Arthropod 2- Molting

What is growth like?

• non continuous

• Exoskeletons make it impossible to grow gradually

Ecdysis definition

Hormone-controlled molting

• much more controlled in arthropods than other ecdysozoans

• Interior tissues grow until the exoskeleton cannot contain them

• Apolysis→ Ecdysis→ new exoskeleton is expanded and hardened

What hormones and organs are involved intermolt phases

• X organ produces Molting Inhibiting Hormone (MIH) and Mandibular Organ-Inhibiting Hormone (MOIH)

• These hormones prevent expression and production of molting stimulation hormones

• Control how long intermolt phases last

What happens in Pre-Ecdysis

1. Mandibular organ produces methyl farnesoate (MF) which stimulates molting process and impacts Y organ

2. Y organ begins to produce ecdysteriods derived from dietary cholesterols

3. Ecdysone especially important→ creates ecdysis triggering hormone receptor molecules

4. These hormones allow build up of ecdysis triggering hormone in cells and sequestration of glycogen and minerals which is energy

How does Pre-Ecdysis show behavioral changes?

• ecdysteroid levels begin to decline which allows ETH to affect central nervous system and behavior

• Animal stops eating to clear out digestive system

• Activity levels decrease→ no energy

• Many go into hiding, most vulnerable stage in their lives

• Will eat a lot→ ETH hits nervous system→ stop eating→ forgut and hindgut clear out

What kind of stage is Apolysis

Pre-molt stage

• separation of old cuticle from epidermis via enzymatic activity

Apolysis steps

1. Space opens up btw the old cuticle and epidermis and inactive molting fluid is secreted into it

2. ETH receptor molecules trigger production of new Epicuticle which is resistant to the molting enzymes because it is sclerotized

3. Molting fluid becomes active and dissolves old Endocuticle, materials re absorbed for reuse

Ecdysis

• Old epicuticle and Exocuticle are thin enough for animal to break through

• Happens through inflation of either air or water to increase pressure and pop through

• Splitting point varies by taxon but happen in areas w/o sclerotized Exocuticle bc too hard

• Once open use muscles to wriggle out of old cuticle

• Bigger organisms take longer to get out

What leaves the animal in ecdysis

All cuticular surfaces

• foregut and hindgut lining

• Gill coverings

• Eye surfaces

• Lining of every sensory pit, groove, seta, and spine

• Usually have a set # of molts in lifetime

What happens post-molt

• water/air is taken up to increase internal volume to its maximum

• Held until the new cuticle is hardened by sclerotization and biomineralization

  • Bursicon= regulates sclerotization

  • Biomineralization is regulated by ecdysteriods

• Once shell is hard extra volume is lost, allowing cells and tissues to grow during new instar

Why are arthropods small

1. The bigger you are the longer it takes to molt- coconut crabs take a month

2. Exoskeletons may not be able to support the weight of larger animals- tube legs can’t bear the weight

3. Insects held back by oxygen delivery via diffusion- only gas exchange through tiny holes that lead to trachea

What allows arthropods to be successful

Exoskeleton!!!

• adapt to many niches via tagmosis

• Exoskeleton allows them to live in less hospitable area