Arthropods

Question 1

What are arthropod exoskeletons made of?

Answer 1

Chitin

Question 2

How many species of arthropod are there?

Answer 2

Approx. 180,000

Question 3

What percentage of all extant organisms are arthropods?

Answer 3

1/2

Question 4

The arthropod group is hugely diverse. What percentage of past arthropod diversity is now extinct?

Answer 4

99%

Question 5

From what period do we have the best fossil record for arthropods?

Answer 5

The Cambrian when rapid diversification occurred.

Question 6

Define a trace fossil.

Answer 6

A fossil that shows evidence of arthropod locomotion, shows their tracks.

Question 7

Most arthropod fossils are trace fossils. True or false?

Answer 7

True.

Question 8

What are the 4 major subphyla of arthropod?

Answer 8

1. Chelicerata
2. Myriapoda
3. Hexapoda
4. Crustacea

Question 9

What organisms does the chelicerata contain?

Answer 9

Spiders, scorpions, horseshoe crabs and sea spiders.

Question 10

What organisms does the myriapoda contain?

Answer 10

Millipedes and centipedes.

Question 11

What organisms does the hexapoda contain?

Answer 11

Insects, silverfish and springtails

Question 12

What organisms does the crustacea contain?

Answer 12

Crabs, lobsters, shrimp, barnacles etc.

Question 13

Define the tracheata.

Answer 13

All arthropods that breathe via a tracheal system.

Question 14

What is another name for the tracheata?

Answer 14

Atelocerata.

Question 15

Which groups does the tracheata contain?

Answer 15

Hexapods and myriapods.

Question 16

How do a) chelicerates and b) crustaceans breathe?

Answer 16

a) Book lungs

b) Gills

Question 17

Define pancrustacea.

Answer 17

A clade that proposes the hexapods and crustaceans are a sistergroup.

Question 18

What is another name for pancrustacea and why?

Answer 18

Tetraconata: based on the ommatidia, which are the optical units of the compound eye. Both hexapods and crustaceans have 4 cone cells. Plus the axons in the neuropil (dense areas of nerves) cross over - this is unique to hexapods and crustaceans.

Question 19

What is pancrustacea based on?

Answer 19

Molecular data: ribosomes, HOX genes and novel microRNA.

Question 20

There are 2 competing hypotheses for the phylogenetic placement of arthropod groups. What are they?

Answer 20

1. Paradoxopoda

2. Mandibulata

Question 21

Explain the paradoxopoda hypothesis.

Answer 21

Proposes that the arthropods are composed of two sister groups: pancrustacea is one and the myriapods and the chelicerates form the other.

Question 22

Explain the mandibulata hypothesis.

Answer 22

Proposes that the pancrustacea and myriapods form a clade called the mandibulata. The chelicerates are proposed as a sistergroup.

Question 23

What similarities between the myriapods and chelicerates serve as support for paradoxopoda? Give 2 reasons.

Answer 23

1. Mandibles of myriapods anf chelicerate fangs share a structural identity
2. Neural tube undergoes post-mitotic division to form the ectoderm in the same way

Question 24

Why is the paradoxopoda theory now widely dismissed?

Answer 24

It is though similarities are due to convergent evolution, not shared ancestry.

Question 25

Explain why the structure of the head serves as support for the mandibulata hypothesis.

Answer 25

In the mandibulata (hexapods, crustaceans and myriapods), the limb on the third head segment has been internalised to form the jaws. In the chelicerates the fangs are external appendages.

Question 26

Describe the structure of the jaw in the madibulata.

Answer 26

The mandible is the crushing mouthpart formed from the appendage on the third head segment. The labrum is the chewing chamber. They chew using the dentate and grinding plate apparatus.

Question 27

What is dachshund and why does it support the mandibulata hypothesis?

Answer 27

1 of the 4 main genes in the arthropod leg: in mandibulata it is expessed at the tip of the leg. In chelicerates it is expressed proximally (near the body).