Microscopic: A small but distinct cephalic vesicle is present . The spicules are long and slender with fused tips. In all except N. battus the male has two sets of parallel rays in each of the main bursal lobes; the female has a truncate tail with a small spine, and the egg is large, ovoid and colourless and twice the size of the typical trichostrongyle egg. IDENTIFICATION N. battus is characterised by having only one set of parallel rays in each bursal lobe while the female worm has a long pointed tail and the large egg is brownish with parallel sides. . The preparasitic phase is almost unique in the trichostrongyloids in that development to the L3 takes place within the egg shell. This development is generally very slow and in temperate climates takes at least two months. Once the L3 is present there is often a lag period before hatching occurs, the duration varying according to the species. LIFE CYCLE In the case of N. battus, the most important species