. Trochanter: A small pivot segment.
3. Femur: Usually the largest and most muscular segment.
4. Tibia: Long, slender segment, often with spines.
5. Tarsus: The "foot," usually subdivided into 2–5 tarsomeres.
6. Pretarsus: The terminal section, usually containing a pair of claws and a pad (arolium or pulvillus) for adhesion.

Modifications of Legs

8. Wings: Venation, Modifications, and Coupling

Insects are the only invertebrates capable of flight. Wings are membranous extensions of the body wall (not appendages like legs).

Wing Venation (Comstock-Needham System)

Venation refers to the arrangement of veins. Veins provide structural strength and house tracheae and nerves.

• Longitudinal Veins (Anterior to Posterior):
  1. Costa (C): Leading edge; unbranched.
  2. Subcosta (Sc): Below Costa.
  3. Radius (R): Strong vein; branches into R1–R5.
  4. Media (M): Middle vein.
  5. Cubitus (Cu): Primary Cu1 and Cu2.
  6. Anal Veins (A): The posterior veins.
• Cross veins: Small veins connecting longitudinal veins (e.g., r-m connects radius to media), creating closed cells.

Wing Modifications

1. Tegmina: Leathery/parchment-like forewings. Protect membranous hindwings.
   • Examples: Grasshoppers, Cockroaches, Mantids.
2. Elytra: Hard, heavily sclerotized forewings. Useless for flight; serve as armor.
   • Examples: Beetles (Coleoptera).
3. Hemelytra: Basal half is thickened/leathery; apical half is membranous.
   • Examples: True Bugs (Hemiptera/Heteroptera).
4. Halteres: Hindwings reduced to club-shaped balancers. Gyroscopic function.
   • Examples: Diptera (Flies, Mosquitoes).
5. Scaly Wings: Covered in flattened setae (scales) which provide color.
   • Examples: Lepidoptera (Butterflies, Moths).
6. Fringed Wings: Reduced venation with long marginal hairs.
   • Examples: Thrips.

Wing Coupling Apparatus

Mechanisms to hook forewings and hindwings together to act as a single aerodynamic unit during flight.

1. Hamuli: A row of small hooks on the costal margin of the hindwing attaches to a fold in the forewing.
   • Example: Hymenoptera (Bees, Wasps).
2. Frenulum-Retinaculum: A spine (frenulum) on the hindwing locks into a catch (retinaculum) on the forewing.
   • Example: Moths.
3. Jugum: A lobe at the base of the forewing overlaps the hindwing.
   • Example: Primitive Lepidoptera.
4. Amplexiform: No physical hook; extensive overlap ensures synchronous movement.
   • Example: Butterflies.

9. The Abdomen

The abdomen contains the visceral organs.

Structure

• Primitive number of segments is 11, plus a terminal telson.
• In advanced insects, segments are often fused or reduced (e.g., 7–8 visible segments).
• Typically lacks walking appendages.
• Contains spiracles (breathing openings) laterally on segments 1–8.

Appendages and Modifications

1. Cerci: Paired sensory appendages on the 11th segment. Can be long (mayflies), pincer-like (earwigs), or reduced.
2. Reproductive Organs (Terminalia):
   • Ovipositor (Female): Modified segments 8 and 9 used for laying eggs. Modified into a sting in bees/wasps.
   • Aedeagus (Male): Copulatory organ located on the 9th segment.
3. Non-Reproductive Appendages (Larvae/Primitive):
   • Prolegs: Fleshy, false legs on caterpillars (Lepidoptera larvae).
   • Collophore, Retinaculum, Furca: Springing mechanism in Collembola (Springtails).
   • Cornicles: Tubes secreting defensive wax/pheromones in Aphids.