Hexapods

Subphylum Hexapoda Members of the

subphylum Hexapoda are named for the presence of six legs. All legs are uniramous.

Hexapods have 3 tagmata: Head Thorax Abdomen

Appendages attach to head and thorax.

Class Insecta

Insects are the most diverse and abundant of all arthropods. 26 orders

Most are terrestrial or inhabit freshwater. Few are marine.

MOST HAVE WINGS!

Class Insecta

Insects have:3 Tagmata: head, thorax, abdomen.3 pairs of legs and usually 2 pairs of wings

on their thorax.

Class Insecta

Insects show a diverse array of morphological variation. Head – compound eyes, one

pair antennae, 3 ocelli, mouthparts (including mandibles & maxillae)

Thorax – 3 segments each with a pair of legs, the last 2 segments usually have wings as well.

Abdomen – 9-11 segments

Class Insecta

Antennae can act as tactile organs, olfactory organs, and sometimes auditory organs.

Class Insecta

Legs have also become highly specialized for walking, grasping, skating over water, and specialized jobs like gathering pollen.

Class Insecta

Insects also have highly variable body forms. Land beetles are

thick and shielded. Aquatic beetles are

streamlined. Cockroaches are

flat and live in crevices.

Class Insecta

Flight is one key to the great success of insects.

An animal that can fly can escape predators, find food, and disperse to new habitats much faster than organisms that can only crawl.

Class Insecta

Insects are the only invertebrates that can fly.

Insect wings not homologous with bird and flying mammal wings.

Class Insecta - Power of Flight

Most have two pairs of wings. Some are ancestrally

wingless – silverfish. Some are secondarily

wingless – fleas. Recent fossil evidence

suggests insects may have evolved fully functional wings over 400 million years ago.

Class Insecta - Modifications of Wings Wings for flight are thin and

membranous. The thick and horny front wings

of beetles are protective. Butterflies have wings covered

with scales. Caddisflies have wings covered

with hairs. Flies & Bees: indirect flight

muscles used, asynchronous, beat at least 100 times per sec.

Class Insecta - Wing Thrust

Direct flight muscles alter the angle of wings to twist leading edge to provide thrust. They are attached directly to the wings.

Indirect flight muscles are not attached to the wings and alter the shape of the thorax.

Figure-8 movement moves insect forward.

Fast flight requires long, narrow wings and a strong tilt, as in dragonflies and horse flies.

Class Insecta

The internal anatomy of an insect includes several complex organ systems.

Insects - Nutrition

Most insects are herbivorous, feeding on plant juices and/or tissues. Some are

specialized, others will eat almost any plant.

Insects - Nutrition

Some insects are predaceous, catching & eating other animals.

Insects - Nutrition

Other insects are scavengers or parasites.

Saprophagous insects- feed on dead animals

Insects - Nutrition

Some insect parasites are parasitized by other insects – hyperparasitism.

Parasitoids are a lethal type of parasite. A tiny wasp lays eggs on the tomato hornworm.

The wasp larvae will consume the hornworm.

Insects - Nutrition Insects have

mouthparts specialized for the many different foods they eat.

Sucking mouthparts (mosquitoes) – form a tube, can pierce animal or plant tissues.

Insects - Nutrition

Sponging mouthparts (house flies) – liquid food is lapped up, food may be liquefied first.

Insects - Nutrition

Chewing mouthparts (grasshoppers) – strong plates can tear food.

Insects – Circulation & Gas Exchange

Insects have an open circulatory system.

Gas exchange is accomplished with a tracheal system. Tracheal trunks open to

the outside by spiracles.

Insects – Circulation & Gas Exchange

Insects & spiders have independently evolved an excretory system of Malpighian tubules – blind tubules opening into the hindgut.

Insects – Nervous System

The nervous system resembles that of larger crustaceans, with fusion of ganglia.

Neurosecretory cells in brain function to control molting and metamorphosis.

Mechanoreception – Mechanical stimuli are received by sensilla (simple or complex) distributed over the antennae, legs, and body.

Insects – Sensory Organs

Auditory Reception – Very sensitive setae or tympanal organs detect vibrations that come through the substrate or the air. Some moths detect

ultrasonic pulses emitted by bats. They drop toward the ground in response to avoid the bats.

