invertebrate zoology E5

What are the key features of the Phylum Arthropoda? 

• jointed, segmented appendages 

• chitinous cuticle

• open circulatory system w/ hemocoel

• ecdysis/molting 

• tagmosis 

How do the 3 major groups of arthropods (chelicerates, crustaceans, and mandibulates) compare in terms of diversity and what are the principal features that distinguish each group?   

Chelicerates:

• no antennae

• chelicerae w/ pedipalps

• 2 body regions: prostoma + opithstoma

Crustaceans: 

• 2 pairs of antennae 

• mandibles and maxillae

• mainly aquatic with gills 

• biramous appendages 

Mandibulates (hexapoda + myriapoda)

• one pair of antennae 

• mandibles 

• uniramous appendages 

What are the 3 factors that have driven the spectacular diversification of insects? 

1. flight

2. symbiotic bacteria / enzymes 

3. holometabolic development 

Describe the structure of the arthropod cuticle and compare the cuticle of terrestrial and marine arthropods.   

layers: epicuticle, exocuticle, endocuticle, epidermis

marine:

• frequently mineralized and heavier 

terrestrial

• waxy epicuticle for waterproofing 

How is molting coordinated and controlled in insects and in crustaceans? 

insects:

• brain releases PTTH → prothoracic gland → release ecdysone

• ecdysone causes molting 

crustaceans:

• driven by ecdysone 

• release inhibited by MIH from x-organ/sinus gland 

• y organ produces molting stage 

What is the functional significance of tagmosis?  Compare the appendages at the front end of the body in insects, crustaceans, centipedes, millipedes, and spiders.  

significance: allows for specialization of regions for different purposes like feeding, locomotion, and reproduction

insects: antennae, mandibles 

crustaceans: 2 pairs antennae, mandibles, maxillae 

millipedes: simple mandibles 

spiders: chelicerae, pedipalps 

How has variation in mouthparts contributed to insect diversity?   

modifications like piercing, chewing, sucking, allows insects to live in diverse feeding niches

How are the walking limbs attached to the body in arthropods?   

at the pleuron on the ventral body wall

Why are there 3 pairs of walking limbs in insects? 

because the thorax is composed of 3 segments, each with one pair of limbs 

How has the exoskeleton affected gas exchange and excretion/osmoregulation in arthropods?  How do these processes vary among terrestrial and aquatic arthropods? 

Terrestrial arthropods

• Gas exchange via tracheae + spiracles

• Waxy epicuticle prevents water loss → excretion via Malpighian tubules with high water conservation

Aquatic arthropods

• Gills for gas exchange

• Ion balance controlled by gill epithelium

What is the principal respiratory pigment in arthropods and how is blood circulated through the body?  How do arthropods avoid mechanical damage/bacterial infection in the gut? 

• Respiratory pigment: hemocyanin

• Circulation: open circulatory system, dorsal heart pumps hemolymph

• Gut protection: peritrophic membrane prevents abrasion & pathogens

What morphological features and muscle groups are involved in generating lifts so that insects can fly?  How are insect musculature and innervation adapted for flight?  What theories account for the origin of wings and what evidence supports these theories? 

Generating lift:

• Dorsoventral & longitudinal indirect flight muscles deform thorax

• Direct muscles adjust wing angle

Innervation:

• Synchronous muscles = 1 nerve impulse per wingbeat

• Asynchronous muscles = stretch-activated, high frequency

Wing origin theories:

1. Paranotal lobe theory = they evolved from simple, fixed lobes on the sides of the thorax 

2. Branchial theory = Gills gave up gas exchange function, changed as wings for gliding function 
   - shared with crustacean gill structures.

What advantages does holometabolous development provide and how has it affected insect diversity?  What mechanisms coordinate and control insect metamorphosis? 

• Larvae & adults occupy different niches → reduced competition

• Allows specialized feeding vs. dispersal stages

14. Molting in insects is mediated by enzymes secreted by epidermal cells.  

 

What specific layer of the cuticle is acted on by these enzymes? 

 

What hormone stimulates the epidermal cells to secrete these enzymes? 

 

What gland in the insect produces the hormone that causes molting? 

Crustaceans utilize the same hormone to regulate molting, but the release of the hormone is controlled by MIH.  What tissue or gland produces MIH in crustaceans? 

• Layer targeted by molting enzymes: Endocuticle

• Hormone that stimulates secretion of these enzymes: Ecdysone

• Gland that produces molt hormone: Prothoracic gland

• Crustacean MIH source: X-organ/sinus gland complex in eyestalk

15. After waste enters a Malpighian tubule, it flows to the hindgut where the water is re-absorbed.  Since waste entered the tubule with the water, why doesn’t it leave the hindgut when the water is re-absorbed -- what happens to the waste to ensure that it remains in the hindgut? 

