Download Introductory Entomology – Embryonic development of Insect

B.Sc. Ag. (Hons) Level 2 Semester II (Introductory Entomology – Embryonic development of Insect)
INTRODUCTORY ENTOMOLOGY (ENTO 253)
EMBRYOLOGY: DEVELOPMENT AND FORMATION OF EMBRYO IN INSECTS
Course Teacher: Dr. Md. Razzab Ali, Associate Professor
Department of Entomology, Sher-e-Bangla Agricultural University, Dhaka-1207
Attention: This note is just only for guideline
Embryology
Embryology is the branch of biology which deals with the growth and development of an embryo of
an organism, commencing with the union of male and female gametes.
Embryology includes the development of the fertilized egg and embryo and the growth of the organ
system.
Embryonic development in insects
Development of an insect from egg to adult can be divided into two partsa. Early embryonic development - takes place inside the egg and
b. Post embryonic development – occurring outside the egg.
Insect eggs
The shapes (Figure 2) and structural parts of an insect egg (Figure 1) are as follows:
a. Mature insect eggs are oval, elongate, may be spherical, disc or barrel like.
b. The eggs are covered by two shells, a tough outer shell called the chorion, and a thinner
inner shell called the vitelline membrane.
c. There is an opening called micropyle at the chorion for the entrance of sperm.
d. The two membranes surround a large nucleus and a mass of cytoplasm.
e. The cytoplasm consists of a large central area of yolk and a peripheral bounding layer, the
periplasm, beneath the vitelline membrane.
f. The egg nucleus lies in a central position and embedded in cytoplasm.
Figure 1. Structural parts of an insect egg
Created by Dr. Md. Razzab Ali
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B.Sc. Ag. (Hons) Level 2 Semester II (Introductory Entomology – Embryonic development of Insect)
Figure 2. Different shapes of insect eggs
Fertilization of egg
The production of male (sperm) and female (ovum) gametes is commonly considered to be the first
phase in insect development. The union of gametes (spermatozoon and ovum) is the second phase
of development, and creates a diploid zygote with the potential to form an entire organism and this
process occurs through the fertilization of ovum with sperms.
The sperm of most of insects remain alive for month to years within the spermatheca. Fertilization
occurs when the eggs are about to be laid and as each passes down the oviduct. The sperm enter into
the eggs through micropyle. Several sperms penetrate into the eggs and fertilization is affected by
one of the sperms to form zygote while the rest sperms degenerate.
Early embryonic development
The early embryonic development is started immediately after fertilization of the eggs by sperms
and it is occurred through a series of events that includesa. Cleavage
b. Formation of blastoderm
c. Vitellophages
d. Formation of germband and
e. Gastrulation
f. Formation of embryonic membrane
g. Blastokinesis
h. Formation of organ system
i. Appendages
Created by Dr. Md. Razzab Ali
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B.Sc. Ag. (Hons) Level 2 Semester II (Introductory Entomology – Embryonic development of Insect)
a. Cleavage
Cleavage is the repeated mitotic divisions of a fertilized ovum zygote. After fertilization, the zygote
nucleus of an egg starts to divide. After fertilization, the egg and sperm nuclei fuse together at the
periphery of the egg to form the diploid fused- nucleus (zygote) and then the zygote migrates to the
centre of the egg. The zygote nucleus divides repeatedly, thus, one cell divides into two daughter
cells called blastomeres, then cleave into four; these cleave into eight, and so on. Ultimately
produces large number of daughter nuclei. Then the daughter nuclei are accompanied by a hollow
mass of cytoplasm forming nucleo-cytoplasm units called energids or cleavage cells (Fig. 3. a & b).
b. Formation of Blastoderm
The energids move and migrate towards the periphery (periplasm) of the egg and arrange in a layer
of circlet within the yolk. The energids may undergo further, one or more mitotic divisions and
retain the distinct cell walls and subsequently form a layer of cells, called the blastoderm. The
blastoderm, in true sense, is the primary germinal epithelium. It lies just beneath the vitelline
membrane (Figure 3. c & d).
c. Vitellophages
In some species of insects, all energids do not migrate to the periphery to form the blastoderm but
some of them lie behind within the yolk are called the yolk cells, merocytes or vitellophages. The
vitellophages carry out breakdown of the yolk and are incorporated in the midgut epithelium
(Figure 3. c & d).
