Animals: Ecdysozoa (Nematodes and Arthropods)

Question 1

True or False

Ecdysozoans are the most species-rich animal group

Answer 1

True

Ecdysozoans are the most species-rich animal group.

Clade Ecdysozoa is defined by molecular phylogeny and consists of 8 phyla.
- Ecdysozoa is a monophyletic group, although the relationships between phyla within the clade are uncertain.

Question 2

What are the two prominent ecdysozoan phyla?

Answer 2

Nematodes are the most numerous animals.

Arthropods are the most biodiverse animals.

Question 3

Ecdysozoan cuticle

Answer 3

Ecdysozoans are covered by a cuticle that is periodically moulted.
- Ecdysozoan shared derived trait.

The cuticle is a tough, non-living, outer layer of organic material deposited outside the epidermis.
- The epidermal cells secrete the cuticle.
- The cuticle is composed primarily of chitin, a long-chain nitrogenous polysaccharide.
Chitin (WC)

Cuticle functions:
- Acts as a barrier against drying out (desiccation) and protects against mechanical injuries.
- Provides structural support by forming a tough external skeleton (exoskeleton) that shapes the body and serves as a rigid framework for muscle contraction.

The cuticle is a non-living outer layer with no capacity for growth or modification.
- Ecdysozoans undergo ecdysis (moulting), shedding their entire cuticle to grow.

Question 4

What are the 3 layers of the Ecdysozoan cuticle?

Answer 4

The cuticle consists of three layers:
- Endocuticle (endo = within)
- Exocuticle (exo = outside)
- Epicuticle (epi = upon): thin, waxy, water-resistant outer layer (no chitin).

Fine ducts run from the epidermis to the top of the cuticle.

Question 5

Role of Ecdysis

Answer 5

Moulting (Shedding their entire cuticle to grow)

Although the cuticle composition varies among ecdysozoans, the process of ecdysis is similar.
- Ecdysis allows ecdysozoans to grow incrementally and facilitates changes between developmental stages, such as metamorphosis, e.g. larval to juvenile; development of wings.
- Ecdysis allows damaged tissue and missing limbs to regenerate and removes external parasites

Question 6

Sclerotization

Answer 6

Sclerotization cross-links the chitin polymers with proteins to create a hardened and rigid cuticle in ecdysozoans (insects, crabs, spiders ect.).

Question 7

True or False

Ecdysozoan cuticles are sclerotized

Answer 7

Trick Question! It does both

The cuticle is often sclerotized (scler = hard) in arthropods.
- Sclerotization cross-links the chitin polymers with proteins to create a hardened and rigid cuticle.
- Only specific regions of the cuticle are sclerotized.
- Called sclerites when shaped like plates.
- Sclerotized cuticles are used as armour plates and hardened appendages that function as tools or wings.

The cuticle between sclerites remains unsclerotized
− The unsclerotized cuticle contains unmodified chitin that is translucent, pliable, and flexible, allowing for movement

Question 8

Steps of Ecdysozoan moulting (ecdysis)

8 Steps

Answer 8

1. The ecdysozoan becomes inactive, and the old cuticle separates from the underlying epidermis. − Apolysis (apo = away from, lys = loosening)
2. Inactive enzymatic moulting fluid is secreted into the separation space, and the epidermis grows (cell proliferation and enlargement), increasing the epidermal surface area (folded).
3. A new proto-epicuticle, a thin protective layer, is deposited on the surface of the epidermis.
4. Moulting fluid enzymes (chitinases and peptidases) are activated (Digests old endocuticle)
5. Moulting fluid reabsorbed
6. The deposition of the new epicuticle is completed
7. The old exocuticle/epicuticle is moulted
8. The outer layer of the new procuticle is sclerotized by secretions via cuticle ducts to form the exocuticle above the unsclerotized endocuticle. − Sclerotization can take between several minutes to several days to complete.

Question 9

True or False

Nematodes are the most numerous animals

Answer 9

True

Question 10

Phylum Nematoda

Populations and size

Answer 10

Nematodes: roundworms or threadworms (nemato = thread).
- Nematodes are the most numerous animals.
- Nematodes inhabit a wide range of environments and serve critical ecological and biological roles.
- Display a wide size range from 0.3 mm to over 8 meters, with most species being <5 mm.
- ~25,000 named species (est. up to 100× more).

