Correlations between integument structure and ecdysteroid titers in fifth-stage nymphs of the tick, Ornithodoros moubata (Murray, 1877; sensu Walton, 1962)

The integument structure of fifth-stage nymphs of the tick Ornithodoros moubata was studied during the molting processes which are induced by the blood meal. In parallel, the ecdysteroid titers in hemolymph and in extracts from whole animals were determined by a radioimmunoassay (RIA). Between Days 2 and 3 after the meal (a.m.) a few procuticle lamellae were deposited and the mitotic period initiated. Ecdysteroid titers were low during the first 3 days a.m. They began to increase between Days 3 and 4 a.m. with the mitotic period ending. The titers rose sharply when apolysis and formation of the exuvial space took place (Day 4–5 a.m.). Highest ecdysteroid concentrations were measured during the deposition of the new epicuticle (Day 5–6 a.m.). Concomitant with the beginning of procuticle deposition and digestion of the old cuticle (Day 6 a.m.) the titers began to drop and reached low values again shortly before ecdysis (Day 9–10 a.m.). A preliminary chemical identification by thin-layer and high-performance liquid chromatography revealed that the majority of the RIA-positive ecdysteroids were composed of ecdysone and mainly ecdysterone. This was confirmed by gas chromatography of the silylated ecdysteroids. The results demonstrate that the titer changes in relation to the molting events in both ticks and insects appear to be very similar. It is thus likely that ecdysteroids are important in the control of tick molting.