1. LON-CAPA Logo
  2. Help
  3. Log In


Text, 7th ed.: Ch. 23, pp. 490-507, 525-528

Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)

possible web site: http://www.ucmp.berkeley.edu/arthropoda/arthropoda.html

poss. HVCC a/v resource: VT 1176,"Life on Earth: Segmented Invertebrates" (to be seen in recitation)

1. Phylum Arthropoda ("jointed foot"): 1,000,000 + species. The animal phylum with the most species. Characteristics of chitinous outer skeleton, jointed legs, and segmented body. Often, metamorphosis (change of form) between young/adult stages (caterpillar/butterfly).

2. The most important characteristic of the phylum is the hard exoskeleton: all other characteristics are interdependent results of that.6 This hard exoskeleton is a cuticle secreted by the epidermis, with chitin (a tough carbohydrate) and protein predominating. The exoskeleton has two layers: an outer waxy layer (epicuticle) to prevent drying out, and an inner layer (procuticle) for protection. This inner layer is often hardened by calcium carbonate in the aquatic crustaceans. The arthropod cuticle is obviously much thicker than the one seen in the annelids.1,2,5

a) How to grow in a suit of armor--periodically shed skeleton as part of life cycle, with this molting controlled by hormones.

b) Size: cannot become too large, as exoskeleton becomes proportionately too heavy.

c) How to move? Jointed limbs, and a segmented, jointed body. The cuticle is softer at these joint regions. Furthermore, motion is now organized among many separate muscles rather than the large sheets of circular and longitudinal muscle seen in the annelids.

d) Tagmatization or specialization of groups of segments into body regions, especially cephalization or head formation.

e) Respiratory and other exchange systems have to be modified to get supplies through an impermeable surface, e.g., spiracles and tracheae. This direct air delivery can be quite effective in land-dwelling arthropods.

f) No longer a need a spacious coelom or body cavity for hydrostatic functions as we saw in the earthworm, and so it can regress to a hemocoel for open circulation of blood.

g) Sense organs have to be altered to use the properties of the available hard materials, e.g., sensory hairs.


An extinct subphylum of Trilobita was the dominant life form from 600 million years ago to 350 million years ago. Dorsal views of fossils indicate that trilobites had three lobes formed by grooves, three tagmata or body regions of head, thorax, and tail, and compound eyes.

The compound eye is seen in many arthropods today (vs. a simple eye). The human eye has a lens which can be changed to focus close or far away. Each retinal cell transmits a tiny bit of picture to brain, but with so many, it seems a continuous whole. In contrast, the arthropod compound eye is made of many individual eyes. The eye does not move, nor can it be focused. Each catches a small part of the scene and leaves a coarse-grained picture. The more facets, the more detail.2

The rarer ventral view of a trilobite fossil demonstrates that they had biramous ("two-branched") appendages. Trilobite appendages had a basal part (protopod), and two branches, an inner branch (endopod) and outer branch (exopod).6 Are these ancient appendages comparable to the uniramous ("one-branched") legs we see on spiders, insects?

Homology is a structural similarity regardless of present day function.2 The three criteria of homology (not specifically in the book but useful to remember):3

a) position; where the organ is borne

b) special qualities; e.g. shape, anatomy

c) serial homology; connection of divergent structures by intermediates


Homology, taken strictly, should refer to a common ancestor, and so when present shows an evolutionary relationship among the organisms and/or structures. Recently, molecular biologists indicate that there may be genetic homologies among the wide variety of arthropod limbs.

For example, the distal-less gene regulates the formation of structures on arthropod (and other) limbs. Branching (or not) of the arthropod limb seems to depend on different patterns of expression of this and related genes. Other genes affect whether crustacean limbs are for feeding or for swimming.4

Four major groups of extant arthropods in 1) Subphylum Chelicerata (horseshoe crabs and arachnids), 2) Subphylum Crustacea (crustaceans), and Subphylum Uniramia (3) insects and 4) myriapods):6

Chelicerates (horseshoe crabs and arachnids [spiders, scorpions, ticks]): 0 antennae, chelicerae ("claw horns") and pedipalps ("foot feelers") for mouthparts; 4 pairs of uniramous legs; two tagmata or body regions of 1) cephalothorax and 2) abdomen

Crustaceans (e.g., lobsters, crabs, crayfish): 2 pairs of antennae; mandibles and maxillae for mouthparts; variable pairs of biramous appendages; variable tagmosis (body region specialization); and a characteristic larval form called a nauplius. Typically aquatic.

