Functional morphology of the thysanopteran pretarsus

Abstract

The functional morphology of the unique pretarsal mechanism of adult Thysanoptera is described. The components of each pretarsus are homologous with those of other insects, but changes in their relative sizes and shapes have arisen through modifications in their function. Among these are the enlargement of the arolium into a protrusible bladder; the reduction of the ungues into spoon-shaped, laterally ensheathing plates; the development of a tibial gland opening to the surface of the arolium through three fine ducts; and the evolution of two elastic "restraining" tendons each inserting into the "head" of the unguitractor apodeme and originating laterally on the walls of the distal end of the tibia.The contraction of the pretarsal depressor muscle causes the ungues to rotate outward on the unguifer and blood pressure, increased by abdominal contraction, everts the arolium. When the depressor muscle relaxes, the restraining tendons contract and the arolium is withdrawn between the closing ungues by sclerites and filaments associated with each. The adults of species in all five families of the order have this mechanism.