1. The final stages of ovarian development in the mosquito Aëdes aegypti appear to be under the control of a hormone which is produced in the head.
2. Decapitation of groups of mosquitoes before a critical period prevents final development of the ovaries; decapitation after the critical period allows development of the ovaries to maturity.
3. Transfusion of haemolymph from donor mosquitoes in the middle of the critical period to recipient mosquitoes decapitated at a stage just before, or very early in the critical period, results in a higher number developing their oocytes to maturity than when transfusion is between donors and recipients at the same stage.
4. Similarly, transfusion from known positive donors (i.e. those which by decapitation and subsequent dissection were known to have already produced hormone at the time of transfusion) results in a higher number of recipients developing oocytes to maturity, than when transfusion is from known negative donors.