1. At the imaginal ecdysis of Calliphora erythrocephala (Meigen) the initiation of normal hardening and darkening is brought about by the release into the blood of an active factor (Cottrell, 1962b).
2. The darkening factor can be detected in fly blood by injecting it into flies decapitated at the moment of emergence. Blood taken from flies at the moment of emergence or 24 hr. after expansion produces little or no reaction, although blood taken between 3 min. and 10 hr. after emergence shows darkening activity. However, extracts of other tissues and many chemicals will also induce darkening though probably unspecifically through damage effects. The assay can therefore be used certainly only for detecting activity in fly blood.
3. The blood-borne darkening factor of blowflies will withstand boiling for 10 min. or drying at 120° C. for 20 min., but it does not retain its activity when kept in solution at room temperature for more than 24 hr. It is non-dialysable, relatively insoluble in organic solvents and is inactivated by ethyl alcohol and the bacterial protease subtilisin. It is probably proteinaceous and is certainly not tyrosine or any of the phenolic compounds at present thought to act as the precursors of the tanning agent.
4. The blood-borne factor of different blowflies (Calliphora, Sarcophaga and Lucilia) is not species-specific and there is some inconclusive evidence that a similar factor is present at the ecdysis of Schistocerca.
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