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How many insects are there?

Estimated 10^18 insects alive at any one time

Over 1,000,000 described species

Constitute ~50% of all known species

Estimates range from 2-30 million species in total


Insect Diversity

Vary in size from sub-milimeter to crow-sized

heaviest extant: 100g

longest extant: 56cm

Habitats: just about everywhere save marine



•Solitary: e.g. preying mantis, locust, bees

•Gregarious (act as individuals but aggregate in groups): e.g. locust, collembola, bees•

Sub-social (most primitive level of interaction involving parents and offspring): e.g. wasps, bees

•Highly social (complex social “caste” system): e.g. ants, bees, termites


Common Features

•  Body divided into: head, thorax, abdomen
•  Head: One pair of antennae and mandibles; 2 pairs of maxillae

•  Thorax: 3 pairs of legs; usually 2 pairs of wings

•  Abdomen: no locomotory appendages; genital opening usually at posterior end

•  Post-embryonic development requires metamorphosis


Classification of Insects

•All insects are in the class ‘Insecta’

•There are two sub-classes:
1.Apterygota  2.   Pterygota

•The sub-class Pterygota has two divisions:
i)   Exopterygota  ii)   Endopterygota

•These two divisions also known as:
Hemimetabola  Holometabola


Sub-class Apterygota

•  No wings – a primitive feature
•  Metamorphosis slight or absent (nymphs/young are effectively miniature adults (can’t reproduce)
•  One (or more) pairs of pre-genital appendages on abdomen
•  Adult mandibles articulate with head capsule at single point
Contains orders: Collembola (springtails)  and  Thysanura (silverfish)


Subclass Pterygota

•Winged (sometimes secondarily wingless)
•Metamorphosis occurs
•Adults have no pre-genital abdominal appendages
•Adult mandible articulates with head capsules at two points
•Contains all the ‘important’ orders


Superorder Exopterygota

(aka hemimetabola)

•Simple metamorphosis
•Immature stages are nymphs – increasingly resemble adult
•Pupa rarely present
•Wings develop externally
•Nymphs (called naiads if aquatic) and adults often feed on same food resources
•Includes: dragonfly, termites, cockroaches, greenfly.


Superorder Endopterygota

•Metamorphosis complex
•Immature stages are larvae – differ from adults in form and function (and often diet)
•Larva assimilates food
•Pupa present: does not feed, tissues ‘re-organised’
•Wings develop internally
•Contains: flies, butterflies and moths, bees, wasps, ants and beetles


Types of Insect Larvae

Larva(e) is a generalised term that refers to all endopterygote orders (and sometimes to exopterygote orders)
Most larvae can be grouped into one of five categories based on physical appearance


Eruciform Larvae


•Well defined segmentation with abdominal prolegs
•Each thoracic segment has a pair of segmented legs
•Well developed head capsule with biting mouthparts
•Head strongly sclerotized; antennae present
•Peripneustic arrangement of spiracles (prothorax and abdominal sections 1-8)
Abdominal prolegs on segments 3, 4, 5, 6 and 10 (in Lepidoptera)


Campodeiform Larvae 

OLIGOPOD: Lack abdominal pro-legs; Have functional thoracic legs
•  Well sclerotised
•  Distinct segmentation
•  Dorso-ventrally flattened
•  Active predators


Scarabaeiform Larvae 

OLIGOPOD: Lack abdominal pro-legs; Have functional thoracic legs

•  Well sclerotised head only
•  Less distinct segmentation
•  slow moving detritivores or phytophages


Apodous Larvae

Larvae without legs – e.g. vermiform larvae of diptera, which are often classified based on head capsule characteristics.

Eucephalous – well sclerotised head capsule.

Hemicephalous – reduced (retractable) head capsule.

