Interaction between the leg and surrounding thorax in the beetle

Interactions between the insect leg and surrounding thoracic epidermis were studied in the beetle, Tenebrio, by grafting the entire larval prothoracic leg into the metathoracic leg site in various orientations. Control grafts simply heal, but A/P reversed grafts lead to regeneration of supernumerary legs of host orientation in A and P positions. M/L reversed grafts also give supernumeraries, again with host orientation but in M (or MP) and L (or LA) positions. The differences in structure between adult prothoracic and metathoracic legs allow the origin of these supernumeraries to be analysed at coxa and tarsus level. The A/P supernumeraries are consistent and complementary in structure, of host origin on the host side and graft origin on the graft side, and with the borders in apparently constant midmedial and midlateral positions. The M/L supernumeraries, however, are variable and often non-complementary in structure. The results of the A/P and M/L reversals are similar to those found at a more distal level in Tenebrio legs and the legs of several hemimetabolous insects, suggesting that the arrangement of positional values and A and P compartments extends from the epidermis of the leg onto the surrounding thorax. The results of a 180 degrees rotation of the entire leg, however, differ from those found at a more distal level in that the grafted leg rarely derotated and two (or occasionally one or three) supernumeraries are formed in a wide variety of positions, some with constant and others with variable orientation. These results are not readily explained by current models of insect leg formation and regeneration.