Insects: 1 pair of antennae; mandibles and maxillae for mouthparts; 3 pairs of uniramous legs; usually 2 pairs of wings; and three tagmata or body regions of head, thorax, and abdomen.

Myriapods (centipedes and millipedes): 1 pair of antennae; mandibles and maxillae (and other possible mouthparts); variable pairs of uniramous legs; two body regions of head and trunk

CRUSTACEANS: all except pill bugs are aquatic. Familiar ones, e.g., lobsters, crayfish, and crabs, have a body plan of a cephalothorax under the protection of a fused shell or carapace. In contrast, the abdomen is highly segmented, and may be of different lengths (very short in crabs). Gills are both off of the body and off of limbs.

Barnacles are crustaceans too, as seen by earlier stages in their life cycle: nauplius, then another larval stage which attaches by an antenna to a surface. Gut becomes U-shaped in filter-feeding adult, with a more calcareous (calcium carbonate) shell than other crustaceans.

CHELICERATES: One group of chelicerates, the horseshoe crabs (Class Merostomata) ["thigh mouth"] are living fossils, not having changed much in millions of years. Their mouthparts of chelicerae and pedipalps are very limb-like.

In contrast to the crustaceans and horseshoe crabs, the arachnids (Class Arachnida) ["spider"] are ready for land. For instance, spiders have book lungs or tracheae to minimize water loss from moist respiratory surfaces. Arachnids often have mating rituals. Development of the young is direct, with no larval stages.

Scorpions: ancient animals, with a stinger, often nocturnal. Pedipalps modified into pincers.

Spiders: chelicerae are tipped with poisonous fangs. Spiders poison their prey, then pour digestive enzymes into the punctured body to create a liquid meal. Pedipalps are modified in male spiders to help with sperm delivery. 8 simple eyes keep watch. A pedicel or waist connects the two body segments. 6 to 8 spinnerets produce silk in the abdomen. The related daddy longlegs (harvestmen) has long legs, but with a body that has less of a pedicel (waist).

Mites and ticks: cephalothorax and abdomen are fused under one carapace. Mites are small and live in a variety of habitats: spider mites on house plants, chiggers, eyelash mites, other skin regions --e.g. scabies. Major component of dust allergies is the feces of mites that eat flaked-off skin. In contrast, the larger ticks feed on blood. Major tick-borne diseases in US are Lyme disease and and Rocky Mountain Spotted Fever (RMSF).


1 R Buchsbaum et al., Animals Without Backbones, 3rd ed. (U of Chicago Press, Chicago, 1987), pp. 320-321

2 CP Hickman et al., Biology of Animals, 7th ed. (WCB McGraw-Hill, Boston, 1998), pp. 95-97, 491-492, 502

3 EM Lord, class notes for BOT 138, Plant Morphology, Spring 1990 quarter, U. of California-Riverside

4 E Pennisi, W Roush,"Developing a new view of evolution," Science 277: 34-37, 1997

5 WD Russell-Hunter, A Life of Invertebrates (Macmillan, NY, 1979), pp. 225-226

  1. P Willmer, Invertebrate Relationships (Cambridge U Press, Cambridge, England, 1990), pp. 272, 280-281, 293-295


|main page| |background| |03028: Physiology| |03048: Anatomy|

|03050: Invertebrate Zoology| |03051: Vertebrate Zoology| |03074: Economic Botany|


Please send comments and questions to: cronewil@hvcc.edu


HVCC home page

Copyright 1999 by Wilson Crone

External and unofficial links are not endorsed by Hudson Valley Community College


This page updated on November 1, 1999