Acephalous – without a head capsule


Larval Types by Order


Endopterygote Pupae

Between their last larval instar and their adult form, Endopterygotes enter a pupal stage


Insect Reproduction

•Parthenogenesis (development from an unfertilised egg)
•Can be obligatory or facultative
•Virtually all orders have at least one asexual representative
•Highly successful means of rapidly exploiting favourable conditions
•Female only eggs = thelytokous e.g. aphids
•Male only eggs = arrhenotokous e.g. Hymenoptera (male drones)
•Male and female eggs = amphitokous e.g. Hymenoptera

•Hermaphroditism (male and female reproductive organs)

•Polyembryony (two or more embryo’s from one egg)


  Oviparity (egg-laying)



•Pseudoplacental viviparity


•Hemocoelus viviparity


•Adenotrophic viviparity



The Insect Head


Insect Mouthparts

Good example of diversity in form and function:
•Specific mouthparts have been adapted to feed on different substrates
•In some cases. Different mouthparts have been modified to feed on the same type of food, e.g. liquids
•Two broad categories:
•mandibulate (chewing)
•haustellate (piercing-sucking, sponging, and siphoning) 


Mandibulate Mouthparts


Haustellate Mouthparts (aphid)


Haustellate Mouthparts (Housefly)


Mouthparts Glossary

Labrum: articulates with the lower margin of the insect's "face" (the clypeus), concealing some or most of the mandibles.

Mandibles: positioned between the labrum and maxillae. Largest mouthparts of chewing insects - used to masticate (cut, tear, crush, chew) food.

Maxillae: (paired) situated beneath the mandibles, manipulate food during mastication, have hairs and “teeth” along their inner margins.

Labium: single structure, although formed from two fused secondary maxillae. Described as ‘floor’ of the mouth.

Proboscis: siphoning mouthpart, e.g. in Lepidoptera.

Stylet: elongated piercing mouthpart, e.g. in Aphids.

Labellum: sponge-like feeding organ with pseudotrachae, e.g. house fly.


Order Collembola 

ancient order – now actually considered to be separate Class within Super-Class Hexapoda.

Apterygote – without wings, no metamorphosis

Two main groups (sub-orders): elongate and globular.

Habitat: soil and leaf litter (100, 000 ind/m3)

Diet: detritus

One of the most abundant macroscopic animals


Leg Morphology (Specifically Bees)

•Distinct modifications of leg segments for pollen collection and transport.
• Pollen is collected by spines on first tarsal segment and transferred to the tibia of opposite leg – the pollen ‘basket’ for transport back to hive.
• A spur at the base of tibia on middle leg dislodges pollen masses from the basket.


Click Beetles

(true beetles)

• Function: to regain normal walking position when tipped over or fall from plants onto back (and deter predators if on back).
• A ‘spine on ventral surface projects back into cavity on the mesothorax.
• Spine pushes quickly into cavity (click noise) and beetle ‘rolls’ to one side and can move onto legs.



Division Holometabola, Order Siphonaptera

• Tube-like mouth-parts adapted to feeding on the blood of their hosts.
• Bodies are laterally compressed permitting easy movement through the hairs or feathers.
• Legs are long, the hind pair well adapted for jumping (around 200 times their own body length)
• Body is hard, polished, and covered with many hairs and short spines directed backward, which also assists its movements on the host.
• Tough body is able to withstand great pressure, likely an adaptation to survive scratching etc



Social Structures in Insects

 Most insects are entirely solitary in life style
• In Exopterygote insects (Hemimetabola), only strongly social insects are the Order Isoptera -  TERMITES

• In Endopterygote insects (Holometabola), social insects are in the order Hymenoptera – BEES, WASPS ANTS


Order Isoptera

Termites - important pest in tropical areas

Highly social insects: Large colonies in mounds – TERMITARIUM
• Up to 4-5m high (several million individuals)
4 Castes

Primary reproductives  (eventually become King and Queen)

  winged adults (♂ and ♀) (swarmers)

  Roles: establish colony

• Secondary (supplementary reproductives if either primary dies)

  wingless adults (♂ and ♀) or reduced wings

• Workers –   wingless and sterile

Roles: foraging, food storage, brood/nest maintenance
• Soldiers –   wingless and sterile

  FUNCTIONAL MORPHOLOGY: mandibulate and nasute

Roles: defense