Invertebrate Zoology Exam 5

Tagmosis

The fusion of segments to form functional units

• arthropods have many segments, but they don’t operate independently, as the tagmata form distinct regions of the body

What are the three groups of Arthropoda?

1. Chelicerata

2. Crustacea

3. Mandibulata

What tagma make up the body of a chelicerate? 

1. Prosoma 

2. Opisthosoma 

Prosoma 

“Head” tagma of chelicerates; composed of 6 segments and bears the mouthparts and walking legs

Opisthosoma

“Body” tagma of chelicerates; contains the reproductive organs and gut

Differences in Hox genes of chelicerates

• no antennae; chelicerae are the first pair of appendages

  • chelicerae are homologous to antennae, not mandibles

• mouth is positioned between the first two segments

• all 6 segments and the ocular fuse to form the prosoma during embryonic development

Crustacean body structure 

• body is divided into head, thorax, and abdomen, although the first two segments are often fused to form the cephalothorax 

  • head: eyes, antennae, mouthparts

  • thorax: maxillipeds, walking limbs

  • abdomen: gut, abdominal appendages

What are the key features of Phylum Arthropoda?

1. Tagmosis 

2. Each segment has jointed appendages with intrinsic musculature 

3. Chitinous cuticle; forms sclerotized plates

Arthropodization

• basically, “a worm-in-a-box” structure

• the “box” is formed by a rigid cuticle that acts as an exoskeleton → this feature affects multiple biological functions

What are the divisions of an arthropod cuticle?

The cuticle is an exoskeleton that is divided into:

• Tergites = dorsal plates

• Sternites = ventral plates

• Pleurites = side walls

Arthropod cardiovascular system

The body cavity is a hemocoel; the blood is pumped through an open circulatory system by a dorsal “heart”

What kind of respiratory pigment do arthropods use?

Hemocyanin

Arthropod musculature 

Muscles attach to the body plates and extend into the limbs, but there are also intrinsic muscles in the limbs to manipulate the segments 

Arthropod cuticle

A non-cellular, extracellular matrix that is secreted by the epidermis.

Key components of the cuticle

1. Epicuticle - continuous outer layer; made up of a waxy, waterproof substance secreted by gland cells and allows for flexibility between sclerotized plates

2. Exocuticle - middle layer; undergoes sclerotization

3. Endocuticle - lower layer; borders the epidermis

Why does the cuticle contain spines/setae?

• some of these are solid, while others contain cells and/or sensory neurons

  • no cilia = not a scalid

• extensions of trichogen cells form the innervated spines, while solid spines are cuticular processes

Sclerotization of the cuticle 

• also called “calcification” in crustaceans 

• this is a process used to harden the cuticle, as it is generally a flimsy material; this involves changes in cuticular proteins and usually results in a darkening of the cuticle (tanning) 

• the extend of sclerotization varies over the body and across life stages

• only the exocuticle undergoes sclerotization

Arthropod molting process

• Begins with a hormonal signal that causes the epidermis to secrete enzymes to dissolve the endocuticle (the dissolved materials are absorbed for later use)

  • this loosens the cuticle and provides a space in which new cuticle can be secreted

• as unsclerotized cuticle is secreted, the old cuticle is cast off

• after a period of time, the new cuticle is sclerotized and the animal can resume normal activity

Hormonal control of molting in insects

Key elements: neurosecretory cells, corpora cardiaca, prothoracic gland (suspended in the hemocoel)

1. Presence of appropriate stimulus → cuticle becomes too tight

2. Signal travels to the central nervous system → neurosecretory cells of the pars intercerebralis produce ecdysiotropin 

3. Ecdysiotropin flows along the neurosecretory cells until it reaches the corpora cardiaca → stimulated to release PTTH (thoracotropic hormone)

4. PTTH travels through the blood and comes into contact with the prothoracic glands in the thorax → release ecdysone 

5. Ecdysone travels through the blood to reach the target cells in the epidermis 

6. The epidermal cells release the enzymes that begin to digest the endocuticle, leading to shedding of the exocuticle

How is crustacean molting different from insect molting?

