Inhibition of Pupariation in Sarcophaga bullata

Abstract

THE classical test used as the bioassay for the insect moulting hormone, ecdysone, is based on the failure of the posterior part of ligatured larvae of Calliphora erythrocephala (or other fly species) to tan unless an adequate dose of ecdysone is administered. It is crucial for the success of the test to select larvae at the right stage of development, and as closely as possible to the critical time for pupariation¹. Larvae ligatured before the critical stage of natural ecdysone release may eventually pupariate in the front part where the hormone originates, but will never do so in the hind part. Post-critically ligatured larvae have had sufficient ecdysone released into the haemolymph previous to ligature to induce tanning in the hind parts. It is evident that at this time the anterior parts must also be post-critical. In larvae ligatured at this stage the anterior parts usually contract to the puparium and tan whereas the posterior parts merely tan without change in shape. The occurrence of tanning in the posterior part alone is therefore a paradox. Fraenkel² along with the description of the “Calliphora assay” also reported that ligatured larvae occasionally tanned only in the posterior region. Several investigators have been subsequently surprised by this phenomenon of “anterior inhibition”, but there has been no agreement as to its cause.