Imaginal discs are very fascinating structures observed in larva of holometabolous insects and can be defined as Clusters of undifferentiated embryonic cells in holometabolous insects that proliferate during larval stages and then differentiate during the pupal stage upon induction by ecdysone but in the absence of juvenile hormone. In one of the recent post dealing with classification of insects , it was mentioned that holometabolous insects are the ones in which the final instar larvae dramatically changes (metamorphoses) into an adult fly which bears no resemblance to worm like appearance of larvae , during a quite pupal phase.
During the event of metamorphoses, organisms need to develop new tissues which were not present during larval phase. Unlike metamorphoses observed in Amphibians where remodeling of existing tissues is observed, insect metamorphosis often involves the destruction of larval tissues by apoptosis and their subsequent replacement by an entirely different population of cells.
There are two different population of cells exists in larvae of an insect : 1) the cells which are used during larval phase of insects ,we can call them larval cells and 2) Group of imaginal cells which are set aside during larval phase and wait for the signal to differentiate into adult structures. Imaginal discs don’t contribute significantly to the larval life but later after metamorphosis contributes to a part or complete adult appendage.
Histoblasts or imaginal cells contribute to the entire structure of adult insect. The precursor cells of the abdomen and the internal organs of the adult, such as the gut, salivary glands and brain, arise from nests or rings of cells intimately associated with larval structures. eg: the salivary gland imaginal rings are embedded in the larval salivary glands. Each segment of the adult abdomen is formed from four pairs of small histoblast nests. Additionally, histoblasts arranged in distinct structures ( imaginal discs) take care of all adult head structures, appendages and genetilia.
Like in many other aspects, majority of information regarding Imaginal discs has come from the study in Drosophila .There are ten pairs of imaginal discs, which construct many of the adult organs, and an unpaired genital disc , making total 21 imaginal discs (if you count eye and antennal discs as separate, which is fused imaginal disc or else the count will be 19 ) in Drosophila larvae. There are pair of imaginal discs of labial, Humeral, Clypeolabral, eye, antenna ,wing , haltere, legs (six in total, one for each leg) and one unpaired genital disc.
1, clypeolabral; 2, eye-antennal; 3, labial; 4, humeral (or prothoracic); 5, first leg; 6, second leg; 7, third leg; 8, wing; 9, haltere; 10, genital. Note that some portions of the head and thorax, including the notum, also originate from imaginal discs.
In D. melanogaster the imaginal disc primordia are formed during embryonic development, rather than during the last instar as they are in some other Holometabola. Each imaginal disc primordium contains 10 to 40 cells, which divide during the three instars to form as many as 60,000 cells by late third instar.
There is an excellent book available on imaginal discs by Lewis held explaining in minute details: how a larva becomes a fully-functioning fly? The book delves into bristle pattern formation and disc development, with entire chapters devoted to the leg, wing, and eye. Extensive appendices include a glossary of protein domains, catalogs of well-studied genes, and an outline of signaling pathways.
Reference and image credit :
Beira, J.V., Paro, R. The legacy of Drosophila imaginal discs. Chromosoma 125, 573–592 (2016). https://doi.org/10.1007/s00412-016-0595-4 — http://creativecommons.org/licenses/by/4.0/
Image modified from – Beira, J.V., Paro, R. The legacy of Drosophila imaginal discs. Chromosoma 125, 573–592 (2016) and free to use under http://creativecommons.org/licenses/by/4.0/