Subphlyum Hexoda: Class Insecta

Introduction

• Nearly 1,000,000 species (80% of all animals)
• Some reasons they are so successful:
• They can fly
• Tremendous reproductive capabilities
• Generally don’t compete for food
• They are a major influence on nature and man

Characteristics

• What makes them different from other arthropods?
• Three pairs of walking legs
• Wings, usually
• Bodies divided into three segments: Head, thorax, abdomen
• One pair of sensory antenna
• Movement
• Three pairs of jointed legs along the thorax.
• Used for movement, but types vary depending on what they are needed for:
• Flies have tiny claws and sticky pads (they can climb on smooth surfaces)
• Bees have fuzzy legs (used for collecting pollen)
• Most have 2 pairs of chitinous wings (although some have 1 pair and a few wingless species exist)
• Only invertebrates that can fly
• Flying styles and speeds vary
• Butterfly flaps wings 5-6 times per second
• Bees flaps wings up to 200 times per second
• Flies fly about 8km/h and dragonflies 40 km/h
• There are four different types of wings that vary based on need and use
• Membranous wings: thin, transparent, and crisscrossed with supporting veins. Most have this type

Membranous Wings

• Scale-covered wings: the flight wings of butterflies and moths are covered with delicate, beautifully colored scales that rub off easily

Scale-covered wings

• Leatherlike wings: grasshopper’s membranous wings covered by a pair of leather like wings 

Leather-like wings

• Horny wings: the front wings of beetles are thick shields that cover most of the dorsal surface of the insect

Horny wings

• Nutrition
• Ingestion:
• Variety of different types of mouthparts called mandibles
• chewing
• sponging
• siphoning
• piercing
• Three main parts of digestive system:
• Foregut: food enters through the mouth, it is moistened with secretions from the salivary glands, passes through to the gizzard where it is thoroughly ground
• Midgut: gizzard opens into the stomach. Digestive juices digest the food.
• Major site of digestion and absorption.
• Hindgut: solid residue left in the midgut after digestion passes through the hind gut, or intestine, and out through the rectum and anus
• Respiration, circulation, and excretion
• Respiratory system of insects is an elaborate system of tubules called tracheae that branch throughout the animal.
• This system is so complete that oxygen is transported throughout the animal without using the circulatory system.
• These tubules open to the outside through a sereis of spiracles, small pores that run along each side of the animal. 
• The insect “breathes” as abdominal contractions pump air in and out of the tracheae.
• The open circulatory system is only involved in the transport of nourishment and collection of waste. No gas exchange.
• Cellular waste is extracted by Malpighian tubes and excreted into the intestine for elimination.
• Irritability
• Anterior “brain” connected to an anterior ganglion and ventral nerve cord.
• Receives information from a variety of sources:
• Smell from antennae.
• Many insects produce chemicals called pheromones to find a mate.
• Antennae can sense humidity and flight speed.
• Taste receptors on the mouth parts.
• Tactile hairs on the antennae, limbs and body
• Compound eyes or several simple eyes
• Some have organs to hear
• Reprodution
• Sexes are separate.
• Females store sperm in seminal receptacle
• As she lays her eggs they are fertilized
• Eggs are protected either by where they are laid (deep in tree bark for example) or with protective coverings.
• Metamorphosis


• Developmental changes in insects is called metamorphosis. 
• This permits various stages to perform different functions.
• Ex. some insects eat only during their immature forms and become adults only to reproduce.
• Only a few species of insects do not exhibit metamorphosis.
• Incomplete metamorphosis (grasshoppers)
• The nymph that hatches looks like a miniature, oddly proportioned adult. 
• As it grows and molts it becomes more like the adult.
• Same diet and habitat for the nymph and adult.
  
  

• Complete metamorphosis (90% of all insects)
• Involves 4 stages: egg, larva, pupa, and adult
• Egg hatches into larva (a segmented wormlike stage)
• Some common names: maggots, grubs, caterpillars
• After period of eating and molting, the larva enters the pupal stage.
• The pupa makes a cocoon around itself (woven silk case).
• Though it is quiet on the outside, it is a period of significant activity and grown. 
• During pupal stage, the organs are dismantled and completely reshaped. 
• Eventually the cocoon opens revealing a fully developed insect. 
• This process is controlled by hormones.