Unit 5: Insect Systematics and Taxonomy I

1. Introduction to Insect Systematics and Taxonomy

Systematics is the scientific study of the kinds and diversity of organisms and of any and all relationships among them. Taxonomy is the theory and practice of classifying organisms.

Importance of Taxonomy in Entomology

1. Identification: Provides the fundamental basis for recognizing pests versus beneficial insects. Without correct identification, pest management strategies (like biological control or chemical application) cannot be accurately applied.
2. Universal Communication: Establishes a standardized language (scientific names) used globally, overcoming language barriers inherent in common names.
3. Predictive Value: Knowledge of a specific group allows researchers to predict the habits, biology, and susceptibility of a newly discovered species based on its relatives.
4. Biodiversity Assessment: Essential for cataloging insect diversity and monitoring ecosystem health.
5. Quarantine and Legislation: Crucial for preventing the entry of invasive alien species across international borders.

History and Development

• Aristotle (384–322 BC): Classified insects based on the presence or absence of wings (Apterygota and Pterygota).
• Carolus Linnaeus (1707–1778): The father of modern taxonomy. In his 10th edition of Systema Naturae (1758), he standardized the binomial system of nomenclature.
• Fabricius: Classified insects based on mouthparts.
• Brauer (1885): Based classification on metamorphosis (Ametabola, Hemimetabola, Holometabola).
• Modern Era: Incorporates numerical taxonomy, cytotaxonomy, and molecular systematics (DNA barcoding) to determine evolutionary relationships.

Binomial Nomenclature

A system of naming species of living things by giving each a name composed of two parts.

• Structure: Genus + species.
• Rules:
  1. The Genus name begins with a Capital letter.
  2. The species name begins with a small letter.
  3. The names must be Latinized.
  4. When typed, they are italicized; when handwritten, they are underlined separately.
  5. The author's name (the person who first described the species) is placed after the specific name (not italicized). Parentheses around the author's name indicate the species was later moved to a different genus.
     • Example: Helicoverpa armigera (Hubner).

2. Taxonomic Definitions and Hierarchy

The hierarchy of classification moves from broad to specific groups.

1. Order: A major division of the Class Insecta based on wing structure, mouthparts, and type of metamorphosis (e.g., Coleoptera, Lepidoptera).
2. Family: A taxonomic group containing one or more related genera. Family names in animals always end in -idae (e.g., Acrididae).
3. Genus: A group of closely related species that share common structural characteristics and a common ancestry.
4. Species: The fundamental unit of taxonomy. A group of potentially interbreeding natural populations that are reproductively isolated from other such groups.
5. Sub-species: Geographically isolated populations of a species that differ morphologically from other populations but can still interbreed if brought together (Trinomial nomenclature is used).
6. Biotype: Populations within a species that are morphologically indistinguishable but differ in their physiological traits, such as host plant preference, ability to transmit diseases, or resistance to pesticides. (e.g., Brown planthopper biotypes).

3. Classification of Class Insecta

The Class Insecta is generally divided into two Subclasses based on the evolutionary presence of wings.

I. Subclass: Apterygota (Ametabola)

Primitively wingless insects; no metamorphosis.

• Order Thysanura: Silverfish
• Order Diplura: Diplurans
• Order Protura: Proturans
• Order Collembola: Springtails

II. Subclass: Pterygota

Winged insects (or secondarily wingless); undergo metamorphosis.

Division 1: Exopterygota (Hemimetabola)
Simple metamorphosis (Egg → Nymph → Adult); wings develop externally.

• Order Ephemeroptera: Mayflies
• Order Odonata: Dragonflies
• Order Orthoptera: Grasshoppers
• Order Dictyoptera: Mantids and Cockroaches
• Order Isoptera: Termites
• Order Hemiptera: Bugs, Aphids
• Order Thysanoptera: Thrips

Division 2: Endopterygota (Holometabola)
Complete metamorphosis (Egg → Larva → Pupa → Adult); wings develop internally during the pupal stage.

