Nematodes often exhibit sexual dimorphism, usually with the males being smaller than the females. An extreme example is found in species of the genus Tetrameres, which live in the glands of the proventriculus of birds. The males look like typical nematodes, long and thin, but the females expand as they become distended with eggs to these bright red, pea-sized individuals that barely resemble nematodes at all, dwarfing the males. As they expand, they can cause pressure necrosis in the tissues of the host.
Post and image contributed by Mike Kinsella.