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THE MOST COMMON ANIMALS

Insects are the most common animals on earth. They can be friends, allies, enemies and aides to humans depending on which one of them  you are considering and under what conditions. They include the most common ones such as cockroaches eg (Periplaneta americana) and the least commonly encountered ones like the petroleum fly named Psilopa petroli found in crude oil. Read the following text excerpted from the book INTRODUCTION TO COMMON ARTHROPODS by Abiodun Denloye. This excerpt is an introduction to the insects belonging to the class of animals called Insecta.



Adult Petroleum fly, Psilopa petroli

Class Insecta
The insects are the most diverse and the commonest arthropods today. They include,
ants, bees, wasps, beetles, butterflies, dragonflies and fleas. They range in size from
microscopic soil-dwelling insects to large beetles and butterflies. They are mostly
terrestrial and some are aquatic. The aquatic species survive inside water by trapping air
bubbles against their respiratory openings (spiracles). Terrestrial insects are one of only
two groups of animals that are truly adapted to terrestrial life, in that they manage water
to an extent that allows them to explore the terrestrial habitat fully (the other group is the
Amniota, which includes Mammalia and Reptilia).
Wingless insects are known from rocks as old as Devonian (from 416 Ma), and the
winged forms appear first in the Pennsylvanian. Most Paleozoic insects are known from
swamp deposits, and most Mesozoic and Cenozoic fossils from air-fall tuffs.

Adult american cockroach, Periplaneta americana

Characteristics of Insects
All insects have an exoskeleton consisting of a firm cuticle. The firm exoskeleton
supports the insect, maintains its shape and protects it from damage and from
evaporation. The rigid exoskeleton prevents insects from growing in a continuous
manner and growth takes place in spurts. The outer layer of cuticle is shed and the
insect expands its body until the new layer of cuticle forms and hardens. This moulting
process (or ecdysis) takes place five or moretimes until the mature insect appears, after
which there is no further growth or ecdysis. Stages between each moult are referred to
as instars.
The body is typically divided into about 20 segments which are grouped into the head (6
fused segments), the thorax (3 segments) and the abdomen (11 segments). The typical
insect body is divided into three distinct tagma: the head (composed of six segments),
thorax (three segments), and the abdomen (11 segments, usually with only 10 being
apparent). The external morphology of insects is illustrated the American Cockroach
The head bears sensory organs and mouthparts. There is a pair of antennae which are
sensitive to touch and smell. There is also a pair of compound eyes made up of
hundreds of separate lenses (ommatidia) and sensory cells. These can detect light,
movement and colour and can form crude images of shapes. In bees and butterflies
these senses allow the insect to find sources of nectar in flowers by their colour, shape
and smell.
There are three sets of mouthparts carried externally on the head. This consists of a pair
of mandibles (jaws) which bites off portions of food and passes them into the mouth. A
pair of maxillae helps the insect in tasting and manipulating the food while the labium (or
lower 'lip') has different functions depending on the species of insect.
The insect thorax is made up of three segments. The first (connecting with the head), the
second and the third are called prothorax, mesothorax and metathorax respectively.
Each of the three thoracic segments has uniramous (unbranched) legs and, in most
insects the mesothorax and metathorax bear two pairs of wings which may be reduced
to one pair. Also, some species have no wing at all.

Characteristically, all insects have three pairs of legs, one pair on each thoracic
segment. Each of the legs has five sections with joints between each section. The joint
enables the leg to move in different directions. The exoskeleton at these joints consists
of a flexible cuticle which permits freedom of movement. In the mature insect the
abdomen carries no appendage other than some apparatus on the final segment to
assist with egg-laying.
Insects are the most successful amongst all animals. Alone, they make up about 76% of
all animals, with several species yet to be discovered. They live in all environments
capable of supporting life. They are therefore found on land (primarily), the sea, and
freshwater. Notably, a dipteran species known as Petroleum fly (Helaemyia syn.
Psilopa petroli  lives in crude oil. Their distribution cuts across every part of the world.
Insect orders
Insects are divided into 29 orders which are listed below..

1. Order Thysanura

2. Order Diplura

3. Order Protura

4. Order Collembola
5. Order Ephemeroptera
6. Order Odonata

7. Order Plecoptera

8. Order Grylloblatodea

9. Order Orthoptera
10. Order Phasmida

11. Order Dermaptera

12. Order Embioptera

13. Order Dictyoptera

14. Order Isoptera
15. Order Zoraptera

16. Order Psocoptera

17. Order Mallophaga

18. Order Siphunculata

19. Order Hemiptera

20. Order Thysanoptera

21. Order Neuroptera

22. Order Mecoptera
.
23. Order Siphonaptera
24. Order Coleoptera

25. Order Strepsiptera
.
26. Order Diptera

27. Order Lepidoptera

28. Order Trichoptera

29. Order Hymenoptera
Success of insects
Insects are successful because:
1. They are small in size
2. They have the ability to fly
3. They produce resistant eggs capable of being dispersed by air water currents
and animals.
4. They are resistant to desiccation
5. They adapt to all conditions.
The study of insects is called entomology. In broad terms entomology is the science that
deals with the biology (structure & function), ecology and importance of insects.
Structure and function relates to the morphology, physiology and genetics. Ecology
deals with the study of the interrelationships between insects and themselves as well as
other organisms and their physical environment.