While the brain still controls the process, the neurosecretory fibers extend into glands in the eyestalks to release a substance that inhibits molting

Effect of molting-inhibiting hormone (MIH) on crustacean molting

• The X-organ produces MIH, which accumulates in the sinus gland and is released into the blood

• MIH travels into the head and inhibits the Y-organ, which ordinarily secretes ecdysone

• As long as MIH is produced, no ecdysone is released and molting does not occur

How does a crustacean molt if it has MIH?

An appropriate stimulus (cuticle tightening) will cause the neurosecretory cells to no longer stimulate the X-organ 

• this depletes the supply of MIH → unable to inhibit the Y-organ 

• Y-organ is able to produce ecdysone and initiate the molting process

Insects have wings on the dorsal surface of their bodies. What is the technical term for the cuticular plate that bears insect wings?

Tergite

Which of the following is NOT a feature of the Phylum Arthropoda?

Lobopods

Which of the following undergoes sclerotization?

Exocuticle

Which of the following secretes ecdysone in an insect?

Prothoracic gland

What is the principle secretion of the X-organ in crustaceans?

Molting-inhibiting hormone (MIH)

Modern chelicerates are almost entirely terrestrial. What is the one (modern) exception?

Horseshoe crab

What is the key feature of chelicerates?

Chelicerae (specialized mouthparts)

Chelicerae

Mouthparts on the prosoma; usually accompanied by poison glands to subdue prey

How do ticks feed?

• Ectoparasites; use their chelicerae to slice open a wound in the skin 

• then, inserts a structure called a hypostome into the wound to anchor the tick while it sucks up blood

How do spiders feed?

• They use their chelicerae to subdue prey

• Once captured, digestive enzymes are secreted onto the prey and digestion take place outside the predator’s body

• Then, they suck it up using a “sucking stomach” → brings the food into the gut

Spider digestive system

• Sucking stomach

• Midgut (lacks a cuticular lining) - expands in the opisthosoma and connects to a stercoral pocket

• Digestive ceca in the prosoma - extends into the legs

What are the two ways that spiders take up oxygen?

1. Tracheae that penetrate into the body wall

2. Book lungs = plate-like extensions of the body wall housed in the opisthosoma

How do “book lungs” work?

• Air enters the chamber and passes over individual lung plates while blood moves through the plates 

• Oxygenated blood is returned to the heart in special veins 

Spider circulatory system 

• The dorsal heart is a contractile vessel surrounded by a pericardium

  • Openings in the heart (ostia) draw in the blood when the heart contracts

• Contraction forces the blood out of the heart in both directions where it travels briefly in vessels until entering the hemocoel

How do spiders get rid of nitrogenous wastes?

Malpighian tubules attach to the gut and extend into the hemocoel, where they apparently take up nitrogenous wastes

• their simple structure suggests that they are not involved in water regulation

Coxal glands (chelicerates)

Excretion and osmoregulatory glands

• these glands are bathed in blood that eliminates wastes

• extremely long tubule ensures that water is retained before wastes are eliminated at the base of the walking limbs on the prosoma

What are the three groups of crustaceans?

1. Branchiopoda

2. Maxillopoda

3. Malacostraca

Branchiopoda

• contains about 1,500 species of mostly freshwater zooplankton

• examples: brine shrimp, water fleas

• key feature: gill appendages

What are the “gill appendages” of branchiopods?

• flattened, paddle-like limbs that serve as gills; greater surface area increases gas exchange

• also used in feeding → when water passes over them, the movement of the appendages traps food particles in suspension and passes them forward to the mouth

  • tiny setae help trap and move food particles

Maxillopoda

• contains roughly 27,000 species, most of which occur in the ocean 

• example: copepods 

• key features: vibrating setae, filter chamber 

How do maxillopods feed?