Question 11

Nematode Body Plan

Answer 11

Nematodes have a morphologically simple body plan.
- Simplicity is a result of secondary simplification from a more complex body design.

No segmentation, appendages, or eyes.
- Cylindrical body plan with tapered ends (anterior end slightly blunt, posterior slightly pointed).

Nematodes have a complete digestive tract but no circulatory system.

Nematodes possess a hemocoel, which serves as their body cavity.
- They do not have a coelom.

Question 12

Nematode Locomotion

Answer 12

Movement is achieved through side-to-side contractions of their longitudinal body-wall muscles

Question 13

Nematode Moulting

Answer 13

Like other ecdysozoans, nematodes moult their cuticle to grow (ecdysis)

Question 14

Nematode Reproduction

Answer 14

Nematode reproduction is primarily sexual.
- Most species have distinct male and female sexes, while some are hermaphrodites.
- They undergo internal fertilization and direct development.

Question 15

Ecological roles of nematodes

Answer 15

Free-living nematodes:
- Found in soil and freshwater/marine sediments, free-living nematodes serve crucial roles in decomposition and nutrient cycling.
- They feed on microorganisms, decaying organic matter, and other small organisms, functioning as predators or microbivores.

Parasitic nematodes:
- Plant-parasitic nematodes are widely distributed in agricultural soils and are responsible for ~10% of global annual crop losses.
- These nematodes infect both foliar and root tissues, with some (e.g. root-knot nematodes) using specialized, needle- like structures called stylets to penetrate plant cells and extract nutrients.
- Animal-parasitic Nematodes are also a major problem

Question 16

Nematode parasites of humans

Answer 16

Animal-parasitic nematodes infect a variety of animal hosts, including humans.
- Humans can host ~50 species of nematodes, such as pinworms and hookworms, that cause significant health problems.
- Humans contract Trichinosis when they consume raw or undercooked meat (e.g. pork) containing juvenile Trichinella spiralis worms encysted in muscle tissues of carnivorous and omnivorous animals.
- Nematodes also cause several significant tropical diseases, including filarial worms and Guinea worm

Question 17

Phylum Arthropoda

Answer 17

Arthropods are the most species-rich phylum, accounting for approximately two-thirds of all animal species.
- ~1 million extant species have been named, with estimates ranging from 3 to 30 million species in total.
- Arthropods exhibit a vast array of body forms and adaptations across terrestrial, freshwater, and marine habitats.
- Arthropods occupy important ecological roles, including as decomposers and pollinators.

Question 18

Arthropod origins

Answer 18

Arthropod body plan consists of:
- A segmented body.
- Jointed appendages.
- A hard exoskeleton (external skeleton).

The arthropod body plan originated in the Cambrian explosion (535–525 mya).
- The earliest arthropods showed little variation from segment to segment, e.g. trilobites.

Arthropod evolution is characterized by: − A decrease in the number of body segments.
- An increase in appendage specialization.
- Changes may have been caused by changes in Hox gene sequences or regulation.

Question 19

What does an arthropod body plan consists of?

Answer 19

Arthropod body plan consists of:
- A segmented body.
- Jointed appendages.
- A hard exoskeleton (external skeleton).

Question 20

Arthropod characteristics

Answer 20

• Segmented body.
• Jointed paired appendages
• Arthropods possess a sclerotized cuticle composed of chitin
• Cephalization
• Reduced coelom
• Open circulatory system

Question 21

Segmented body

Arthropod characteristics

Answer 21

All arthropod embryos are segmented.
- The last common ancestor of arthropods probably had numerous undifferentiated segments, each with a pair of appendages that functioned as limbs.

Body segments and limbs are arranged into functional units (tagmata) with specialized functions.
- e.g. the three-part appearance of many insect bodies; the two-part appearance of spiders

Question 22

Jointed paired appendages

Arthropod characteristics

Answer 22

arthros = joint, pod = foot

Jointed appendages are the most versatile part of the arthropod body, undergoing evolutionary modification for specialized functions such as:
- swimming
- walking
- feeding
- sensory perception
- reproduction
- defence

Appendage specialization varies across tagmata, with distinct roles in the head, thorax, and abdomen:
- Head appendages: feeding, sensory.
- Thoracic appendages: locomotion.
- Abdominal appendages serve a variety of functions, e.g. swimming or reproduction; often lost.

