PERITROPHIC ENVELOPE PERMEABILITY IN HERBIVOROUS INSECTS

A sensitive method was developed for measuring the size and flux of marker substances that permeate the peritrophic envelopes (PEs) of insects. Seven species of herbivorous insects were fed polydisperse fluorescein isothiocyanate-labeled dextrans (FITC-dextrans), and the average size and amount of FITC-dextrans that diffused through their PEs were measured using high-performance size-exclusion chromatography. The diameters of FITC-dextrans that diffused through the PEs of four species of larval Lepidoptera and three species of Orthoptera ranged from 21 to 29 and from 24 to 36 nm, respectively. The flux of FITC-dextrans through the PEs of Lepidoptera and Orthoptera ranged from 32 to 159 and from 91 to 2821 ng/h, respectively. We conclude that: (1) on average, the PEs of Orthoptera are more permeable than those of Lepidoptera; (2) that pores in the peritrophic membranes of Lepidoptera and Orthoptera are too large to allow the PE to function as an effective size-dependent ultrafilter for digestive enzymes and free plant allelochemicals; and (3) that the permeability of the PE to some digestive enzymes and ingested plant allelochemicals is determined by the chemical properties of the matrix.