that the inhibition process in O. ostertagi depends on the weather conditions to which the infective L3 larvae are exposed on pasture, and the length of time that L3 larvae spend on pasture. In temperate regions of the northern hemisphere, hypobiosis occurs in autumn and early winter, but in the southern hemisphere there is evidence that this phenomenon occurs in spring and early summer. Morphology O. ostertagi adults are slender reddish-brown worms. Adult males are long, adult females long, and the eggs long. Identification of adult medium stomach worms is based on the structure of the bursa, genital cone, and spicules in males and on the dimensions of the oesophageal valve and the configuration of the synlophe in males and females. The cuticle in the anterior region is striated transversely whereas the rest of the body is unstriated and bears around 36 longitudinal ridges. The Ostertaginae are characterized by a reduced buccal capsule and square head, and well-developed copulatory bursa in the male. The genital cone ventrally bears two small slender papillae and dorsally bears two long slender papillae surrounded by an accessory bursal membrane. The lateral rays of the copulatory bursa are in a pattern of 2-1-2 or 2-2-1 pattern. The key to morphologically identifying O. ostertagi males is a prominent proconus and the remainder of the genital cone is not prominent. The spicules are of equal length and shape, tapering towards the distal end. The key to morphologically identifying O. ostertagi females is the lateral synlophe with one pair of ridges ending next to the lateral ridge between cervical papilla and the posterior end of the oesophagus. The cervical papillae are prominent and thorn-like. A large number of surface cuticular ridges (synlophe) are perpendicular to the surface. The infective L3 larvae of O. ostertagi can also be identified morphologically. The L3 will have an external sheath. The larva head is rounded and can be distinguished from Cooperia spp. as it lacks the two refractive bodies in the head that is seen in this species. The tail of the larvae within the sheath is rounded and the body contains 16 intestinal cells. The external sheath of the O. ostertagi larvae also has a much blunter tip then that of Copperia spp. or Haemonchus placei. O. ostertagi can also be distinguished from the L3 larvae of Trichostrongylus spp. as it has a much longer sheath tail extension. Molecular biology The