• The second antennae and mouthparts contain vibrating setae that create swirls to direct water into a midline filter chamber

• the second maxillae trap items in the filter chamber → setae brush the particles forward to the mouth

• copepods are able to discriminate between low- and high-nutrient food items, and their feeding rate is higher when given more nutritious food items

Malacostraca

• examples: shrimp, lobster, crab

• key features: maxillipeds on thoracic appendages, cheliped (large, claw-bearing appendage), walking limbs

What is the body structure of malacostracans?

• divided into three tagma: head, thorax, abdomen 

  • sometimes, the head and thorax are fused into a cephalothorax

• head has 2 pairs of antennae, mandibles, and 2 pairs of maxillae

• thoracic appendages include maxillipeds and a large claw-bearing appendage (cheliped), as well as walking limbs

  • in most cases, the abdomen does not have jointed appendages

  • usually <8 pairs of thoracic limbs

What are the two chambers of a malacostracan stomach?

1. Cardiac stomach → contains the gastric mill 

2. Pyloric stomach → contains the gland filter and filter press (setae rows)

What is the gastric mill?

Found in the cardiac stomach; grinds up food items to optimize enzymatic action and to minimize damage from sharp objects

Gas exchange of crustaceans

• smaller species are able to conduct gas exchange directly across the body wall, while larger species use gills

• gills = branches of the walking limbs that are partially enclosed within the carapace; housed inside the gill chamber

  • biramous appendages → have two branches 

Gill bailers

Located on the second maxillae; used to create water flow

• this causes water to enter the chamber and pass over the gills 

• oxygen is carried in solution in the body by hemocyanin

Coxal glands (crustaceans) 

• also called “green glands”

• special excretory/osmoregulatory organs in large crustaceans that process liquid wastes from the hemolymph 

  • located beneath the mouthparts → wastes are pushed away by the flickering appendages 

• essential solutes are reabsorbed as the liquid moves down the tubule before elimination at the front of the animal body 

Ommatidia 

Individual units that make up the compound eyes of crustaceans. 

• each unit has its own cornea to focus incoming light

• the entire unit forms mosaic images

Barnacle larva

Called a nauplius.

• appendages of the larva represent the first 3 segments of the head: 1st antennae, 2nd antennae, and mandibles → these appendages are used for swimming

Labrum

“Upper lip” found in nauplius larva; aids in holding food in the mouth (not an appendage)

Reproduction of barnacles 

They are hermaphroditic; one extends a “groping penis” to fertilize the nearest individual

Brachiopod reproduction

Reproduce by parthenogenesis; have a resting egg stage

• other freshwater and terrestrial crustaceans carry the young in brood chambers and release juveniles

Shrimp development

Indirect development, including “post-larval” stages.

• shrimp cast their eggs on the seafloor

• the nauplius molts to a zoea → molts to a post-larval mysis

• mysis migrates into estuaries/shoreline habitats; these nurseries provide protection for the juvenile shrimp

Lobster development  

• lobsters carry their eggs through early development 

• post-larval stage = mysis; more fully-developed

  • the mysis has tons of setae for swimming through plankton

Crab development

• the zoea molts into a post-larval stage, megalops, which has compound eyes and tiny claws

• as segments and appendages are added, the abdomen folds under the body in a process called carcinization

Comparison between malacostracan body forms:

Shrimp:

• cylindrical, flexible; build for swimming and crawling

• ~900 species

Lobster:

• cylindrical; stout limbs are built for walking

• ~4,000 species

Crab:

• carries the abdomen beneath the thorax → more efficient locomotion

• ~9,500 species

Features of carcinization in crabs

• Carapace is wider than it is long 

• Sternites are fused into a single plate 

• Abdomen is bent beneath the thorax

What is the advantage of carcinization in crab evolution?

• molds the body into a flattened and widened form that is able to move laterally, thereby avoiding front-facing predators

• it also allows the animal to exploit habitats that provide hiding places from other predators

What prevents sharp food items from entering the crustacean mid-gut, which is not lined with cuticle?