Figure 3. Cleavage (a & b) and formation of blastoderm (c & d)
Created by Dr. Md. Razzab Ali
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B.Sc. Ag. (Hons) Level 2 Semester II (Introductory Entomology – Embryonic development of Insect)
d. Formation of Germ Band
Initially Blastoderm forms a thin layer of cuboid cells subsequently they become columnar and
thicker in the ventral region. This thickening is called embryonic primordia or germ band (Figure 4)
which develops future embryo. The rest blastoderm remains as extra-embryonic membrane called
serosa.
Figure 4. Formation of germ band
e. Gastrulation
Gastrulation is the process by which the mesoderm and endoderm are invaginated within the
ectoderm. The germ band becomes differentiated into a median area called middle plates and two
lateral areas called lateral plates. The gastrulation stage begins when the mesoderm is formed from
the middle in one of the three waysi.
by an invagination of the middle plates (Figure 5.a),
ii.
by growing lateral plates over middle plate (Figure 5.b) or
iii.
by proliferation of cells from the inner surface (Figure 5.c).
Cells proliferation from each end of the mesoderm (derived from middle plate) and eventually grow
around the yolk. These represent the beginning of the endoderm (derived from vitellophages), and
they form the lining of what will be the future mid gut of the insect.
Figure 5. a. Gastrulation by an invagination of the middle plates
Created by Dr. Md. Razzab Ali
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B.Sc. Ag. (Hons) Level 2 Semester II (Introductory Entomology – Embryonic development of Insect)
Figure 5. b. Gastrulation by growing lateral plates over middle plate
Figure 5. c. Gastrulation by proliferation of cells from the inner surface
Created by Dr. Md. Razzab Ali
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B.Sc. Ag. (Hons) Level 2 Semester II (Introductory Entomology – Embryonic development of Insect)
f. Formation of the embryonic membrane
The germ band becomes covered by one or more embryonic membranes. Soon after formation of
germ band, the serosa from either side extend until both extensions meet and fuse in the ventral mid
line. Small cavity forms on the ventral surface of the germ band called amniotic cavity. The
amniotic cavity is bound by a membrane is called amnion (Figure 6).
Figure 6. Formation of embryonic membrane
g. Blastokinesis
The embryo begins to move within the yolk, undergoing rotation, revolutions and marked
displacement, the phenomenon is called blastokinesis (Figure 7). The movements taking place from
the posterior to anterior pole of egg are termed as the anatrepsis, whereas those from ventral to
dorsal surface of the egg are called katatrepsis.
Figure 7. Blastokinesis of future embryo
Created by Dr. Md. Razzab Ali
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B.Sc. Ag. (Hons) Level 2 Semester II (Introductory Entomology – Embryonic development of Insect)
h. Development of organ system
From the three germ layer- ectoderm, mesoderm, and endoderm- the various organs and tissues of
the insect develop.
i. The ectoderm gives rise to the body wall, tracheal system, nervous system, the malpighian
tubles, foregut and hindgut;
ii. The mesoderm gives rise to the muscular system, heart and gonads;
iii. The endoderm gives rise to midgut.
i. Appendages
Body segmentation starts in early embryonic development. It involves ectoderm and mesoderm, but
not endoderm. Appendages appear soon after segmentation (Figure 8).
i.
Transverse furrows and bilateral evaginations of ectoderm create various appendages.
ii.
In front of stomodaeum is the labrum, either side of the protocephalon are antennal
rudiments.
iii.
Protocorm becomes segmented, each segment laterally to form rudiment of appendages.
iv.
Behind the protocephalon are the rudiments of mandibles, maxillae, and labium.
v.
Appendages of next three segments form walking legs.
vi.
Abdominal appendages disappear except eight and nine which form ovipositor and
eleven form the cerci.
Figure 8. Different appendages of insect embryo
Created by Dr. Md. Razzab Ali
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