Waste becomes precipitated or bound into insoluble uric acid crystals, which cannot diffuse back with reabsorbed water → remains in hindgut until excreted.

There is no acid-filled stomach to destroy potential pathogens in the digestive system of insects.  In fact, the midgut consists of a single layer of epithelial cells.  What structure of the insect midgut prevents infection by bacteria or mechanical damage by food items? 

peritrophic membrane in the midgut

a Paleozoic mandibulate called an arthropleurid. It grew to a length of 8 feet or more (Wow!) and behaved like a millipede, feeding on detritus.  

Where would you expect the limbs of 

an arthropleurid to attach to the 

segments of its body? 

 

   A) at the paranotal lobes 

   B) on the ventral surface of the body 

   C) on the ventral surface, but attached at the sides 

B

 

17. Arthropods and nematodes are both members of the ecdysozoa, but arthropods are clearly different because they have jointed appendages.  What significant feature do arthropods and nematodes share that justifies placing them on the same branch of the animal “tree of life”?  

• ecdysis

• chitnious / collagenous cuticle does not grow with animal

Some arthropods feed using appendages that are not directly associated with the mouth (i.e., not on the “head”).  Name an organism that uses each of the following appendages to apprehend and/or manipulate food items. 

• prehensorial claw 

• maxilliped

• Prehensorial claw: centipedes (forcipules)

• Maxilliped: crustaceans like crabs, lobsters, mantis shrimp

The wingbeat frequency in this animal is very low (e.g., 25 beats/sec). Would you expect this insect to have synchronous muscles, or asynchronous muscles? 

synchronous

The following figures are cross-sections of an insect body at 2 different points during flight (A and B). What muscle group is contracted in A and B?  (This item is asking for a specific group of muscles, not the terms “direct” or “indirect”.) 

A = dorsoventral muscles

B = longitudinal muscles

While many animals take up oxygen by diffusion across the body wall, the arthropod cuticle makes that impossible.  Describe how oxygen enters the body of a terrestrial arthropod (e.g., a grasshopper) and then describe the process for a crayfish. 

• grasshopper 

• crayfish 

Grasshopper

• Air enters spiracles → tracheae → tracheoles → direct diffusion to tissues

• Does NOT rely on blood for oxygen transport

Crayfish

• Water passes over gills → hemolymph picks up O₂ (with hemocyanin) → circulated via open circulatory system

We know that the earliest insects had mouthparts that could be used to feed on spores and reproductive structures in plants, but the insect had to climb the plant to get to the food source.  This creates a risk of falling.  Although these early insects couldn’t fly, they did have structures that would allow them to glide safely to the ground.  What were these structures?   

paranotal lobes 

Mandibulata 2 subgroups:

• hexapoda

• myriapoda 

gas exchange in insects, spiders, crustaceans 

insects: tracheae

spiders: book lungs 

crustacean: gills

holometabolous vs hemimetabolous metamorphosis 

• Holometabolous = full metamorphosis  

• Hemimetabolous = half metamorphosis  

• ^ important for development of wings  

hemimetabolous development

• molt several times to increase size and wings 

• sexually immature stages = nymph 

• after las instar = insect molts to adult stage with fully formed wings and functioning gonads 

Juvenile hormone: 

• secreted by what?

• what does it result in?

• how does it affect adult form?

• corpora allata, right next to corpora cardiaca 

• results in an extra larval stage 

• appearance of adult form linked to JH

what happens during the pupal stage of holometabolous insects?

larval structures are replaced by adult structures and the insect that emerges from the pupa is sexually mature and has fully functional wings

steps of holometabolous insects?

1. several larval stages 

2. pupal stage before adult = tissues in larval stage digested and replaced by adult tissue 

3. adult stage emerges with fully functional wings and is sexually mature

what are imaginal discs?

• Imaginal discs are clusters of cells that direct the development in adult structures  

• Some of the gene activated by JH prevent the genetic information in these cells from being expressed  

• The “fate” of imaginal discs is confirmed through transplants of tissue in pupae  

where have we seen imaginal discs?

nemertine larva = pilidium larva

difference between maxillipeds and maxillae

Maxilliped = walking limbs coopted to help w feeding  

Maxillae = feeding parts on front  

who has a peritrophic membrane? what other organism has something similar?

• millipedes and insects

• crabs have a gastric mill in the cardiac stomach to crush and grind food

millipede vs centipede leg attachment:

• Millipede = diplo segments, 2 pairs of walking limbs midventral (belly) of body  

• Centipede = attached on actual sides of body, able to move quicker because they are predators