Prof. Dr. Badr El-Sabah Entomology Lecture 1

Lecture 1
Introduction
To Phylum Arthropoda
Class : Insecta
Prof.Dr. Badr El-Sabah / Lecture 1
1

Introduction To Phylum Arthropoda
• The name Arthropoda would mean animals with jointed legs. (arthro:
jointed; poda: legs).
• The animals of this phylum have occupied almost all possible niches in
this planet, be it land or water or soil; and there are parasites too.
• In terms of numbers also they are the largest group – there are about
900, 000 species.
• Their success could be attributed to a variety of factors:
– Widespread distribution
– Variety of habitats occupied
– Varied food habits
– Adaptations to overcome the harmful effects of the environment –
particularly the development of an impermeable exoskeleton –the cuticle.
– Ability to adapt themselves to changing environment
Prof.Dr. Badr El-Sabah / Lecture 1 2

General Characters of Phylum Arthropoda
• The body is bilaterally symmetrical, triploblastic and metamerically
segmented.
• Jointed appendages which lie as one pair to a somite, and with
varied functions as jaws, gills, legs, etc.
• The exoskeleton is composed of dead chitinous cuticle that is shed at
intervals, called ecdysis or molting, for growth and development.
• The body is divisible into head, thorax and abdomen. Head and thorax
are often fused to form a cephalothorax.
• True coelom is reduced and is largely replaced by a blood-filled
haemocoel.
• The muscles are mostly striated, usually capable of rapid contraction.

4Prof.Dr. Badr El-Sabah / Lecture 1

General characters of Class Insecta
1) The head contains
– a pair of compound eyes,
three frontal simple eyes
called ocelli
– a pair of antennae
– a terminal mouth
– Mouth parts modified for
different feeding habits.
– The 4 main mouthparts
are the labrum,
mandibles, maxillae
(plural maxilla) and
labium.
2) The thorax
• The insect thorax is
divided into three parts:
I. prothorax (pro=first)
II. mesothorax
(meso=middle)
III. metathorax (meta=last).
• Each of the three
thoracic segments
contains one pair of
legs.
• Wings are found only
on the meso- and
metathoracic segments.
3) The abdomen
• The abdomen contains
the reproductive organs
and the majority of the
organ systems
• Spiracles usually can be
found in the conjunctive
tissue between the terga
and sterna of abdominal
segments 1-8
• Reproductive structures
are located on the 9th
segment in males and on
the 8th and 9th
abdominal segments in
females .
• The segmented body is divided into three typical regions, namely a head(6 fused segments), a
thorax (3 segments) and an abdomen(11 segments)
5

• Each segment consists of hardened plates,
or sclerites. The dorsal plate is called
tergum and the ventral plate is called
sternum. lateral plates are called pleura
(singular pleuron)
• The respiration is by trachea. Spiracles are
present laterally.
• Excretion by Malpighian tubules.
• Unisexual and fertilization is internal.
• Development usually takes place with
metamorphosis.
General characters of Class Insecta(cont)

Complete Metamorphosis
(Holometabolous)
• Each of the developmental stages is
structurally and functionally very different
• The egg develops into an immature larva;
eats voraciously
• Followed by a transitional stage - pupa,
contained within cocoon
Metamorphosis occurs within the pupal
exoskeleton, yielding a sexually mature
adult
(egg --> larva --> pupa --> adult).
Early developmental stages are very
similar to the adults.
only the wings and the reproductive
structures gradually develop
• The immature stages are called
nymphs
(egg --> nymph --> adult)
Incomplete Metamorphosis
(Hemimetabolous)
7

Basic Definition:
1. external skeleton
2. 3 distinct body regions
3. 3 pairs legs on 3 thoracic segments
4. 1 pair sensory antennae
5. (usually in adult) 1 pair compound eyes
6. (usually in adult) 1 or 2 pair of wings
What Is an Insect?

Other Defining Features:
7. External mouthparts (mandible, maxillae, labium)
8. Eyes primitively with 2 pigment cells
9. Thoracic legs with maximum of 6 segments
10. Abdomen primitively with 11 segments
11. Antennae with 2 musculated segments (scape &
flagellum)
12. Ocelli (in addition to compound eyes)
13. Gonopore on segment 8 or 9, cerci on seg. 11
14. Well developed Malpighian tubules
15. Embryonic development by superficial cleavage
16. Epimorphic (cf. Anamorphic), number of
segments fixed
(What Is an Insect?)

THE IMPORTANCE OF ARTHROPODS
>>Natural World<<
Effects are “Neutral”
CONTRIBUTIONS OF INSECTS TO:
• Community structure - nutrient cycling, vegetation structure
(phytophagy, pollination, seed dispersal)
• Population dynamics - predators & parasites, disease
transmission
• Food webs - food for other animals.

POLLINATION – FLOWERE POLLINATORS

Negative Influences
Agriculture: competition for food & fiber apple maggot, bark
beetles
Domestic: damage to property & goods termites, silverfish,
dermestids
Medicine: parasites & disease malaria (via mosquitoes), bot
flies, screw worm fly
THE IMPORTANCE OF ARTHROPODS
>>Anthropocentric World<<
•Defined best by Human Culture & $Economy$
•Negative & Positive Effects

Positive Influences
Agriculture: biocontrol, pollination lady beetles, honey bees, solitary bees
Medicine: antibiotics, chronic disease treatment, maggot debridement,
honey bee venom, maggots
Commerce: products, cochineal, silk, wax, honey scale insects, silk
moth, honey bees
Science: ideal subjects for experimentation, genetics (Drosophila)
ecology, physiology vinegar “fruit” flies, tobacco hornworm,
honey bees
Aesthetics: art & inspiration butterflies, beetles, fireflies
Food: nutrition, environmental, economic soundness 500 species, 17
families, especially grubs & caterpillars

Beekeeping
Silk Industry
Biological
Control

Insect Evolutionary/Ecological Success:
SPECIES RICHNESS & NUMBERS
Why are insects so diverse?
Possible reasons:
 Relative size (more niches)
 Short generation time (rapid evolution)
 Holometabolism (specialization)
 Powered flight
 Sensory & motor neuron sophistication
 Preadapted for coevolution with other organisms
 Geologic age
 Sexual selection
 Phytophagy

External
Morphology
Of
insects

Tagmosis in general body regions. Insect fundamental
segment number: 6, 3, 11.

