ARTHROPODS(ZOO 101 Lecture)

Abiodun A. Denloye, PhDDepartment of Zoology and Environmental Biology,

Faculty of Science, LASU

Meaning• The Arthropoda – • Bilateral protostomes

eucoelomate • eumetazoa • metamerically segmented body• each segment bears jointed appendages.

The name is derived from a fusion of two Greek words (i) arthron ie “joint” and (ii) podos ie “foot”. In other words it is the group of invertebrates having

jointed legs.

• How did the name come about? • The contributions of the following four are well known 1. Aristotle described a few crabs and other arthropods, 2. Linnaeus included all such animals in the group he

called "Insecta" 3. Lamarck divided this group into three classes namely

Crustacea, Hexapoda and Arachnida

4. Von Seibold (1845) was the one who actually established the phylum Arthropoda for these animals for the first time.

ABUNDANCE - 80% of all animals

Reasons for Success1. The body is divided into somites, which bear jointed

appendages. 2. Their mode of respiration allows air to pass directly

to the cells (terrestrial arthropods use racheal system, the aquatic species use gills.

3. Developed sense organs, e.g., compound eyes, chemoreceptors on appendages etc.

4. Complex innate behaviors (eg Waggle dance in bees) 5. Development is through metamorphosis). 6. Versatile integument (outer covering).

Characteristics of Arthropods

1. They are bilaterally symmetrical and triploblastic invertebrates 2. Their integument has hard, chitinous exoskeleton having basement membrane,

epidermis and the cuticle. 3. They grow through periodic shedding of the cuticle – moulting (ecdysis).. 4. Each segment bears lateral jointed appendages 5. The Muscular system has muscles arranged only in discrete bundles (Striated). 6. The central nervous system (CNS) is made up of the dorsal brain, connected to the

solid ganglionated nerve cord on ventral side. . 7. They have the complete type of digestive tract. 8. They have open circulatory system with a dorsal, tubular heart made of many

chambers 9. Respiration is carried out using gills in aquatic arthropods, or tracheae or book lungs

in terrestrial species. Respiration may also be carried out by diffusion through body surface in some species.

10. They have Compound eyes. 11. The main organ of excretion and osmoregulation in some are the Malpighian

tubules. 12. The nervous system basically consists of a brain-ring.13. Sexes are separate.

Group Features

Phylum Trilobita• The are all extinct. • They are Paleozoic fossils (542 – 251 Mya) • Found throughout the world. • The most advanced among arthropods• Body divided transversely into three tagma: the cephalon or head,

thorax, and the pygidium (or tail).

Phylum Crustacea - Crustaceans1. They are mostly aquatic, found in fresh, brackish and marine water2. Some are terrestrial isopods and semi-terrestrial crabs being the

only exceptions. 3. They use gills for respiration. 4. Body Divided to Cephalothorax (head fused with thorax) and

abdomen. 5. The cephalothorax is usually covered dorsally by a hard chitinous

cerapace. 6. They have two-segmented (biramous) appendages7. There are always two pairs of antennae and one pair of antennules. 8. All appendages, except the antennules, are biramous, 9. The sexes are separate, although some are hermaphrodites. 10. They undergo primitive development mediated through nauplius

larva stage. 11. Excretion is by means of green glands

Examples of crustaceans. Photo Credit: Visual Dictionary online

Phylum Chelicerata – The Chelicerates

• They are one of the most successful animal phyla. • Members include spiders, scorpions, ticks and mites. • They are distinguished generally by having the first two tagma

fused, and they generally have eight limbs.• The first one or two pairs of limbs are often modified for

feeding. • There are two classes namely Merostomata and Arachnida.

Class Merostomata • They are aquatic arthropods. • The body is typically divided into two parts; a cephalothorax (or prosoma),

and an abdominal tagma, the opisthosoma. • They possess lateral compound eyes, appendages with gills and sharp

talons. • They have anterior claws (chelicerae), and a spike-like extension at the

posterior end called telson. • The opisthosomal appendages are biramous, with one branch serving as a

gill. • They include extinct eurypterids (subclass Euryptida), e.g., giant water

scorpion and the xiphosurids, e.g. horseshoe crabs

Class Arachnida• The body is divided into a cephalothorax and an abdomen, except ticks

and mites, where they are fused together. • The cephalothorax bears six pairs of uniramous appendages • The first two appendages are modified for feeding, and the last four for

locomotion • They have four pairs of legs• They are primarily terrestrial, and respiration is by the use of lung books

and trachea. • Excretion is through gills or coxal glands. • They have a dorsal bilobed brain• They only have simple eyes, no compound eyes. • The sexes are separate. And with no true metamorphosis, some oviparous. • They undergo direct development. • They are the second largest group, next to the Insecta. • Examples are spiders, scorpions, ticks, mites etc.

Examples of Arachnids

Phylum Uniramia• This is the phylum of the millipedes, centipedes and insects. • It is named Uniramia to connote their exclusively uniramous

appendages• . Two classes namely Class Myriapoda (with subclasses Diplopoda

and Chilopoda) and Class insecta are recognized here.

Subclass Diplopoda:

• These are worm-like arthropods. • The common millipedes • Found under rocks and logs in fairly moist

environments • They are cylindrical and bear two pairs of legs

on each segment. • They were one of the first animals to venture

onto dry land.

Subclass Chilopoda

• Chilopods are the centipedes.• They differ from the diplopods (millipedes)

mainly in having only one pair of appendages on each segment.

• The first pair of appendages behind the head is claw-like and functions as a set of poisoned pincers.

Class Insecta • All insects have an exoskeleton consisting of a firm cuticle. • The body is typically three tagma, with about 20 segments grouped

into the head (6 fused segments), the thorax (3 segments) and the abdomen (11 segments).

• The head bears sensory organs (antennae and eyes) and mouthparts. (mandibles, maxillae, labrum.

• The have three pairs of legs, one pair on each thoracic segment

Dorsal view (L), entral View (R) of Cockroach – A typical insect

Economic Importance of Arthropods

1. They serve as food2. They are predators and so help in biological control of

pests3. They help in pollination of flowers4. They can bite resulting in pain or death5. They serve as useful tools for scientific research6. The serve as sources of income7. They are medicinal8. They are used as ornaments.9. They help in improving soil fertility10. They break down organic matter.

THANK YOU• Any question?