The gastric mill

Which of the following appendages is found in a nauplius larva?

All of these are found in the nauplius

Which of the following is NOT a post-larval stage in crustaceans?

Nauplius

Mandibulata 

• almost entirely terrestrial

• includes millipedes, centipedes, a group of wingless hexapods called Entognatha, and the insects 

• key features: mandibles, maxilla, 1 pair of antennae 

What are the two clades of Mandibulata?

1. Myriapoda → “many-footed”

2. Hexapoda → “six-footed”

What are the two groups of Myriapoda?

1. Chilopoda → centipedes; 3,000 species

2. Diplopoda → millipedes; 11,000 species

What are the two groups of Hexapoda?

1. Entognatha → interior mouthparts; 10,000 species 

2. Insecta → exterior mouthparts; >1 million species (largest species diversity of all arthropods) 

Chilopod body structure

• Divided into two tagma: head and trunk

• in addition to the standard mandibulate mouthparts, they have a prehensorial claw on the first trunk segment and walking limbs on all remaining segments

• maxillae cover the mandible, which lies beneath the first maxillae

  • second maxillae function as sensory palps

Prehensorial claws

• attach to the first trunk segment of a chilopod and extend underneath the head (they are not mouthparts)

• contain poison glands used to subdue their prey

How do chilopods grind up food items?

• they have a gizzard → rigid “teeth” tear and chew food items before it travels to the midgut 

• additionally, the midgut produces a perforated peritrophic membrane to contain the food items and protect the gut lining from sharp objects and microbes 

Chilopod excretion/osmoregulation

Uses a pair of Malpighian tubules in the hemocoel.

• moves nitrogenous waste out of the hemocoel, where it can be processed and exit the body

How do chilopods breathe?

• rows of spiracles along their back draw air into tubules called tracheae that deliver oxygen directly to tissues and organs

• a dorsal blood vessel acts as a pump to circulate blood through the body

  • hemocyanin is present in solution to aid in transport of oxygen

Chilopod reproduction and development:

• Male centipedes produce spermatophores → deposited on the soil; female passes over it and reproductive structures draw it up inside

• centipedes exhibit direct development, and their young hatch with most of the segments and legs of the adult 

  • there are some species that add segments and legs with additional molts

• some centipedes brood the young after hatching

Diplopod body structure 

• mandibulate arthropods; 2 tagma: head and trunk (similar to chilopods)

• trunk segments are fused to form diplosegments, each of which bears 2 pairs of legs 

• cuticle is typically well-sclerotized and forms a hard body covering

• some species produce repellent fluids to deter predation

Gnathochilarium 

The first pair of maxillae in diplopods are fused to form this structure; creates a “shelf” beneath the mandibles 

• millipedes lack the second pair of maxillae 

Collum

A leg-less segment behind the head of a diplopod; acts as a blade as the millipede “bulldozes” through the detritus

Gut bacteria of millipedes

• Millipedes are detritivores and herbivores → the gut is a lengthy tube, as is typical for animals that eat plant material

• some have suggested that gut bacteria contributed to millipede nutrition by aiding in digestion of cellulose, but experimental studies show no differences in growth rates when bacteria are not present in the gut

  • this suggests that the bacteria may be providing other services, like de-toxification of secondary compounds

Millipede circulatory system 

The entire circulatory system is a dorsal vessel.

• ostia in this vessel draw in blood from the hemolymph and pump it forward into the head 

• millipedes also have hemocyanin to transport oxygen throughout the body 

Movement of millipedes

• Millipedes have short, stout legs that attach to the ventral surface of the body 

• Not all of the legs are in motion when a millipede moves → most remain in contact with the soil 

  • This concentrates the energy of movement in power to move the millipede through the dense leaf litter on the forest floor 

• Movement style is appropriate for the detritivorous habit of millipedes

Movement of centipedes

• Centipede legs are much longer than millipede legs; attached to the sides of the body 

  • This arrangement suspends the body above the surface to make movement easier

• When a centipede is running, only a few of the legs are in contact with the soil

  • As with annelids, a significant amount of energy is wasted in lateral movement when moving at high speeds

Entognatha 

Non-insectan, soil-dwelling (edaphic) hexapods.