Question 23

Sclerotized cuticle composed of chitin

Arthropod characteristics

Answer 23

Arthropods possess a sclerotized cuticle composed of chitin.

The cuticle forms a hard, lightweight, external skeleton (exoskeleton), that provides a supportive, rigid surface for muscles to act against.

Cuticle sclerotization sometimes involves the incorporation of minerals (biomineralization), e.g. calcium carbonate in crustaceans.

When an arthropod grows, it moults its entire exoskeleton

Question 24

Cephalization

Arthropod characteristics

Answer 24

The head region concentrates neural tissues and is composed of several segments specialized for feeding and sensory perception.
- Arthropod heads often have eyes, olfactory receptors, and antennae that function in smell and touch.

Question 25

Reduced coelom | Arthropod characteristics

Answer 25

The strong, segmented limbs of arthropods eliminate the need for a hydrostatic skeleton (the main ancestral function of coeloms). Arthropods’ primary internal cavity is a **hemocoel**. - The hemocoel supports the internal organs

Question 26

Open circulatory system and respitory systems | Arthropod characteristics

Answer 26

**Hemolymph circulates in the hemocoel surrounding tissues and organs**. - Hemolymph is the circulatory fluid in the open circulatory system of arthropods; ± respiratory pigments for oxygen transport. Arthropods have evolved a **variety of respiratory systems**. - Gas exchange in small aquatic arthropods is by **diffusion across the body surface**. - Aquatic arthropods (crustaceans) usually have **gills** that are **modified appendages**, whereas terrestrial arthropods have **book lungs or tracheal systems**.

Question 27

What are the three major arthropod lineages?

Answer 27

Morphological and molecular evidence suggests three major arthropod lineages: 1. **Subphylum Chelicerata** (e.g. spiders, scorpions, ticks). 2. **Subphylum Myriapoda** (e.g. centipedes, millipedes). 3. **Clade Pancrustaceans** (e.g. crustaceans, insects).

Question 28

Arthropod phylogeny

Answer 28

The evolutionary relationships among arthropods are complex and are subject to ongoing revision. Historically, hexapods (insects and their relatives) and myriapods were grouped due to shared morphological characteristics. Molecular studies have shown that **crustaceans are paraphyletic: hexapods evolved within crustaceans**. - **Clade Pancrustaceans** include crustaceans and hexapods. - Recent results have shown that the insects are nested within lineages of aquatic crustaceans, with remipedians identified as a potential sister group to hexapods. - This relationship suggests insects originated from an aquatic ancestor and later adapted to terrestrial environments.

Question 29

Subphylum Trilobita

Answer 29

Trilobites are a **diverse group of extinct marine arthropods**. - Abundant and diverse in the Cambrian, thriving for almost 270 million years. - Became extinct during the Permian mass extinction (252 mya). Trilobite compound eye - >17k fossil species (hard, easily fossilized exoskeleton). Segmented body with **three distinctive tagmata**: - Cephalon (head); thorax (body); pygidium (tail). - Cephalon sensory organs: - Many trilobites had **compound eyes, with numerous lenses acting together to create a single image**. - One pair of flexible preoral antennae, likely used for sensory detection. Each thoracic segment had a pair of undifferentiated **biramous (two-branched) limbs**. - The ventral branch was likely used for locomotion (walking or swimming), while the dorsal branch functioned as a gill for respiration. - This biramous limb structure is thought to be ancestral to all arthropods. - In more derived arthropod lineages, the dorsal branch was lost, resulting in **uniramous (single-branched) limbs**. - Uniramous limbs evolved independently in several arthropod lineages.

Question 30

Chelicerata

Answer 30

Chelicerates (subphylum Chelicerata) are named for claw-like feeding appendages called **chelicerae**. - Chelicerates include diverse taxa such as **spiders, scorpions, and mites** Chelicerata originated as marine animals during the early Paleozoic era. - The earliest chelicerates were large (up to 2.5m), widespread **eurypterids** (water scorpions). - Most marine chelicerates (including eurypterids) are extinct, but some marine species survive today, including **horseshoe crabs**.

Question 31

What are the two main tagmata of Chelicerates? | Arthropods

Answer 31

Remember this is the group arachnids are from **Anterior cephalothorax**. - Entirely or partially covered by a carapace, a dorsal exoskeleton section. - **6 pairs of appendages**: chelicerae, pedipalps, 4 pairs of legs - Pedipalps are jointed sensory appendages. - No antennae. - Uniramous appendages/limbs. - Eyes, if present, are located on the cephalothorax. **Posterior abdomen** - No appendages. - Contains digestive and reproductive organs.

