Insect Ancestry

Question 1

What was the most abundant fossil insect?

Answer 1

Palaeodictyoptera - found in the Carboniferous
•wasn’t an active flyer - instead glider, had long and broad wings
•behaviourally different to modern insects, but morphologically similar

Question 2

Insects all have shared characteristics like…

Answer 2

• jointed legs
• external articulated skeleton
• compound eyes
• distinct head

Question 3

2 possible phylogenies

Answer 3

1. They have several origins - polyphyletic from a worm ancestor
2. Have a single origin - monophyletic from a crustacean ancestor

Question 4

Worm origin assumptions

Answer 4

• would have uniramous limbs (single branch)

* suggested they are all similar due to the constraints imposed by a rigid exoskeleton… so focuses on differences

Question 5

Crustacean ancestor assumptions

Answer 5

• insects would be biramous

* if they don’t show this will have been lost in evolutionary time

Question 6

Fossil record evidence

Answer 6

•fossil record shows insects as uniramous
•in Carboniferous insects diversified and radiated - not a period conducive to fossilisation
-may be that no biramous fossils were found bc of this
•fossils from Soviet Union - saw vestigial legs attached to normal on fossilised insect
=monophyly ?!?

Question 7

Neuroanatomical techniques

Answer 7

Comparing neuroanatomical wiring - the layout of the visual system of insects, to worms and crustacea
•brain and visual system wiring more similar to crustacea
=monophyly?!?

Question 8

Developmental genetics - EvoDevo

Answer 8

Looking at distal-less which determines limb branching
-won’t have this gene if from worms, as will not need it
•insects do have this gene, just don’t express it
•when switched on will cause biramous limbs
=monophyly?!?

Question 9

mtDNA evidence

Answer 9

Changes more slowly over time than normal DNA
•insects more similar to crustacea
=monophyly?!?
•showed Collembola branched off before insects - consistent with polyphyly?

Question 10

The ancestor of the insects…

Answer 10

•invaded around 500mya
•crustacean fossil found in Australia - similar to woodlouse, a Euthycarcinoid
-favoured bridging habitats
-had more than 6 legs
-segments very similar to those of insects
-lived in shallow marine habitats - scavenging

Question 11

Overcoming problems of invading land

Answer 11

1. Water loss (and osmoregulation) - large SA:V
   •evolution of cuticle, initially for preventing entrapment in water film
2. Respiring in air
   •tracheal system
3. Reproduction and fertilisation
   •internal fertilisation
   •some crustacea already produced spermatophores
4. Supporting body weight on land
   •selects for small size (more powerful muscles etc.)
   •have a stable, tripod gait (any 3 legs on ground at one time)

Question 12

The first insects

Answer 12

Apterygotes (wingless)
•silver fish, collembola
•small, restrictive, primitive and primitive jaws
•scavengers of dead material
•300-350mya diverse communities everywhere
-no evidence in much changing in 60mys