Differentiation capacities of the labial imaginal disc ofDrosophila melanogaster.

The mature labial disc, when implanted into a larva of the same age, undergoes metamorphosis along with the host and produces one lateral half of the medi- and distiproboscis. On the basis of results obtained from transplanted disc halves (including the separate peripodial membrane) a tentative fate map of the labial disc was constructed, which shows most of the presumptive mediproboscis to be located in the dorsal, and most of the presumptive distiproboscis in the ventral part of the disc. The distal protion of the peripodial membrane also contains imaginal anlagen, viz. part of the mediproboscis, prementum, and labellar cap anlagen. The involvement of this part of the peripodial membrane was checked by a careful histological analysis of labial disc development during the first ten hours after prepupation. The results were compared with the situation described forCalliphora imaginal discs.In addition, a detailed morphological analysis was made of the proboscis of the homoeotic mutantproboscipedia (pb). At 27°C,pb changes the distiproboscis into a "telopodite" (leg segments distal to the coxa); the (unchanged) prementum may therefore correspond to the coxa. At 15° C, the tarsus of this homoeotic "telopodite" is replaced to a greater or lesser extent by an arista. The present analysis thus confirms (a) the fundamental morphological correspondence of the medi- and distiproboscis with the labium of other insects, and (b) the fundamental developmental correspondence of the labial, antennal, and leg discs.