Question 32

Pedipalps

Answer 32

Pedipalps are jointed sensory appendages (The little face feelers on spiders and other Chelicerates)

Question 33

Chelicerae

Answer 33

Chelicerae serve as mouthparts adapted for predation or defence. Chelicerae are modified appendages that appear before the mouth. - Homologous to antennae in other arthropods. - 2 or 3-segmented fangs or pincers (chel = claw, ceros = horn). The **chelicerae of spiders are hollow**. - Contain venom glands that inject venom into prey or threats.

Question 34

Arachnids | Chelicerata

Answer 34

**Arachnids** are the dominant chelicerate group, **including spiders, scorpions, ticks, and mites**. - ~100k species, mostly terrestrial with diverse ecological roles. - Most are **fluid-feeding predators** (do not ingest solid particles). - Simple eyespots (**ocelli**) that focus light through a single lens. - Arachnids have an abdomen and a cephalothorax with six pairs of appendages (chelicerae, pedipalps, and four pairs of walking legs). - Separate sexes

Question 35

Spider Structure | Chelicerata: Arachnids

Answer 35

Spiders (>40k extant species) Spiders are **fluid-feeding predators**. - 2-segment chelicerae with **poison glands** that empty through the fangs of chelicerae. - Some dangerous to humans, e.g. black widows. Tight constriction between cephalothorax and abdomen. Gas exchange is via abdominal respiratory organs called **book lungs**. 0 – 4 pairs of ocelli (simple eyes)

Question 36

Spider hunting, nurturing, and courting behaviour

Answer 36

**Spinnerets**: abdominal glands that produce **silk**, a liquid protein. - The silk is used to capture prey, escape danger, transport, line burrows, and wrap eggs. Many spiders exhibit **maternal care** of eggs, and sometimes the young. Complex courtship, especially in species with good vision

Question 37

Scorpions | Chelicerata: Arachnids

Answer 37

~1,750 species, **primarily found in deserts or rainforests**. Their **pedipalps** evolved into **grasping pincers**. **The abdomen is segmented** - Terminates in a poisonous sting. - Many species are dangerous to humans. Scorpions **crush or sting their prey to death**. - They externally digest the body contents of their prey and ingest the digested fluid. They engage in **complex courtship**, with the male leading the female in a **mating ‘dance’.** The **female broods her fertilized eggs** beneath her - Hatchlings are carried on her back.

Question 38

Mites | Chelicerata: Arachnids

Answer 38

Mites (>40k named species) - They are mostly microscopic (size range: 80 μm to 3 cm) - **No separation** between the **cephalothorax and abdomen**. - **They resemble a ball with legs**. - No external evidence of segmentation - 2 or 3-segmented chelicerae. - 0–2 pairs of ocelli Diverse ecology: - Includes predators, detritivores, herbivores, and parasites. - Found in soil, water, plants, and on or in other animals. - Three species inhabit human skin! - **Some are fluid-feeders**, while o**thers are detritivores** that ingest solid particles. mportant arachnids in terms of economic impact : - Crop pests, such as spider mites that feed on 100’s of plant species. - Predatory mites are utilized as **biocontrol agents**. - **Medically significant species**, including chiggers, ticks, and dust mites.

Question 39

Myriapods | Anthropods

Answer 39

Myriapods (subphylum Myriapoda) include **millipedes** and **centipedes**. Myriapod characteristics: - **Terrestrial** (no aquatic myriapods). - Less tagmatized body plan: - Head with **simple eyes** (ocelli), **three pairs of mouthparts**, incl. mandibles for main mouthparts (biting/chewing), and **one pair of antennae**. - **Numerous (>3) pairs of legs** (myria = a great many): long trunk with numerous pairs of jointed **uniramous** legs. - Gas exchange via tracheal system (respiratory tubes). - Air enters lateral **spiracles**, through the **trachea**, into tracheoles throughout the body; most lack O2-transporting respiratory pigments in the hemolymph. - Direct development (without larval stages)

Question 40

Millipedes | Myriapods

Answer 40

Millipedes (~12k species, mostly tropical). - **Detritivores** or **herbivores** (eat decaying leaves and plant matter). - **Two pairs** of **jointed legs per body segment**. - Diplosegments result from the fusion of adjacent pairs of segments (class Diplopoda, diplo = double).

