Biology and Redescription of Pheromermis Pachysoma (V. Linstow) N. Gen., N. Comb. (Nematoda: Mermithidae), a Parasite of Yellowjackets (Hymenoptera: Vespidae)

Pheromermis pachysoma (von Linstow) n. gen., n. comb., a parasite of the yellowjacket, Vespula pensylvanica (Saussure), is described from California. The genus, Pheromermis, is characterized by the presence of four submedian cephalic papillae; large anteriorly placed cup-shaped amphids; an S-shaped vagina not bent in a transverse plane to the body; six hypodermal cords; paired, short, separate spicules; cuticle with cross fibers; and eggs lacking processes. The development of P. pachysoma is unique because a paratenic or transport host is required for completion of the life cycle. The adult nematodes occur in water or saturated soil and the eggs are fully embryonated at oviposition. The eggs hatch in the gut of various insects and infective stage juveniles penetrate the gut wall and enter a quiescent state in the tissues of these paratenic hosts. Wasp larvae are probably infected when they are fed paratenic hosts captured by worker yellowjackets. Postparasitic juveniles of P. pachysoma emerge from adult wasps when the latter visit wet sites after their fall emergence from the nest. The ant parasite, Mermis myrmecophila Baylis, is transferred to the genus Pheromermis.