• Order Coleoptera: Beetles
• Order Lepidoptera: Moths and Butterflies
• Order Diptera: True flies
• Order Hymenoptera: Bees, Wasps, Ants
• Order Neuroptera: Lacewings

4. Orders and Families of Agricultural Importance

Order: Orthoptera (Straight Wings)

• Mouthparts: Chewing/Biting.
• Wings: Forewings are leathery (Tegmina); hindwings are membranous.
• Legs: Hind legs usually modified for jumping (Saltatorial).
• Metamorphosis: Hemimetabolous (Nymphs resemble adults).

1. Family: Acrididae (Short-horned Grasshoppers / Locusts)

• Antennae: Shorter than the body.
• Ovipositor: Short and horny.
• Tympanum: Auditory organ located on the first abdominal segment.
• Example: Desert Locust (Schistocerca gregaria), Rice grasshopper.

2. Family: Tettigoniidae (Long-horned Grasshoppers / Katydids)

• Antennae: Filiform, longer than the body.
• Ovipositor: Sword-shaped (ensiform) or sickle-shaped.
• Tympanum: Located on the base of the fore-tibia.
• Habit: Mostly nocturnal.

3. Family: Gryllidae (Crickets)

• Antennae: Long and filiform.
• Wings: Flat on the back; lateral edges bent down abruptly.
• Ovipositor: Long, needle-like (lanceolate).
• Example: House cricket.

4. Family: Gryllotalpidae (Mole Crickets)

• Legs: Forelegs heavily modified for digging (Fossorial).
• Habit: Subterranean; feed on roots.
• Example: Gryllotalpa orientalis.

Order: Dictyoptera

Includes cockroaches and mantids. Usually characterized by the production of an Ootheca (egg case).

1. Family: Mantidae (Praying Mantids)

• Legs: Forelegs modified for grasping prey (Raptorial).
• Head: Triangular and highly mobile.
• Prothorax: greatly elongated.
• Importance: Highly beneficial predators of other insects.

2. Family: Blattidae (Cockroaches)

• Body: Dorso-ventrally flattened; head concealed under the pronotum.
• Legs: Adapted for running (Cursorial).
• Importance: Household pests, mechanical vectors of pathogens.
• Example: Periplaneta americana.

Order: Odonata (Toothed ones)

• Common Names: Dragonflies (Anisoptera) and Damselflies (Zygoptera).
• Wings: Two pairs of similar, membranous wings with a dark spot (Pterostigma) near the tip.
• Mouthparts: Chewing; adults catch prey in flight (basket legs).
• Metamorphosis: Hemimetabolous with aquatic nymphs (Naiads) possessing a prehensile labium (mask).
• Importance: Predators of mosquitoes and crop pests.

Order: Isoptera (Equal Wings)

• Common Name: Termites (White ants).
• Social Structure: Eusocial (Castes: Queen, King, Soldiers, Workers).
• Food: Cellulose (wood).
• Wings: Alates (reproductives) have two pairs of similar wings which are shed after nuptial flight.

1. Family: Termitidae (Higher Termites)

• Characteristics: Largest family of termites; cultivate fungus gardens.
• Importance: Major pests of structural timber and crops (sugar cane, wheat). Mound builders.
• Example: Odontotermes obesus.

Order: Thysanoptera (Fringe Wings)

• Common Name: Thrips.
• Wings: Narrow, fringed with long hairs.
• Mouthparts: Rasping-sucking (asymmetrical; right mandible absent).

1. Family: Thripidae

• Ovipositor: Saw-like, curves downward.
• Importance: Sap feeders causing "silvery" streaks on leaves; vectors of Tospoviruses (e.g., Tomato Spotted Wilt Virus).
• Example: Scirtothrips dorsalis (Chilli thrips), Thrips tabaci.