• mouthparts are internal and confined within the head capsule

• typically small and wingless

• lack a waxy covering of the epicuticle, so they have to live in moist, underground environments

• examples: collembola (“springtails”), diplura 

Entognatha mouthparts

• The mouthparts have limited mobility, as they are internal

  • structures: mandible, superlingua, maxilla, hypopharynx

  • cavity enclosed by oral folds

• Entognathans include some carnivores, but detritivory is the dominant feeding strategy

Features of collembola

• Furcula = abdominal appendage; used as a catapult to launch themselves away from predators 

• Collophore = a structure used to take up water

What are the two subclasses of Insecta?

1. Apterygota → wingless insects; ~4,500 species

2. Pterygota → winged insects; ~995,500 species

What are the two infraclasses of Pterygota?

1. Paleoptera → primitive, non-folding wings; ~7,500 species

2. Neoptera → folding wings; ~988,000 species

What are the three superorders of Neoptera?

1. Orthopterodea → no metamorphosis; ~33,000 species 

2. Hemipterodea → partial metamorphosis; ~90,000 species 

3. Holometabola → complete/complex metamorphosis; ~865,000 species

Apterygota

Wingless insects; include silverfish and bristletails

• they are not part of Entognatha because they have external mouthparts

At what time period did insects start to evolve wings?

Around 350 million years ago

When did the earliest entognath evolve?

Around 474 million years ago

When did the earliest insect evolve?

Around 420-400 million years ago 

Insect digestive system

• Foregut and hindgut are lined with cuticle, while a peritrophic membrane is formed in the midgut

• crop = food storage organ; leads into proventriculus

• proventriculus = acts as a gizzard and regulates food entry into the midgut; contains spines to grind up food

What are “bio-reactors”?

Symbiotic microorganisms in the hindgut of termites → break down cellulose in a fermentation process

• when they molt, they lose the symbionts and have to lick each other to re-obtain them

How do insects deal with wastes?

Malpighian tubules → extend into the hemocoel and are bathed by hemolymph

• blind sacs; attach to the hindgut 

• remove solutes, but as the material passes into the hindgut, water is removed so only solid waste remains 

• converted into uric acid; passed out in feces 

Respiratory system of insects

• take in oxygen through spiracles → lead to tracheae → branch into finer tubules (tracheoles) to deliver gas to tissues

  • concentration of tracheae in the thorax is associated with the wing muscles

How do aquatic insect larvae breathe?

They utilize gills or “anal respiration” → takes up oxygen and passes it through a tracheal system 

• rings in the tracheae prevent collapse and the presence of fluid in the tracheoles regulates gas movement 

• tracheoles fill with fluid → blocks access to muscle → brings it closer to greater mitochondrial concentration

Insect circulation

• Elongate dorsal vessel pumps blood forward into the head

  • additional pumps are associated with the wings

• After it reaches the head, blood enters the hemocoel and flows to the rear, with side trips into the legs

  • septa within the leg segments allows blood to be delivered efficiently

Did insects co-evolve with flowering plants?

Probably, not as most trophic diversity in insects occurred before angiosperms even developed 

• mouthpart diversity pre-dates taxonomic diversity

• the herbivorous group Paraneoptera displayed rapid diversification in late Paleozoic and early Mesozoic → made the greatest contribution to insect diversity during this time period 

If not angiosperms, what caused the increase in insect diversity during the Cenozoic era?

Development of a constriction region (CR) between two sections of the midgut

• this CR blocks passage into the next section for all materials except specific bacterial species

• these bacteria take up residence in this section and their activity produces essential amino acids and nutrients

Elytra

In beetles, the forewings are modified to form this hardened covering to protect the hindwings

• additionally, beetle larvae do not have external wing buds and are wormlike for burrowing/mining