What Is A Spider?

What is a spider?

The Order Araneae (Spiders) is the largest order within Arachnida, and spiders are found in every part of the world, except Antarctica.  Although many people think of spiders as being insects or “bugs”, they are in fact different in many ways from Insects.

Spiders	Insects
2 body parts (cephalothorax & abdomen)	3 body parts (head, thorax, abdomen)
4 pairs of legs	3 pairs of legs
2-8 simple eyes	Compound eyes
Do not have wings	Most have wings or vestigial wings
Carnivorous	Herbivorous and carnivorous
Ecdyse from spiderlings to adults	Complete and incomplete metamorphosis

Spiders have 8 jointed legs, 2 distinct body parts, 2-8 simple eyes, exoskeletons, silk glands (and spinnerets) and all but one family have venom glands.  There are approximately 79 families of spiders in Africa, representing about 72% of all the spider families in the world, and they can be split up into two groups: Araneomorphae and Mygalomorphae.

Araneomorphae:

Araneomorphs are what most people think of when they imagine a true spider.  Many Araneomorphs are conspicuous, easy to find, and build webs.  The quintessential orb-webs and cob-webs we all know are made by Araneomorphs.  However, not all Araneomorphs build webs; there are also several types of burrowing and free-living Araneomorphs.

Mygalomorphae:

Most African Mygalomorphs are large, robust ground/burrow dwelling spiders, most of which, despite their ‘scary’ appearance, are harmless to humans.  They have small venom glands and are often not aggressive.  Baboon and Trapdoor spiders and their relatives are Mygalomorphs

Araneomorphae	Mygalomorphae
More highly evolved	More primitive
1 pair of booklungs	2 pairs of booklungs
Chelicerae strike horizontally (against each other)	Chelicerae strike vertically (with each other)
Web-bound, burrowing, or free living	Most are burrowing or ground dwelling.

Araneomorph - In Web: Tetragnathidae -
Silver MarshSpider

Mygalomorph - Burrowing: Theraphosidae -
 Baboon Spider

Definition – Booklungs: Several thin plates of permeable membrane where gas exchange takes place with oxygen on one side of the plate and blood on the other.

Spider ecology:

Despite the negative image many people have of spiders, they play an important role in a functioning, healthy ecosystem.  With almost 8,000 different species from 79 different families represented in Africa, spiders inhabit every type of ecosystem and biome on the continent, and are an important food source for a diverse number of larger (and smaller) predators.  As spiders are carnivorous themselves, they are also important biological controllers of their prey species, keeping insect numbers in check. 

 - If every spider on the continent were to suddenly disappear, we would drown in a sea of insects!

Spiders are a fascinating group of organisms to study; behavioural differences and colour variations are as numerous as the species themselves.