Arthropoda

INTRODUCTION

The Phylum arthropods contains more species than all other known phyla added together. Arthropods are characterised by having segmented bodies, a hard cuticle covering their bodies, called an exoskeleton, and jointed limbs. The name Arthropoda actually means jointed limbs. Arthropods have more advanced sense organs than any other phyla discussed so far in this course. The arthropods do possess a limited nervous system.

The phylum arthropods are divisible into at least seven classes:

Oncycophora;
- Crustacea;
- Insects;
- Arachnids;
- Chilopoda;
- Diplopods; and
- Pynogonida.

In this Lecture, only three of these will be discussed and only two in significant detail.

Crustacea: These are mainly marine organisms such as shrimps and lobsters. They possess a shell that protects their bodies. They do have an economic value in certain areas. However, they are of no agricultural value.

Insecta: These are the six legged arthropods and are also referred to as Hexapods, meaning six- legged. These are commonly known as the insects. Many insects are winged, in fact possessing two pairs of wings. However, not all insects are winged, and some have even lost the ability of flight. This class includes many agriculturally important species.

2. ARACHNIDS

This class makes up the eight-legged arthropods, and includes ticks, mites, spiders and scorpions. This class includes some very important species.

The class of Arachnida can be divided into three orders:

Scorpionidae;
- Arinea; and
- Acarina.

As agriculturists, we are only concerned with the order Acarina, these are ticks and mites. The first two orders are the scorpions and spiders respectively. Spiders are important because they predate upon other harmful arthropods. The order Acarina is divided into ticks on one hand, and the mites on the other. Ticks are further divided into hard ticks (Ixodidae) and soft ticks (Argasidae). The ixodidae possess a hard shield called a scutum whereas the argasidae do not. The table below should help the student to appreciate the classification so far discussed:

Table 1: Classification of Arachnida

TICKS

The Argasidae (soft ticks) are mainly parasites of poultry, whereas the hard ticks prefer the larger vertebrates, mainly mammals. All ticks are blood sucking, and they are important pests of livestock. Ticks inflict their damage in a number of ways:

they transmit diseases;
- they can cause physical skin damage which may lead to ulceration, or even infection;
- they may cause severe anaemia due to loss of blood in the host; and
- some hard ticks have toxic saliva and cause a condition known as sweating sickness.

Figure 1 shows a typical hard tick. The most familiar ticks are those that parasitise cattle and transmit diseases to them. Listed below are some of the diseases transmitted by ticks:

Redwater (Babesiasis);
- Gall Sickness (anaplasmosis);
- East Coast Fever;
- Theileriosis; and
- Heart water.

Figure 1 and 2: Hard Ticks, Male (left) and Engorged Female (right)

Figure 2: Life Cycle of One Host Hard Tick

The life-cycles of hard ticks depend upon the species. There are essentially three types of life cycle and this fact is used in classifying the ticks. These are known as:

One host ticks;
- Two host ticks; and

Defoliation: remove leaves from a tree, plant, or area of land, for agricultural purposes. Deformities: the state of being deformed or misshapen.

Three host ticks.

All ticks have 4 pairs of legs, except the larval stages where they have only 3 pairs of legs. (Figure 2 shows the Life cycle of the one host ticks). All life cycles of hard ticks go through basically the same stages, but appear on different hosts.

ONE HOST TICKS

The larval, nymphal and adult stages of these ticks remain on one individual host throughout the entire life cycle. The adult tick eventually drops off, fully engorged with blood. The female lays a mass of eggs, and these hatch on the ground into six- legged larvae. They soon attach to a suitable host, and continue through the life cycle on the one individual.

TWO HOST TICKS

In this group, the larva attaches to a host animal, proceeds through to the nymph stage, drops off, moults into an adult and then finds another host to eventually drop off as an engorged adult.

THREE HOST TICKS

The larva, nymph and adult all feed on different individuals dropping off after each feed. All hard ticks mate on the host. The male never becomes as engorged as the female does. The time span of the various cycles varies immensely. Dipping programmes are planned to break the life cycle and to minimise the spread of tick-borne diseases.

The soft ticks are mainly parasites of birds and they live in the cracks and crevices of the birds’ environment. The adults can live for up to 5 years without food. These ticks feed for a few hours at a time.

MITES

Mites are microscopic animals. They attack plants and animals and can cause severe damage on the former and severe irritation on the latter. They are in the case of animals, parasites and are predominantly blood-sucking. Mites cause Mange in many mammals. In plants, Mites cause defoliation, resulting in yield loss and also fruit deformities as in citrus fruits. Examples of pest mites are:

Red Fowl Mite – blood sucking in hens;
- Red Spider Mite – pest of cotton; and
- Citrus Bud Mite – pest of oranges and lemons.

3. HEXAPODA

Insects are very diverse organisms, all possessing 6 legs. They possess three segments; Head, Thorax and Abdomen. The changing in shape and form is called metamorphosis. Refer to Figure 3.

Insects (Class)
*Apterygota*	Pterygota (Sub-classes)
(Primitive wingless)	(Winged)
*Endopterygota*	*Exopterygota*
These insects have larval, pupa and adult stages in their life cycles. Only the adults have wings. The Pupa is a dormant phase. (See Figure 4)	These insects only have nymphal stages. They all bear resemblance to the adult. However, only the adults have functional wings. (See Figure 5)
Orders	Orders
Coleoptera	–	Beetles	Hemiptera	–	Bugs, aphids
Hymenoptera	–	Bees, wasps, ants (not white ants)	Isoptera	–	Termites, white ants
Lepidoptera	–	Butterflies, moths	Orthoptera	–	Locusts, grasshoppers, Crickets
Diptera	–	Two-winged flies

Contrary to popular belief, not all insects are undesirable as some are in fact desirable. The main example of a desirable insect is the Honey Bee (Apis melifera). The honey bee is absolutely essential to agriculture because it pollinates many crops and enables the farmer to reap fruit, which would otherwise not have been formed. Probably the most notorious insect pest is the Locust of which there are many species. These insects are leaf-eaters, and fly in swarms and in a few hours they can completely destroy a crop. Many insects transmit animal and plant diseases, and for this reason they must be controlled. The larvae of many moths and butterflies (caterpillars) are very damaging and eat the leaves and fruits of numerous crops. The adults are often actually quite harmless. Figure 6 shows the general appearance of the main insect orders and the names of a few agriculturally significant insects.

4. CONTROL OF PESTS

Chemicals can be used to kill insect pests, but these have two major disadvantages:

They tend to kill beneficial insects; and
- Insects build up resistance to them. Chemicals also tend to pollute the environment and man eventually suffers.

Many insects, birds and other animals predate on insects and these can be used to keep pest populations down to an acceptable level. This method is clean and healthy and when used in conjunction with selective insecticides, forms a formidable and economic method of pest control. Insect diseases can also be used to kill off pests without affecting anything else. This natural form of control is called Biological Pest Control, and is now being used by many agricultural concerns. An example is using the Trichoplusia Virus to kill loopers on soya beans. A solution of the virus is made from ‘infected loopers’ and this is sprayed onto the soya bean crop.

Figure 3: An Insect