1. The cross-striation in the indirect flight myofibrils of Calliphora has been studied by phase contrast and polarised light microscopy. The band pattern at rest-length has been determined in flies killed in osmium tetroxide vapour while their wings remained in the resting position. All other observations have been made on unfixed fibrils. Although length changes in situ are probably very slight (about 2 per cent), isolated fibrils, by treatment with crude muscle extract or with ATP, can be induced to elongate to 104 per cent rest-length, or to shorten by 8 per cent but no more. Over the range 98 to 104 per cent rest-length, experimentally induced length changes are reversible. The fibrils can also be stretched beyond 104 per cent rest-length, but the process is irreversible. During the course of glycerol extraction the fibrils elongate to 104 per cent rest-length.
2. The changes in band pattern observed over the range 104 to 92 per cent rest-length are qualitatively the same as the changes observed over a wider range (about 130 to 40 per cent rest-length) in the skeletal myofibrils of rabbits. The earlier stages of shortening appear to be effected by retraction of the I bands into the A bands where they fill up the H zones. No evidence has been found that any changes in band pattern are due to a migration of the A substance.
3. Two components of the sarcomere can be extracted from it and a third component remains behind. These three components, which have also been demonstrated in skeletal myofibrils of the rabbit, where they behave in the same way, are: (a) the A substance which does not change its position as the fibril changes its length, and which can be extracted by the same procedures as remove myosin (shown elsewhere to be the A substance) from rabbit fibrils; (b) a material which extends from the Z lines to the borders of the H zone and which moves inwards during contraction and outwards during elongation; it can capture rabbit myosin from solution and form with it a contractile system, and it is thought to be actin; (c) a "backbone" or stroma bearing Z and M lines.
4. Since all these features of the cross-striation are the same in the insect fibrils as in rabbit fibrils, it is considered very probable that the sarcomere is similarly organised in both types of muscle and contracts by essentially the same mechanism.