Arthropods Flashcards
How are the phylum arthropoda described?
1,000,000 spp
Represented in virtually in every habitat
Especially successful in terrestrial environments
Protostomes
Segmented
Exoskeleton
How is a generalised arthropod described?
Nervous system similar to annelids (dorsal brain)
Circulatory system open circuit
Hemocyanin is usual respiratory pigment
Compound eyes are common
Jointed cuticle (exoskeleton) with flexible membrane at joints
Cuticle extends into front
Segments often specialised to form distinct body regions- Tagmata
How are the cuticles structured in the arthropoda?
Cuticle of each segment with four plates (but often modified)
Rigid cuticle necessitates periodic moulting
Coelom very reduced in adults
How does moulting work in the arthropoda?
New exoskeleton is formed below the old exoskeleton
Some materials are re-absorbed from the old exoskeleton
Exoskeleton begins to split along specific lines of weakness
Body swells as it emerges from the splits in the old exoskeleton
Body remains soft to enable it to expand further. Vulnerable to predation (tend to hide)
How are the arthropods appendages described?
Jointed appendages specialised for different functions
Movement of jointed appendages by antagonistic muscle system
Primitive condition of one pair appendages per segment
Appendages branched (biramous) in many arthropods
Muscle faster acting than annelid muscles
What sub-phyla exist in the phylum Arthropoda?
Sub phyla:
- Trilobita- the trilobites (extinct)
- Chelicerata- spiders, scorpions etc
- Crustacea- crabs, barnacles, shrimp etc
- Uniramia- millipedes, centipedes, insects
How are the Trilobita described?
Extinct since end of paleozoic (-240 mya)
3900 spp described
Mostly 3-10cm
Evidence of burrowing, epibenthic, pelagic and nektonic forms
How are the Trilobita structured?
Three lobes- refers to the axial and two lateral lobes
Body composed of cephalon, thorax and pygidium
What did the Trilobita have that was the first appearance of that structure?
Compound eyes- first appearance in fossil record
What does each segment of the Trilobita bear and what are they capable of?
Each segment bears a pair of branched appendages
These are fairly similar on different segments
Some capable to rolling into protective
How are the Chelicerata described?
Lack antennae
Body divided into cephalothorax and abdomen
1st pair appendages are chelicerae and have a feeding function
2nd pair appendages are pedipalps- modified for different functions
Normally followed by 4 pairs of legs
What classes exist in the Chelicerata?
Class Merotomata- horseshoe crabs and sea scorpions (extinct)
Class Arachnida- spiders, scorpions, mites etc
Class Pycnogonida- sea spiders
How are the class Merostomata described?
Five or six pairs abdominal appendages modified as gills
Spike-like telson at posterior end of body
What sub classes exist in the class Merostomata?
Sub class Xiphosura- horseshoe crabs
Sub class Eurypterida- sea scorpions (extinct)
How are the sub class Xiphosura described?
Four living species
Compound eyes (no other living chelicerates have these)
Live in shallow water in soft sediments
Large carapace gives protection when ploughing through sediment
How are the sub class Eurypterida described?
Extinct since end of the paleozoic (240 mya)
Similar body plan to horseshoe crabs (cephalothorax is smaller and abdomen divided into separate segments)
Legs show various modifications
Last pair of legs modified as paddles- active swimmers
Predatory- one species >2m long
Marine and brackish water- possibly terrestrial
May be ancestral to arachnida
How are the class Arachnida described?
Very large group but mostly terrestrial
Spiders, scorpions, mites, ticks and several other groups of spider like or scorpion-like terrestrial arthropods
Marine representatives found amongst the mites in the family Halacaridae
-200 spp- intertidal to abyssal- epibenthic and interstitial- often more prominent component of meiofauna
How are the class Pycnogonida described?
1000 spp
Aberrant group- relationship to other chelicerates uncertain
Narrow body, anterior proboscis, four pairs of long legs
Often found on hydroids and bryozoans
Carnivorous, detrital feeders or herbivorous
No gas exchange or excretory organs
How are the Uniramia described?
Single pair of antennae and appendages are unbranched
Massively diverse group but almost entirely terrestrial
Includes Myriapods (centipedes and millipedes) and the insects
Evolved on land- none are fully marine
May visit intertidal zone to feed but few live there
Apterygotes (insects lacking wings) are notable exception
What are some examples of the Uniramia?
Heteroptera- Halobates spp.
The most fully marine known insects- live water surface of open sea in warm climates, predatory
Apterygotes- Thysanura (silverfish) e.g. Petrobius spp.
Diptera- Chironomid (midge) larvae
How are the crustacea described?
38,000 spp- most are marine
Very diverse structurally and ecologically
Two pairs antennae (unique in arthropods)
One pair mandibles and two pairs maxillae
Carapace commonly present
Appendages typically biramous but often highly modified
Gills are normally present and associated with appendages
Two types of eye- pair of compound eyes and small median naupliar eye
What classes exist in the crustacea?
Remipedia
Cephalocarida
Branchiopoda
Ostracoda
Copepoda
Mystacocarida
Tantulocarida
Branchiura
Cirripedia
Malacostraca
How are the class Remipedia?
Primitive body plan- numerous un-fused segments each with a pair similar biramous appendages
First described in 1981
All from anchialine caves
Elongate body resembling a polychaete
Active free-swimming predators
How are the class Cephalocarida described?
Primitive body plan- first 8 segments each bearing a pair of similar biramous appendages
First described in 1953
Benthic sediments
Horseshoe shaped head and elongate trunk of 20 similar segments
Selective deposit feeders on sediment surface
Meiofaunal
