Arachnids

Question 1

Arachnida basics

Answer 1

● ~93,000 species, but likely many hundreds of
thousands yet to be described
● Group receives bad press (blood-sucking parasites, venomous etc…

Question 2

Excretionof nitrogenous waste

Answer 2

Hexapods & Arachnids secrete uric acid into hindgut via Malpighian tubules
● In arachnids the tubules are endodermal & arise from gut
● In the Hexapods & Myriapods they are ectodermal & arise from hindgut
↳ Convergent

Question 3

Scorpiones

Answer 3

● Generally nocturnal
● Relatively large; between 3 – 9 cm
● Often seek refuge in dwellings

Question 4

Scorpiones body plan

Answer 4

● The body consists of a cephalothorax covered by a carapace and long abdomen and sting
● In the middle of the carapace are a pair of large median eyes
● Small anterior chelicerae and enlarged pedipalps
that form pincers

Question 5

Araneae (spiders)

Answer 5

● ~53,000 species described
● Complex phylogeny that’s poorly resolved

Question 6

Araneae body plan

Answer 6

● Spiders have cephalothorax and abdomen – which is connected with a pedicel
● The chelicera are modified to consist a fang which is equipped with venom glands to attack prey
● The pedipalps are short and leglike but in males these have become modified copulatory
organs
● Have book lungs like other pulmonate arachnids

Question 7

Spider eyes

Answer 7

Spiders have multiple different eye layouts, all variants of “simple” (camera-type) eyes, with a single lens that focuses light onto an array of photoreceptors
Secondary eyes typically have a reflective tapetum behind the photoreceptors
(causes eye-shine and makes eyes appear pale)

Question 8

Silk

Answer 8

● Each spinneret has many openings from the silk gland
● Silk is made of protein and is emitted as liquid that hardens when drawn out of the abdomen (either by walking or using legs)

Question 9

Feeding

Answer 9

● Predatory
● Chelicerae contain venom gland & fang
● Food pre-digested by injecting enzymes &
ingesting liquid OR chopping with chelicerae and sucking up goo from pre-oral cavity
● First joint of pedipalp (palpal coxa) chews food or filters liquid food through hairs (similar to ancestral gnathobase)

Question 10

What are Intromittent organs

Answer 10

Their modified
pedipalps that semen is sucked up into to be modifies during reproduction.
The palp has a
bulblike reservoir for sperm and then a penis-like
projection called the embolus