The timing of release of prothoracicotropic hormone in relation to the head critical period during larval-adult development in Rhodnius prolixus (Hemiptera): Evidence from head transplantation experiments

Abstract

This paper examines the timing of release of prothoracicotropic activity from heads of various stages of last larval instar Rhodnius prolixus by their ability to stimulate the haemolymph ecdysteroid titre of a host body (decapitated on day 5 after a blood meal) onto which they were transplanted. The ecdysteroid titre of normal animals at day 5 is constant, but in decapitated controls it begins a progressive decline due to the injury, not to removal of the source of prothoracicotropic activity. Stimulation by transplanted heads is therefore measured in relation to this declining titre of decapitated controls. Activity was found to emanate from heads of female larvae on days 4 and 5 after a blood meal and from male heads on days 5 and 6. Heads of both younger and older developmental stages had no effect on the host titre. It is concluded that prothoracicotropic activity is released from the head for a period of about 48 h in both sexes, but this period occurs one day earlier in females than in males. This period of release encompasses the classical head critical period, which also occurs one day earlier in females than in males. However, activity is detectable from heads about one day prior to the beginning of the head critical period. It is argued that release of prothoracicotropic activity prior to the head critical period occurs in the most rapidly developing animals in the population and ceases in these animals at the beginning of the head critical period. In the most slowly developing animals, release ceases at the end of the head critical period. It is concluded that the head critical period for the population represents the range of times over which release of prothoracicotropic activity ceases in the population. The significance of this interpretation is discussed in relation to the classical literature and work with other insects.