Removal of the sheath of the ensheathed infective juvenile of Trichostrongylus colubriformis prevents capture by the nematophagous fungus Arthrobotrys oligospora. Exposure of the trap hyphae to a variety of saccharides, which may block a recognition system based on lectin/carbohydrate binding, failed to prevent capture but some saccharides did inhibit penetration and invasion by the fungus. Capture and penetration thus appear to be two distinct processes with capture being less specific than penetration. Carbohydrate residues were absent from the outer surface of the cuticle and the sheath but were present on the inner surface of the sheath. The limited accessibility of these lectin-binding sites may explain the slow process of infection of the infective juvenile by the fungus. The sheath does not protect the infective juvenile against attack by this nematophagous fungus.

The nematode-trapping fungus Arthrobotrys oligospora traps and invades all the free-living juvenile stages of the tri-chostrongyle nematode Trichostrongylus colubriformis. The processes of capture and invasion of the 1st-stage juveniles are described using differential interference contrast optics and 3-D reconstruction techniques. The adhesive responsible for capture is well preserved using a freeze-substitution technique for scanning electron microscopy. The invasion process of the ensheathed 3rd-stage juvenile of T. colubriformis takes much longer than in the 1st- or 2nd-stage juvenile and involves the formation of secondary infection pegs between the sheath and the cuticle which appear to penetrate the cuticle by physical pressure.