Chelicerata Flashcards
Chelicerata is composed of three classes: Pycnogonida, Merostomata, Arachnida
They are defined by their chelicerae
The Arachnida are the most diverse, comprising the spiders, scorpions, ticks, pseudoscorpions, solifuges, whip spiders and whip scorpions
..
Chelicerate
- Have chelicerae (pair of appendages in front of the mouth, usually modified as pincerlike claws)
Not the same as the chelae (pincers)
Chelicerata classification
- subphylum of arthropoda
Chelicerata classification 3 classes
- Class Arachnida
- Class Merostomata
- Class Pycnogonida
Chelicerata body plan
2 points
Body divided into two regions (May have a clear connector or pedicel):
- Prosoma (or cephalothorax)
1st pair of appendages: chelicerae
2nd pair of appendages : pedipalps
3rd to 6th pairs of appendages : walking legs - Opisthosoma
Up to 13 segments and a tail spine
2nd segment has gonopore
Respiration
Book gills
2 points
- Thin leaves of tissue filled with haemolymph
2. Pulsation of opisthosoma causes circulation
Respiration
Book lungs
3 points
- Spiracle opens into a chamber
- Hollow flat plates through which air circulates
- Haemolymph flows over inner surface and exchanges respiratory gases
Respiration
Trachea
- Spiracle leads into cuticle-lined tubes suspended in haemolymph
- Unbranched tracheae carry oxygen direct to organs
- May have tracheal lungs – clusters of tracheae in the haemocoel
Respiration
Haemocyanin
Copper-based respiratory pigment dissolved in plasma
Circulation
4 points
- Standard Arthropod system with dorsal contractile heart
- May have arteries which direct the flow of the haemolymph
- After emerging from aorta in head, haemolymph is routed to the rear via the book lungs
- Spiders only have short hearts in the opisthosoma
Aorta takes blood through pedicel into prosoma
Nervous system
6 points
- Basic arthropod system of ventral nerve trunk
Often fused into single nerve
- A nerve collar or nerve ring surrounds the oesophagus
Most nerves to other parts of body arise from here - One or more ganglia may be present
- Brain
Protocerebrum associated with visual organs
Tritocerebrum associated with chelicerae
Sub-oesophageal ganglion
- Well developed tactile senses
Every bristle is a sense organ
6. Good visual system Simple eyes (ocelli)
Digestion
4 points
- Variable between groups
- Nearly all depend on a pumping system for fluid food
Part of the foregut, between the nerve ring and the mouth
Pharyngeal pump – chamber with dorsal folds, compressor and dilator muscles work antagonistically to pull liquids into pharynx
- Solid food is broken up
Using chelicerae, aided by pedipalps or legs
Extra-oral digestion due to enzymes in saliva or midgut excretions
Arachnids have no jaws and a narrow oesophagus
- Midgut digests food and absorbs nutrients
Arachnids have diverticula – highly folded and branched
Excretion 3 points
- Coxal glands
Sacculus – extract wast water
Labyrinth – convoluted tubule where nitrogenous waste is added to the contents
Excretory pore – at base of each leg
- Malpighian tubules
Arachnids only
Blind tubes attached to union of midgut and hindgut
Branched and tangle with diverticula of gut
- Primary nitrogenous product is guanin
Reproduction
4 points
- Arachnid males have pedipalps modified into copulatory organs
Charge with sperm - Various strategies for avoiding becoming a post-coital snack
Courtship dances
Nuptial gifts
Restraining the female
- Spiders create an egg cocoon out of silk
Protects from injury and desiccation - May guard or carry the egg sac; carry young after hatching; or even feed the young.
