ARTHROPODA (VI): Trilobitomorpha and Uniramia (H. Proctor) (Other arthropod study images ;
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a) extinct for approx. 200 million years, but once among most diverse and abundant animals
b) ranged in length from 0.5 mm to 40 cm
c) had three tagmata: head (= cephalon), thorax, and pygidium
d) majority were benthic scavengers, a few were predatory, some swam or were planktonic
e) compound eyes, one pair antennae, biramous limbs
f) larval stage looked like 'head' without abdomen; grew by adding segments (anamorphic)
a) General: primarily terrestrial with a fair number of freshwater species and only a few marine ones
- characterised by a single pair of antennae, mandibles for mouthparts, sessile compound eyes, spiracular/tracheal system for respiration, and (mostly) uniramous appendages
- includes both multi-legged and six-legged taxa
b) SuperCl. MYRIAPODA (="thousand legs"; millipedes, centipedes, pauropods and symphylans); about 13,000 spp. in the world
- entirely terrestrial except for a few millipedes that don't mind being submerged occasionally
- size ranges from <1 mm to 30 cm
- only two tagmata: head and trunk; have from 9 to 90 pairs of legs
- some experts consider the myriapods to be para- or polyphyletic
- lack compound eyes except for scutigeromorph centipedes
- millipedes (Cl. DIPLOPODA) have two pairs of legs/segment (= diplosegment); all other myriapods have one pair of legs/segment
- centipedes (Cl. CHILOPODA) are predatory with poison glands in their highly modified front legs (= 'poison claws')
- anamorphic development, additional segments added over time; however, some centipedes are epimorphic and hatch with their entire adult complement of segments.
c) SuperCl. HEXAPODA
- the 6-legged atelocerates
- one group includes taxa with internalised mouthparts (entognathous hexapods) and another has external mouthparts (ectognathous hexapods, the 'true' Class INSECTA)
- the largest class of entognaths is the Cl. COLLEMBOLA or springtails. Their common name derives from their 'tail', which can be bent under the body and flung back, causing the collembolan to leap upwards
- INSECTA: the most species-rich group of organisms with around 800,000 named species and some estimates of up to 30 million species in total
- only group of invertebrates to evolve powered flight, but not all insects have wings (APTERYGOTE vs PTERYGOTE)
- Hexapods show three main types of development:
(i) AMETABOLY: wings absent, juvenile stages look almost exactly like the adults
(ii) HEMIMETABOLY: juveniles and adults look similar, wings develop as external wing pads over several moults
(iii) HOLOMETABOLY: juveniles and adults morphologically and ecologically different (e.g. caterpillar vs. butterfly) and wings develop in a special stage called the pupa
Copyright (c) 2004 by A. Richard Palmer. All rights reserved.
(revised Mar. 25, 2004)