Order: Hemiptera (Half Wings)

• Mouthparts: Piercing and sucking type.
• Metamorphosis: Hemimetabolous.
• Suborders: Traditionally Heteroptera (True bugs) and Homoptera (Hoppers, Aphids, Scales).

A. Suborder Heteroptera (Basal half of forewing leathery, apical half membranous - Hemelytra)

1. Family: Pentatomidae (Stink Bugs)

• Body: Shield-shaped.
• Scutellum: Large and triangular.
• Odor: Produce offensive odor from thoracic glands.
• Example: Nezara viridula (Green stink bug).

2. Family: Coreidae (Squash Bugs / Leaf-footed Bugs)

• Wings: Many veins in the membranous part of the hemelytra.
• Legs: Hind tibia often leaf-like.
• Example: Rice earhead bug (Leptocorisa acuta).

3. Family: Cimicidae (Bed Bugs)

• Wings: Vestigial (wingless).
• Body: Dorso-ventrally flattened.
• Habit: Ectoparasites on blood of mammals/birds.
• Example: Cimex lectularius.

4. Family: Pyrrhocoridae (Red Cotton Bugs)

• Coloration: Bright red and black warning colors.
• Ocelli: Absent.
• Example: Dysdercus cingulatus (Cotton stainer).

5. Family: Lygaeidae (Seed Bugs)

• Ocelli: Present.
• Wings: Only 4-5 veins in the membrane of hemelytra.
• Example: Nysius spp.

B. Suborder Homoptera (Wings uniform in texture; held tent-like over body)

1. Family: Cicadellidae (Leafhoppers / Jassids)

• Legs: Hind tibia has a double row of spines.
• Movement: Walk sideways or jump.
• Importance: Transmit viral diseases; cause "hopper burn."
• Example: Amrasca biguttula (Cotton jassid), Nephotettix virescens (Green leafhopper).

2. Family: Delphacidae (Planthoppers)

• Legs: Large, movable spur (calcar) at the apex of the hind tibia.
• Importance: Serious rice pests; vectors of rice viruses.
• Example: Nilaparvata lugens (Brown Planthopper).

3. Family: Aphididae (Aphids)

• Body: Soft, pear-shaped.
• Abdomen: Possess a pair of cornicles (siphunculi) for excreting alarm pheromones.
• Excretion: Excrete honeydew (attracts ants, causes sooty mold).
• Example: Aphis gossypii (Cotton aphid).

4. Family: Coccidae (Soft Scales)

• Sexual Dimorphism: Females wingless, legless, sedentary, covered in wax/scale; Males winged.
• Example: Coccus viridis (Coffee green scale).

5. Family: Lophophidae

• Characteristics: Moderate size, head produced forward; hind tibia with apical spines but no large spur.
• Example: Pyrilla perpusilla (Sugarcane leafhopper).

6. Family: Aleurodidae (Whiteflies)

• Wings: Covered with white, powdery wax.
• Metamorphosis: Complex (Pseudo-pupa stage).
• Importance: Major vectors of mosaic viruses.
• Example: Bemisia tabaci (Sweet potato whitefly).

7. Family: Pseudococcidae (Mealybugs)

• Body: Covered in mealy/powdery wax secretion.
• Features: Often have lateral filaments.
• Example: Phenacoccus solenopsis (Cotton mealybug).

Order: Neuroptera (Nerve Wings)

• Wings: Two pairs of similar membranous wings with net-like venation (held roof-like).
• Mouthparts: Chewing.
• Metamorphosis: Holometabolous.

1. Family: Chrysopidae (Green Lacewings)

• Appearance: Green body, copper-colored eyes.
• Eggs: Stalked eggs (laid on pedicels to prevent cannibalism).
• Larvae: Called "Aphid Lions" (active predators).
• Importance: Highly beneficial biological control agents used against aphids and soft-bodied pests.
• Example: Chrysoperla carnea.