The document discusses the muscular system of insects. It describes the three main types of muscle tissue - striated, cardiac, and smooth muscle. It notes that insects only have striated muscle. The structure and organization of insect muscle fibers and myofibrils are explained. Insect muscles are classified as either synchronous or asynchronous skeletal muscles. The mechanisms of muscle attachment to the exoskeleton via tonofibrillae and apodemes are outlined. Finally, the key functions of the muscular system in insects are listed.

1. Kingdom of Saudi Arabia
Al Imam Mohammad Ibn Saud Islamic University
College of Science
Department of Biology
Entomology
353 Bio
T. Amani Alsharidah
1441 – 2019

2. Lecture 9
Insect muscular system

3. Introduction
 Muscles power all the movements, external and internal, in insects.
 Insect muscles show high levels of homology to these vertebrate
 muscles in their structure, protein content, contractility and regulation.
 Insect muscles are mostly translucent, colourless or grey, though the flight
muscles often show a yellowish or brown tinge.
 In most skeletel muscles, especially those of the appendages, one end of the
muscle is attached to a movable part.
 Cuticular invaginations or apodemes, in the form of cords, bands or plate like
structures, may provide the true sites of attachment.

4. The cells that constitute this tissue are referred to as muscle fibers .
Classification of muscle tissue:
Three types of muscles are usually distinguished according to striations into
striated (skeletal and cardiac)and unstriated (smooth).
1. The most familiar is the skeletal ,voluntary .This type is named skeletal muscle
because it is connected to the skeleton and is concerned with body
movements.
2. The second type forms the contractile portion of the heart and is therefore
called cardiac muscle .It is striated but involuntary in action. They contract
spontaneously.
3. The third type is also involuntary and because it does not exhibit striations, it
is known as smooth muscle. It is found as part of the walls of the viscera.
The muscular Tissue

5. The muscle type

6. The muscular Tissue
 Each type of muscle has its own particular characteristics, but all types of
muscles exhibit common features which are:
 Muscle cells are elongated in shape.
1. The plasma membrane of the muscle fiber is called sarcolemma and its
cytoplasm is called sarcoplasm.
2. Organoids and inclusions are present in the cytoplasm (mitochondria and
smooth ER which is called sarcoplasmic reticulum).Glycogen is the main
muscle inclusions.
3. Muscle fibers are mesodermal in origin.

7. Insect muscle
 Vertebrates and many non-insect invertebrates have striated and smooth muscles, but
insects have only striated muscles.
 The entire muscle is enclosed by a sheath of dense CT called epimysium .
 Thin fibrous septa ,that constitute the perimysium exetnd the epimysium and divide the
muscle into bundles of muscles fibers called fascicles .
 The epimysium sends delicate CT sheets to surround individual muscle fibers in each
fascicle. These connective tissue sheets constitute the endomysium.
 Blood vessels and nerves penetrate the epimysium to the perimysium and endomysium
but Lymphatic vessels are only present in the epimysium and perimysium.

8. Insect muscle
 Skeletal muscle fibers :
 Long cylindrical cells, enclosed by the sarcolemma.
 The fibers are multinucleated ,pale oval nuclei that lie under its sarcolemma.
 The sarcoplasm is filled with numerous myofibrils.
 The myofibrils, Two principal types of myofilaments have been identified ;thick
and thin ones .
1. The thick filament is composed of a protein called myosin.
2. The thin myofilaments are attached to the Z line and are composed of a protein
called actin together with two other associated protein called tropomyosin and
troponin.
 Muscle contraction involves the sliding of filaments past each other, stimulated
by nerve impulses.

9. Insect muscle
striated (skeletal) muscle

10. Classification of skeletal muscle in insect
There are several different muscle types but in general it classified to:
1. Synchronous Skeletal Muscles :-
Vast majority of insect muscles are synchronous muscles. Each contraction is driven
by a single neural stimulus. The form and arrangement of the myofibrils in
synchronus muscles is very variable.
2. Asynchronous Skeletal Muscles :-
In these neural stimulation is asynchronous with respect to contraction. The
specialized asynchronous muscles typically have large cylindrical myofibrils, that’s
why they are sometimes called fibrillar.

11. Muscle attachments
 Vertebrates’ muscles work against an internal skeleton, but the muscles of insects
must attach to the inner surface of an external skeleton.
 As musculature is mesodermal and the exoskeleton is of ectodermal origin, fusion
must take place.
 This occurs by the growth of tonofibrillae, fine connecting fibrils that link the
epidermal end of the muscle to the epidermal layer.
 At the site of tonofibrillar attachment, the inner cuticle often is strengthened
through ridges or apodemes, which, when elongated into arms, are termed
apophyses.
 These muscle-attachment sites, often include resilin to give an elasticity that
resembles that of vertebrate tendons.

13. Function of muscular system
1. Support of the body.
2. Helps maintain posture.
3. Movement of the limbs, including ovipositor.
4. Movement of the wings-insects.
5. Movement of the viscera.
6. Locomotion.
7. Closure of spiracles.
8. Operation of various pumps such as cibarial pump and the pumping of the
poison glands.
9. Generation of heat by ‘shivering’.