Fig. 2.7, Gullen & Cranston 2000
a) insect larva
b) “secondary’ segmentation
c) “derived” secondary seg.
d) advanced winged insect
Increasing sophistication of body
segmentation

1
2
6
5
4
Insect head segmentation.
post antennal segment (compound)3

Generalized insect MOUTHPARTS
! 20Prof.Dr. Badr El-Sabah / Lecture 1

Insect Body-parts
A brief guide to the
main external
features of insects
and the terminology
used to describe the
various body-parts.

The insect body is divided into three main parts - head,
thorax and abdomen - as shown above.
Head. The head bears the mouthparts, the eyes and a
pair of antennae (singular: antenna). Most adult
insects and the immature stages (nymphs) of
Exopterygota have a pair of large, multi-faceted
compound eyes (labelled 'eye' in the diagram above).
Many of these insects also have two or more small,
single-faceted eyes, called simple eyes or ocelli
(singular: ocellus), usually placed on the top or front of
the head between the compound eyes

In some insects, particularly those
belonging to the Apterygota, and in the
immature stages (larvae) of all
Endopterygota, compound eyes are
lacking and only ocelli are present, with
one or more of these simple eyes on
each side of the head. In a few adult
insects and among many larval forms,
eyes are completely absent.

Thorax. The thorax consists of three segments - the
prothorax (front), the mesothorax (middle) and the
metathorax (back). Each of these segments bears a
pair of jointed legs. Each leg consists of four main
parts - the coxa, the femur, the tibia and the tarsus
(plural: tarsi) - with a small, often inconspicuous
segment between the coxa and femur, called the
trochanter. The tarsus usually consists of several
small joints, the last of which generally carries a pair
of terminal claws. The last tarsal segment may be
extended between the claws to form a pad-like
organ - the arolium (as in (1) below).

Some Diptera (true flies) have two additional pads - the
pulvilli (singular: pulvillus) - lying below the claws on
either side of the arolium (as in (2) below), although in
most flies, including the common housefly and its
relatives, the arolium is replaced by a stout central
bristle - the empodium (as in (3) below). In many
insects, the other tarsal segments also have ventral
pulvillus-like organs, called plantulae. These various
structures - the arolium, pulvilli and plantulae - produce
a sticky secretion and act like 'suction-pads', enabling
the insect to climb smooth or steep surfaces. This is the
secret of the fly's ability to walk up-side-down on the
ceiling!

Terminal Segments of the Tarsus (see text above for
details)
(1) (2)
(3)
(a = arolium, c = claw, e = empodium, p = pulvillus)
In winged insects, the meso- and metathoracic segments each bear a
pair of wings (the forewings and hindwings, respectively), although in
many insects one or both pairs of wings have been lost, or modified into
other structures, during the course of evolution. Special wing
modifications include the structures called halteres found in the Diptera
(true flies), and the structures called elytra (singular: elytron) found in
the Coleoptera (beetles) and Dermaptera (earwigs):

Halteres - small, club-shaped
organs of balance found in
Diptera (flies) on each side of
the thorax (metathorax) just
behind the forewings and
thought to be modified
vestigial hindwings.

Elytra (singular: elytron) - the
hard, leathery forewings of
Coleoptera (beetles) and
Dermaptera (earwigs), that serve
as protective coverings or 'wing-
cases' for the membranous
hindwings. The hindwings (if
present) are usually folded out of
sight under the elytra when at rest.
The diagram shows a green tiger
beetle (Cicindella campestris) with
the elytra open and hindwings
outstretched, as the beetle
appears when in flight.

Abdomen. The abdomen is made up of
several, more or less similar segments
and often bears a pair of jointed, terminal
appendages called the cerci (singular:
cercus). Typical cerci are illustrated in
(a) below, although in some insects they
consist of many more joints than in the
example shown.

Abdominal Appendages of Adult Insects
(a)
typical cerci
(b)
cerci and tail
filament
(c)
cerci modified
as
forceps e.g.
earwigs
(d)
paired
cornicles
e.g. some
aphids

In the immature stages (larvae) of some
Endopterygota, e.g., those of many
butterflies and moths (Lepidoptera),
scorpion flies (Mecoptera) and sawflies
(Hymenoptera), the abdomen carries
several pairs of appendages called false-
legs or prolegs. These are unjointed, fleshy
structures different from, and additional to,
the jointed legs of the thorax. Larvae with
prolegs are commonly called caterpillars.

Caterpillar of a Sawfly -
side-view showing: (a) the
jointed legs (i - iii) of the
thorax, and (b) the fleshy
prolegs (i - vii) of the
abdomen. There are always
three pairs of thoracic legs
(as characteristic of insects
generally), but the pairs of
abdominal prolegs may vary
in number depending on
species.

THANKS

Prof. Dr. Badr El-Sabah Entomology Lecture 1

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