The maggot, or larva, of the fruitfly is made from a fertilised egg. About twenty-four hours after the mother fly lays an egg, a larva emerges. This little animal, about half a millimetre long, feeds voraciously on rotting fruit, changes its skin twice in a five-day period after it hatches and then begins a dramatic transformation. The well-fed maggot stops crawling and eating; and over another five days metamorphoses to emerge as a beautiful adult fly.
 

What we See --- A Segmented Animal

    The maggot, to even the untrained observer staring at it closely with a magnifying glass or a simple microscope, appears to be intricately designed.

    It has a front and a rear-end, and the bulk of its body seems to be made of repetitive units or segments. The segmentation in the head is not easy to see --- many segments are compressed together --- but the partitioning of the animal is easier seen in the thorax (the T1-3 segments) and in the abdomen (A1-8). If you were to start to design a maggot, it may be natural to assume that the segment is the most convenient unit of construction. You could divide the length of an un-patterned animal into the correct number of segments and then give the segments identities according to their ordering from front to rear.

    The maggot is indeed made in by creating repetitive units, but these units are not the segments so easily seen by us. The units that the maggot uses to develop is called a parasegment. This unit comprises parts of two adjacent segments. Thus parasegment 5 is made up of the rear of segment T2 and the front of segment T3. We know that parasegments are key units in making the maggot because of the pattern of labels that give cells properties. These labels mark domains that are the size of a segment, but are out of register with segments, in the manner seen in the figure above.
 

Numbering the Parts

    The parts that make up an animal, the segments that we see, or the parasegments biologists suggest to be important functionally, have other features.  Each unit, knows its position along the front-rear axis of the animal. Each parasegment knows its identity because of the effect of inheritable labels. These labels overlap partially in their domains. Each label has a defined “beginning” towards the front-end of the animal and its presence extends all the way to the rear.

Labels Act Together to Number a Part

    All the labels present in a parasegment act together to give it an identity. It appears that labels that “begin” their expression  towards the front-end are “repressed” from acting by labels that begin their expression further rear-wards. Said another way, “longer” labels as seen in the figure above are rendered less effective by “shorter” labels in regions where their domains overlap. 

    This can be visualised by simply overlaying different labels. Within a set of parts defined by the dominance of one label, such as parasegments 5 and 6, the specific identity of each parasegment is defined by the quantity of, and interactions between, the labels.
 

Parts Are Made in Two Kinds --- Odd and Even-Numbered

    The fourteen parasegments along the length of a developing maggot are defined by the presence of alternating stripes of two classes of labels. Early during the development of the animal, seven stripes of one kind of a label, that define the odd-numbered parasegments and seven that define even-numbered parasegments are seen.

    Parasegments are defined in this manner, before they acquire labels that give each of them a specific identity. The two sets of seven parasegments are formed by a sequence of events that begins in the mother. We shall see later how this is done.
 

Each Part is Patterned into Front and Rear

    The odd and even-numbered labels act to generate new labels within each parasegment. The front of each parasegment, a region of about a quarter of this domain, has one label. The remaining three-quarter of the segment has another label.

End of the Making Maggots page