The Imaginal Ecdysis of Blowflies. Observations On the Hydrostatic Mechanisms Involved in Digging and Expansion

1. The digging movements and the expansion process of newly emerged blowflies involve the production of positive internal pressures. 2. During digging newly emerged blowflies produce a characteristic cycle of internal hydrostatic pressure changes which in Sarcophaga reach, at their maximum, 6-12 cm. of mercury. 3. During expansion, two different pressure phenomena are detectable. First, there is a gradual rise and fall in the basic haemolymph pressure which reaches a maximum (of 6 cm. of mercury in Calhphora and 9-5 cm. in Sarcophaga) a few minutes after full wing extension and then falls to atmospheric pressure in the next 20 min. Secondly, superimposed on the basic rise there is a series of brief rhythmic pressure pulses which gradually decline and then cease about the time of full wing extension. 4. Evidence obtained by blocking the proboscis or denervating the abdominal muscles of newly emerged flies indicates that the gradual rise in haemolymph pressure s attributable to air-swallowing and the pressure pulses to the performance of ‘muscular efforts’ (i.e. simultaneous contractions of both the ptilinal and abdominal muscles). 5. The wing expansion of flies which have had their abdominal muscles denervated is abnormal, suggesting that at least some of these muscles play a part in bringing about normal expansion. 6. After full wing extension has occurred (or after an operated fly has reached a point in the air-pumping cycle at which expansion would have occurred) ‘muscular efforts’ cease and digging movements can no longer be stimulated. 7. This cessation is not due to the hardening of the cuticle and it is suggested that it is due to loss of the ability to excite the muscles concerned. 8. Within a few days of emergence many (perhaps all) of the muscles concerned in the production of digging movements ‘muscular efforts’ degenerate. 9. Immediately after expansion flies eliminate large quantities of fluid via the anus. This appears to be correlated with the change from a combined hydrostatic and external skeleton to a hard exoskeleton.