b.sc. ag. (hons) level 2 semester ii (introductory entomology – … - development and... · b.sc....

B.Sc. Ag. (Hons) Level 2 Semester II (Introductory Entomology – Embryonic development of Insect)

Created by Dr. Md. Razzab Ali of 7

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 parts-

a. 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



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 includes-

a. 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

Figure 2. Different shapes of insect eggs



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)



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. 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 ways-

i. 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 4. Formation of germ band

Figure 5. a. Gastrulation by an invagination of the middle plates



Figure 5. b. Gastrulation by growing lateral plates over middle plate

Figure 5. c. Gastrulation by proliferation of cells from the inner surface



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).

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 6. Formation of embryonic membrane

Figure 7. Blastokinesis of future embryo



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

b.sc. ag. (hons) level 2 semester ii (introductory entomology – … - development and... · b.sc....

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