Insects – Sensory Organs

Chemoreception – Chemoreceptors for taste and smell are located in sensory pits on the mouthparts, antennae or sometimes the legs.

Visual Reception – Simple eyes (ocelli) are used to monitor light intensity, they do not form images.

Compound eyes in insects, similar to those of crustaceans, consist of thousands of ommatidia, each having its own pigment cells and lens.

Compound Eye

Insects – Sensory Organs

Different insects have different capability to see color.Bees can distinguish most colors (they don’t

see red) beginning in the ultraviolet range.To us a flower may look uniformly colored, but to

the bee there are lines that appear in the UV range that act as nectar guides.

Other insects, like butterflies, can see red.

Insects - Reproduction

Sexes are separate, some are parthenogenetic. Fertilization is

internal. In some, like

butterflies, nutrients are passed to the female as well as sperm.

Insects - Reproduction

Insects have a variety of methods for attracting mates.Pheremones (Compounds secreted by

one that affects the behavior of another) play an important role in many species.

Fireflies communicate using light flashes.Crickets communicate using sound.

Insects - Reproduction

Female insects deposit eggs on a specific habitat that will provide food for larvae. Monarch butterflies lay eggs on milkweed plants. Parasitoid wasp species lay eggs on tomato

hornworms. Mosquitoes lay eggs in standing water where the

larvae will live as filter feeders.

Insects - Metamorphosis

Many insects undergo metamorphosis during their development.Each stage between molts is called an

instar.

Insects - Metamorphosis

Insects with complete metamorphosis have larval stages specialized for eating and growing that are known by such names as maggot, grub, or caterpillar.

The larval stage looks entirely different from the adult stage.

HOLOMETABOLOUS

Insects - Metamorphosis

Female butterflies lay eggs on the plant that the caterpillars will feed on.

After the eggs hatch, the larvae (caterpillars) eat and grow, molting many times.

When it reaches a certain size, the larva will molt one more time, becoming a pupa (chrysalis in butterflies).

Insects - Metamorphosis

Metamorphosis from the larval stage to the adult stage occurs during a pupal stage.

Insects - Metamorphosis

In incomplete metamorphosis, the young, called nymphs resemble adults but are smaller and go through a series of molts until they reach full size.

HEMIMETABOLOUS

Insects - Metamorphosis

Direct DevelopmentSilverfish and springtails have young similar

to adults except in size and sexual maturation.

Stages are egg-juveniles-adult.Wingless insects.

Insect Behavior

Insects exhibit a wide range of behaviors involving innate behaviors, pheromones, and learning.

Trophallaxis: social insects- pass pheromones between individuals for mutual feeding

Insect Behavior

Fireflies use bioluminescence to signal each other. The female firefly attracts males by using a particular flash pattern. Another firefly species

mimics the call of the female and then eats the males that arrive.

Social Insects

Honey bees, ants and termites have complex social groups.

In honeybees: The queen is the

reproductive female. Workers are non-

reproductive females. Drones are haploid males.

(Developed from unfertilized eggs)

Social Insects

Ants have fascinating societies where they “farm” fungi, herd “ant cows” (aphids which they keep for the honeydew that they secrete), sew their nests with silk, and even use tools.

Insects and Humans

Insects can be beneficial, preying on harmful insects, fertilizing crops etc.

Insects are critical components of most food chains and an important food source for many fish and birds

Insects and Humans

Or, they can be harmful, spreading disease, eating crops etc.

Care must be taken when controlling pests that beneficial insects are not harmed.

Insects and Humans

Control of Insects:Broad-spectrum insecticides damage

beneficial insect populations along with targeted pest.

Some chemical pesticides persist in the environment and accumulate as they move up the food chain.

Some strains of insects have evolved a resistance to common insecticides.

Insects and Humans

Biological control – use of natural agents, including diseases, to suppress an insect population.

Bacillus thuringiensis - bacterium that controls lepidopteran pests. Gene coding for the “B.t.” toxin has been introduced to other

bacteria and transferred to crop plants. Some viruses and fungi may be economical

pesticides. Natural predators or parasites of insect pests can be

raised and released to control pest. Release of sterile males can eradicate the few insect

species that only mate once.

Insects and Humans

Integrated pest management - combined use of all possible, practical techniques listed above, to reduce reliance on chemical insecticides.