Embryogenesis is a developmental process that usually begins once the egg has been fertilized. It involves multiplication of cells (by mitosis) and their subsequent growth, movement, and differentiation into all the tissues and organs of a living insect. The field of insect embryology has recently yielded stunning insights into the developmental processes of humans and other vertebrate organisms. There is remarkable similarity in genes responsible for organizing the fundamental body plan in vertebrates and invertebrates. For example, eyeless, a gene needed for development of an insect's compound eyes is also necessary for development of a mouse's vertebrate eyes!
Although much of insect embryology is still a mystery, there has been remarkable progress in knowledge over the past few years thanks to new methods in molecular biology and genetic engineering. Fruit flies, silkworms, and hornworms are proving to be a "rosetta stone" for embryology.
An insect's egg is much too large and full of yolk to simply divide in half like a human egg during its initial stages of development (imagine how much time and energy it would take just to build new cell membranes!). Birds have this same problem -- think of the yolk in a chicken's egg. Birds solve the problem by having the embryo develop within a tiny spot of cytoplasm (the blastodisc) on the surface of the yolk. Insects solve the problem by "cloning" the zygote nucleus (mitosis without cytokinesis) through 12-13 division cycles to yield about 5000 daughter nuclei. This process of nuclear division is known as superficial cleavage (in "true" cleavage entire cells divide). As they form, the cleavage nuclei (often called "energids") migrate through the yolk toward the perimeter of the egg. They settle in the band of periplasm where they engineer the construction of membranes to form individual cells. The end result of "cleavage" is the blastoderm -- a one-cell-thick layer of cells surrounding the yolk.
The first cleavage nuclei to reach the vicinity of the öosome are "reserved" for future reproductive purposes -- they do not travel to the periplasm and do not form any part of the blastoderm. Instead, they stop dividing and form germ cells that remain segregated thoughout much of embryogenesis. These cells will eventually migrate into the developing gonads (ovaries or testes) to become primary öocytes or spermatocytes. Only when the adult insect finally reaches sexual maturity will these cells begin dividing (by meiosis) to form gametes of the next generation (eggs or sperm). Germ cells never grow or divide during embryogenesis, so DNA for the next generation is "conserved" from the very beginning of development. This strategy has a clear selective advantage: it minimizes the risk that an error in replication (a genetic defect) will accidently be passed on to the next generation.
Blastoderm cells on one side of the egg begin to enlarge and multiply. This region, known as the germ band (or ventral plate), is where the embryo's body will develop. The rest of the cells in the blastoderm become part of a membrane (the serosa) that forms the yolk sac. Cells from the serosa grow around the germ band, enclosing the embryo in an amniotic membrane.
At this stage of development, when the embryo is not much more than a single layer of cells, a group of control genes (called homeotic selector genes) become active. These genes encode for proteins that contain a special active site (the homeobox) for binding with DNA. They interact with specific locations in the genome where they function as switches for activating (or inhibiting) the expression of other genes. Basically, each selector gene controls the expression of certain other genes within a restricted domain of cells based on their location in the germ band.
By regulating activity within a suite of genes that produce hormone-like "organizer" chemicals, cell-surface receptors, and structural elements, the selector genes guide the development of individual cells and channel them into different "career paths". This process, called differentiation, continues until the fundamental body plan is mapped out -- first into general regions along an anterio-posterior axis, then into individual segments, and finally into specialized structures or appendages.
|Developmental Fate of Insect Germ Layers
|Ectoderm:||Epidermis, exocrine glands, brain and nervous system, sense organs, foregut and hindgut, respiratory system, external genitalia.|
|Mesoderm:||Heart, blood, circulatory system, muscles, endocrine glands, fat body, gonads (ovaries and testes).|
As the germ band enlarges, it begins to lengthen and fold into a sausage shape with one layer of cells on the outside (the ectoderm) and another layer of cells on the inside (the mesoderm). An important developmental milestone, called dorsal closure, occurs when the lateral edges of the germ band meet and fuse along the dorsal midline of the embryo's body. Ectoderm cells grow and differentiate to form the epidermis, the brain and nervous system, and most of the insect's respiratory (tracheal) system. In addition, the ectoderm invaginates (folds inward) at the front and rear of the embryo's body to create front and rear portions of the digestive system (foregut and hindgut). Mesoderm cells differentiate to form other internal structures such as muscles, glands, heart, blood, fat body, and reproductive organs. The midgut develops from a third germ layer (the endoderm) that arises near the fore- and hindgut invaginations and eventually fuses with them to complete the alimentary canal.
During its early development, the embryo's body is rather worm-like in appearance. Individual segments first become visible near the anterior end (the protocephalon) where ectodermal tissue differentiates into the brain and compound eyes. Bud-like swellings develop in front of the mouth opening. They will eventually grow to form the labrum (front lip of mouthparts) and the antennae. Segments behind the mouth also develop bud-like swellings. Each of the first three post-oral segments form paired appendages that become mouthparts: mandibles, maxillae, and labium. The next three post-oral segments develop into the thorax -- they form appendages that become walking legs. Segments of the abdomen also develop limb buds but these soon shrink and disappear -- perhaps they are vestigal remnants of abdominal appendages found in more primitive arthropods (like millipedes and centipedes). Another pair of vestigal buds appears on the head, between the antennae and the mouthparts. This pair, called the intercalaries, may be remnants of a second pair of antennae (found in members of the class Crustacea).
In general, the rate of embryonic development depends on temperature (insects are poikilothermic) and on species-specific characteristics of development. Embryogenesis ends when the yolk's contents have been consumed: the immature insect is fully formed and ready to hatch from the egg. During the hatching process (often called eclosion) the young insect may chew its way through the egg's chorion or it may swell in size by imbibing air until the egg shell "cracks" along a predetermined line of weakness. Once the hatchling emerges, it is called a first instar nymph (or larva). As it grows, it will continue to develop and mature. These post-embryonic changes are known as morphogenesis.