Summary
The Arachnida are the most
diverse class comprising: spiders, scorpions, ticks, pseudoscorpions, solifuges, whip spiders whip scorpions
Summary
Chelicerata are define by their
Chelicerae
Class Arachnida (spiders) 6 points
- Unsegmented prosoma, typically with solid carapace
- Chelicerae and pedipalps
- Four pairs of legs
- Opisthosoma lacks appendages, often heavily modified
- Principally carnivorous
- Sensory hairs (primary for most arachnids), eyes, slit sense organs (detect air pressure)
Class Arachnida
Scorpiones (scorpions)
3 points
1. Prosoma Chelate pedipalps (claws)
Single shield covers whole prosoma
- Opisthosoma
Mesosoma of 7 broad segments
Metasoma of 5 narrow segments
Telson modified into venom gland and sting
- Stinging apparatus
Venom used in subdual of prey and defence
Neurotoxic and painful
Usually specialised for invertebrate prey
Strong venom correlates with weak pedipalps
Class Arachnida
Opiliones (harvestmen)
4 points
- all habitats, omnivorous
- Undivided prosoma
- 9 segmented opisthosoma
- Long, leg-like pedipalps
- Single pair of simple eyes in middle of prosoma
Class Arachnida
Pseudoscorpiones (book scorpions)
- Claw-like pedipalps with venom glands
- Chelicerae have silk glands
- 0-2 pairs of simple eyes
- Flattened opisthosoma
- Phoresy – carried by other organisms
Class Arachnida
Solifugae (camel spiders, sun spiders, wind scorpions)
5 points
- desert and semi-desert,
- predatory on large invertebrates and small vertebrates
- Large prosomal shield
- Two-jointed chelicerae, no venom
- Leg-like adhesive pedipalps
Class Arachnida
Thelyphonida (Whip scorpions, vinegaroons, uropygids)
3 points
- Pedipalps have crushing jaws
- Elongated 1st legs used as sensory appendages
- Telson (tail) is sensory organ
Acetic acid as defence mechanism
Class Arachnida
Amblypygi (Whip spiders, tailless whip scorpions)
2 points
- Pedipalps are raptorial
2. Elongated 1st legs used as sensory appendages
Class Arachnida
Araneae (Spiders)
- Prosomal segments fused
- Opisthosoma of 12 segments attached with narrow stalk
- Jointed chelicerae with venom glands
- Leg-like pedipalps for food handling and insemination (in males)
- Up to eight eyes
- Silk glands
Class Arachnida
Araneae (Spiders) silk glands
4 points
- Silk is a protein extruded in liquid form
- Produced by a silk gland and emitted by spinnerets
1-4 pairs (usually 3); only abdominal appendages in Araneae - Many different types of silk
Stiff for construction
Sticky for prey capture
Woolly to catch insect bristles
4. Uses: Prey capture Prey immobilisation Dispersal via ballooning Spermatophores and egg sacs Nest construction Guide lines and drop lines Alarm lines Pheromone trails
Acari
Mites and ticks
5 points
- Fusion of prosoma and opisthosoma
- A capitulum formed from pedipalps and chelicerae
- Opilioacariformes
Superficially similar to harvestmen
10 genera, believed to be basal to the Parasitiformes - Acariformes
House dust mites, mange mites, feather mites
Spider mites, gall mites, chiggers - Parasitiformes
Hard ticks and soft ticks - probably polyphyletic
Class Pycnogonida (Sea spiders or pycnogonids) 6 points
- marine benthos, up to 70cm leg span
- Mouth on a proboscis, mostly carnivorous on sessile, soft-bodied invertebrates
- Caecae of midgut protrude down legs
- Narrow prosoma
4 to 6 pairs of legs
Ovigers: modified legs used for grooming and (in males) for carrying eggs - Vestigial opisthosoma
- No organs for gas exchange or excretion
Class Merostomata, Order Xiphosura (Horseshoe crab)
- coastal waters
- Prosoma covered by large dorsal shield
- 5-6 pairs of respiratory appendages on opisthosoma
“book gills” - Well-developed spine (telson)
- Pair of compound eyes, pair of simple eyes
