Effects of decapitation at the head critical period for moulting on haemolymph ecdysteroid titres in final-instar male and female Rhodnius prolixus (Hemiptera)

Decapitation experiments performed on large populations of carefully staged last-instar Rhodnius prolixus indicate that there is a 24 h sex difference in the time of the head critical period. Males pass the head critical period over a 36 h period beginning at 1200 A.Z.T. (Arbitrary Zeitgeber Time) day 6 after feeding, while females pass the head critical period over a 36 h period beginning at 1200 A.Z.T. day 5 after feeding. Radioimmunoassay determinations of haemolymph ecdysteroid titres show that there is also a 24 h sex difference in the time of the major increase in ecdysteroid titre that commences at the head critical period. These results imply that there is a sex difference in the time at which the release of prothoracicotropic hormone (PTTH) associated with the head critical period occurs. Decapitation prior to the head critical period results in a rapid, permanent decrease in ecdysteroid titre, while decapitation after the head critical period does not prevent normal increases in ecdysteroid levels. We infer that the presence of the head is essential until just prior to the head critical period and its associated increase in at the head critical period also indicates that the head critical period represents a change in the activity of the prothoracic glands indicative of a response to a significant release of PTTH at this time. The significance of these results is discussed in relation to both classical and recent evidence concerning the timing of PTTH release and changes in ecdysteroid titres in Rhodnius prolixus.