External body surfaces of hosts challenge parasites with a variety of physical conditions. Hair, feathers, gills and scales present textured surfaces, sometimes hard, sometimes soft. The parasites are in an exposed location that exposes them to grooming or preening by the host, to abrasion, or to strong water currents. Each species of ectoparasite has put together a package of features that enable it to attach to its host yet undertake other activities needed for its survival. Each package is different. Some of them are highlighted below.
The copepod Phrixocephalus attaches to the skin of its host, in this case the eye, using an embedded bulla. This is a permanent attachment, so the parasite must choose its initial attachment site carefully. Another feature that maintains attachment is the streamlined body shape of the parasite.
Argulus uses suckers modified from its mandibles to attach to the skin of its fish host. It frequently leaves the host, and the suckers are released easily.
Ergasilus uses claws modified from its antennae to attach to the gills of its fish host.
The monogenean Tetraonchus attaches to gills. It has a small body size that allows it to rest close to the gill lamellae and avoid strong water currents. It attaches using its large opisthaptor, which contains anchors and marginal hooklets that can grip the gills effectively. Tetraonchus needs to be able to move over the gills as it feeds, so its opisthaptor does not attach permanently. It can move it, using its prohaptor and mouth to attach while repositioning the opisthaptor,
Fleas have a laterally-compressed body that allows them to move easily between the hairs of their host, yet resist dislodgment by grooming. The body and legs of the flea are covered with setae, directed posteriorly, that do not hinder its movement but resist grooming. There are also small claws on the ends of the legs to grip the host.
Ticks have flattened bodies that allow themselves to rest close to the host's skin, reducing the chances that they can be groomed effectively. Claws on the legs provide attachment while moving, and the hypostome anchors the tick securely while feeding. The engorged females become rounded and large, but even if groomed by the host they are not seriously affected because they are ready to leave the host to deposit their eggs.