Question 41

Centipedes | Myriapods

Answer 41

Centipedes (>3k species, mostly tropical). - **Fast-moving predators**. - **One pair** of **jointed legs per segment**. - One pair of modified legs on the first trunk segment. - Form venomous claws to subdue prey and fight predators (class Chilopoda, cheilos = lip, pod = foot). - A few species are poisonous to humans

Question 42

Crustaceans | Arthropods (Name the 3 major types)

Answer 42

Crustaceans (subphylum Crustacea; paraphyletic) include crabs, lobsters, shrimp, and their relatives. - 40–50k extant species. - **Mostly marine**, but many freshwater and some terrestrial species. - Crustacean morphology is highly variable. - Huge size range: microscopic zooplankton to giant crabs. - **Isopods** include terrestrial, freshwater, and marine species. - Pill bugs are a well-known group of terrestrial isopods. - **Copepods** are small crustaceans, many of which are planktonic. - **Decapods** are relatively large crustaceans, including lobsters, crabs, crayfish, and shrimp

Question 43

Crustacean body plan

Answer 43

The crustacean body plan includes: - Three tagmata: - **Head, thorax,** and **abdomen**. - The head and thorax often merge in a **cephalothorax** (cephalo = head). - Cephalothorax is usually covered by a **carapace** - An exoskeleton shield that covers dorsal and lateral sides - Carapace sometimes also covers the abdomen. - **Biramous limbs**. - **Two pairs** of **antennae**. - **Compound eyes** (often stalked).

Question 44

Crustacean Limbs

Answer 44

Crustacean tagmata typically have branched (biramous) limbs adapted for **specific functions**: - **Head: two pairs** of **antennae**, paired segmented **mandibles** (main mouthparts; biting/chewing), and **2 or more pairs of maxillae** (tasting and manipulating food). - **Thoracic limbs** are usually used for **locomotion**. - Crustaceans >4 pairs of thoracic limbs. - **Abdominal limbs** are used for swimming, gas exchange, filter feeding, or holding eggs

Question 45

Crustacean exoskeleton

Answer 45

**The exoskeleton** is often strengthened by **biomineralization**. - Calcium carbonate incorporated into the cuticle (crust = crunchy). - In barnacles, a group of mostly sessile crustaceans, the cuticle is hardened into a shell.

Question 46

Crustacean respiration

Answer 46

Small crustaceans exchange gases **through the cuticle**; larger crustaceans have **gills**. - Gills are usually modified appendages. - Plate-like or fluffy, well-vascularized, thin unsclerotized cuticle. - O2 is transported by respiratory pigments (e.g. hemocyanin) in the hemolymph. A few terrestrial crustaceans use **tracheae**. - Branched tunnels from openings in the body wall delivers oxygen directly into internal tissues. e.g. woodlice/pill bugs

Question 47

Crustacean life cycle

Answer 47

Crustacean life cycles are highly variable (hard to generalize). - Most crustaceans have **separate sexes** and **reproduce sexually**. - Except for sessile barnacles, which are hermaphrodites, but rarely self-fertilize. - Asexual parthenogenesis (embryonic development without fertilization of diploid eggs) is widespread among crustaceans. - Sperm transfer occurs via **copulation** (sexual intercourse). - The male places sperm on or in the female’s sperm-receiving structure. - Copulation occurs via intromission or spermatophore deposition (sperm capsule deposited at the female’s genital opening). - Some female crustaceans brood their eggs until they hatch. - **Direct (e.g. Daphnia) or, most commonly, indirect development** with one or more larval stages.

Question 48

Crustacean ecology and importance

Answer 48

- Crustaceans play crucial roles as **detritivores, scavengers, and predators** in various ecosystems. - Planktonic crustaceans, such as copepods and larval crustaceans, serve as food for many vertebrates, including fish and whales. - Some crustaceans engage in filter feeding; this includes both motile species, like **krill**, and sessile species, such as most **barnacles**. - Some crustaceans **scavenge** on plant or animal matter, while **others are active predators**. - Additionally, some crustaceans are **parasitic**, targeting both vertebrates and invertebrates. - Examples include isopod “whale lice” and copepod “sea lice” that infect salmon.

