In D. tryoni three larval instars occur, distinguishable by their mouth hooks and anterior and posterior spiracles. In the first two instars, the imaginal discs established during embryonic development grow slowly, only the cephalic discs showing marked change. Paired labial and lateral genital discs invaginate during the second half of the 2nd instar. During the 3rd instar, the imaginal discs increase further in size, first rapidly, then more slowly. Each shows attenuation of the lateral wall as a peripodial membrane, thickening and folding of the median wall without histodifferentiation and, in the wing, haltere, leg, and genital discs, formation of mesoderm from cells in the outer part of the median wall. Disc morphogenesis is largely completed by 120 hr after hatching. A single pair of lymph glands and 13-15 pairs of pericardial cells lie along the heart. Both show cell enlargement during larval life but no proliferation. Haemocytes in the blood arise directly from embryonic mesoderm as two main types, numerous spheroids and less numerous hexagons. They circulate freely in the living larva, but tend to occur in irregular clumps in fixed material, especially after 120 hr. No epigenetic relationship between haemocytes, imaginal discs, and lymph glands such as that described for Drosophila melanogaster by el-Shatoury and Waddington can be discerned in D. tryoni. The haemocytes do not arise from the lymph glands. The disc mesoderm does not arise from the haemocytes. The larval development of the discs is largely completed before the haemocytes become numerous.