Introduction to Arachnids

1 – Game Ranging / Field Guiding Course

Invertebrates

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Module # 2 – Component # 1

Introduction to Arachnids

Objectives:

To achieve an understanding of the wide diversity of animals within the Class Arachnida.

Expected Outcomes:

To be able to recognise all arachnids based on their external anatomy To gain insight into their general physiology To become familiar with the different ecologies of arachnids

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Introduction

To begin with, we need to take a brief step back to explain what an Arthropod (Phylum Arthropoda) is, and where it fits in under the classification.

(For an in depth look at this, please see Module # 1, Component # 1 – Classification and Taxonomy).

To be classified as an Arthropod you need to satisfy several requirements:

You must be an invertebrate You need to have an exoskeleton You need to have jointed legs

There are many different creatures that satisfy these requirements, including the Insects. This makes the Phylum the largest on earth in terms of species diversity.

Arachnids also have the unwarranted reputation as being the most disliked group of arthropods as far as many people are concerned. They have even been responsible for their own unique psychological disorder – “Arachnophobia” or the fear of spiders.

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Phylum: Arthropoda

Class: Arachnida

To be admitted into the Class Arachnida, one further important anatomical feature is a prerequisite – 8 limbs or legs.

The class Arachnida encompasses the following animals:

Scorpions Spiders Ticks Mites

The group has an extremely long fossil record, but scorpions in particular, date from the Silurian Epoch of 400 million years ago, (For a look at the geological time scale. Please see Module # 1, Component # 2, Evolution).

Most of today’s forms are terrestrial, but some fossil species were known to be aquatic. These early species would have breathed by means of gills, and probably had soft bodies.

The movement of Arachnids onto land required the development of lungs, modification of the limbs, and a general hardening of the epicuticle to prevent dehydration. Later, developments such as silk and venom apparatus were perfected as adaptations to their terrestrial, predatory way of life.

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External Anatomy

Despite a great diversity of body form among this group, arachnids all show fusion of the thorax and head into one unit, known as the cephalothorax. The abdomen is segmented in the scorpions, but has fused into one smooth unit in the spiders. The abdomen of scorpions is divided into two sections viz. the pre- abdomen (body section of the abdomen) and the post-abdomen (sting and tail.)

Their mouthparts consist of chelicerae (fangs / biting mouth parts) and pedipalps (grasping mouthparts). In some groups, these may be modified into biting and chewing apparatus (scorpions), or into sucking apparatus (spiders). All members of this group have four pairs of legs. None possess wings.

Poecilotheria rufilata - Endemic to South Western Ghats of India

Nervous system

Three types of sense organs are common to most arachnids. Eyes, sensory hairs and sensory slit organs. Sensory hairs may be olfactory setae (stiff bristle like hair); movable setae located all over the body; or fine hairs restricted to their appendages.

For most arachnids, their sensory hairs are their primary sense organs. Slit sense organs are usually very small, consisting of a slit-like pit covered by a thin membrane. These slits may occur in great numbers, with some spider species having more than 3000. Most arachnids are only endowed with simple eyes.

Food and Feeding

Most species in this class are predatory, and in the case of ticks they readily include humans as part of their daily diet, when available.

The fact that many arachnids are capable of being vectors of disease when feeding makes them an exceptionally important group of invertebrates.

They are also a significant group in terms of their ability to inflict medically important bites and stings – some with the potential of becoming fatal if left untreated.

The identification, treatment and management of victims of arachnids inflicted injuries – in covered in Module # 11 Component # 5.

One further striking feature common to many arachnids, is their mode of digestion. Preliminary digestion takes place outside the body. After prey has been captured and immobilised by the chelicerae and pedipalps, digestive juices are poured from the midgut onto the open wounds caused by the mouthparts. The soft internal parts of the prey are rapidly digested into a “soup” that is then sucked into the mouth and alimentary canal, where it is further processed.

The following three Components examine the complete lifestyle and ecology of the various Arachnid groups – Scorpions, Ticks & Mites and Spiders.