Question 49

Hexapods | Arthropods

Answer 49

Subphylum Hexapoda is an enormous clade that includes insects and their relatives. - **6-legged arthropods (hex = six)**. - Three pairs of legs on thoracic segments; no appendages on the abdomen. Class Insecta - Most diverse of all animal groups; >1 million described extant species. - Insects inhabit nearly all terrestrial habitats and some freshwater habitats. - Insects are the only group of invertebrates that evolved **wings** and **flight**. - Insects diversified several times (**adaptive radiation**), following the evolution of flight, adaptation to feeding on gymnosperms, and the expansion of flowering plants (insect pollinators). Three other basal classes of wingless hexapods: - Protura (coneheads); - Collembola (springtails); - Diplura (two-pronged bristletails).

Question 50

Hexapod characteristics

Answer 50

Consistent tagmatization (specialization of body parts): - **Head** with **one pair of antennae** and **3 pairs of mouthparts**, incl. mandibles for main mouthparts. - **Thorax** with **3 pairs** of **jointed uniramous legs ± wings**. - Abdomen without paired jointed appendages. Gas exchange occurs through a **tracheal system** via lateral **spiracles**. - Branched cuticle-lined tubes carry oxygen directly to tissues. - Most hexapods lack O2-transporting respiratory pigments in their hemolymp

Question 51

Insect Wings

Answer 51

Wings represent a pivotal evolutionary innovation in insects. Several basal insect groups lack wings. - Paraphyletic Apterygota (a = without), e.g. silverfish. Most insects have wings as adults. - **Subclass Pterygota** (pter = wing); 99.9% of all insects. - 1-2 pairs of wings on thoracic segments. - Insect wings are an extension of the cuticle. - Wings evolved only once within this clade. - **Wings** are a **shared derived trait of clade Pterygota**. - How and why insect wings evolved is not well understood. - **Flight** is a key factor in the **adaptive radiation of insects**. - Animals that can fly can evade predators, locate food sources, and colonize new habitats efficiently. - Some Pterygota have secondarily lost their wings. - e.g.lice, bed bugs, and fleas.

Question 52

Metamorphosis | Insect Life Cycle (Incomplete vs Complete)

Answer 52

Many insects undergo metamorphosis (indirect development), a process involving distinct larval and adult stages with differing morphology, behaviour, and ecology. - In **incomplete metamorphosis**, the juveniles, called **nymphs**, resemble adults but are smaller and **undergo multiple moults before reaching full size**. - Wings develop slowly over several moults, e.g. grasshoppers and dragonflies. - Insects with **complete metamorphosis** progress through larval stages known as maggots, grubs, or caterpillars. - The larval stage has different morphology and ecology than the adult, e.g. caterpillar vs. butterfly, maggot vs. fly. - Wings develop all at once during a specialized **pupal stage**

Question 53

Hexapod Reproduction

Answer 53

Most insects have **separate males and females and reproduce sexually** (no hermaphrodites). Non-insect hexapods and **apterygote insects transfer sperm indirectly** via a **spermatophore** (an encapsulated package of sperm). **All pterygote insects copulate** (sexual intercourse). - The male places sperm directly on or in the female’s sperm- receiving structure. - Individuals find and recognize members of their species by colours, sounds, or odours.

Question 54

Insect Moulting

Answer 54

During moulting, insects shed their entire exoskeleton. - Includes cuticle invaginations, e.g. linings of foregut, hindgut, and trachea/tracheoles. - The new cuticle is produced before moulting and is soft and wrinkled. - After the moult, the insect swallows air or water to expand the new exoskeleton

Question 55

Insect ecology and importance

Answer 55

Insects **primarily inhabit terrestrial habitats** but often have aquatic larval stages. Insects are crucial as **detritivores**, helping with decomposition and nutrient recycling. **Insects are vital food sources for many animals**, including birds and fish. **Beneficial insects**: - Some insects, like honeybees, are vital **pollinators** for crops and plants. - Others, such as ladybugs and lacewings, act as **biological control agents** by preying on pest species. - Some insects are parasites of other insects, regulating pest populations in ecosystems. **Harmful insects**: - Insects are the leading biotic factor limiting agricultural production. - The larvae stages of many insects are **major crop pests** causing significant damage. - Some **insects**, such as mosquitoes, **transmit diseases** that threaten human health.