Morphology

Lice are small (0.4—10 mm in the adult stage), wingless, dorso-ventrally flattened insects. The elongate abdomen possesses sclerotized dorsal, ventral, and/or lateral plates in many lice (Fig. 4.2); these provide some rigidity to the abdomen when it is distended by a blood meal or other food source. In adult lice the abdomen has 11 segments and terminates in genitalia and associated sclerotized plates. In females, the genitalia are accompanied by finger-like ¿jonopods, which serve to guide, manipulate, and glue eggs onto host hair or feathers. The abdomen is adorned with numerous setae in most lice. Immature lice closely resemble adults but are smaller, have fewer setae,

Louse Morphology

caecum . . ... esophagus modifications, members of the chewing louse genus Trochilocoetes (parasites of humming birds) have evolved mouthparts that can function as sucking organs.

The thorax in chewing lice usually appears as two, and occasionally three, segments. Chewing lice possess one or two simple claws on each leg; species that parasitize highly mobile hosts, especially birds, typically have two claws.

In sucking lice (Figs. 4.2 and 4.7) the head is slender and narrower than the thorax. Anoplura have three- to five-segmented antennae and lack maxillary palps. Eyes are reduced or absent in most sucking lice but are well developed in the medically important genera Pediculus and Pthirus (Fig, 4.7A, B), and ocular points, or eyeless projections posterior to the antennae, are characteristic of sucking lice in the genus Haematopinus (Fig. 4.7E).

As indicated by their name, anopluran mouthparts function as sucking devices during blood feeding caecum . . ... esophagus

Haematopinus Suis Organs

aedeagus

FIGURE 4,3 Internal abdominal anatomy of a male human body louse (Pediculus humanus humanus), (From Ferris, 1951.)

aedeagus

FIGURE 4,3 Internal abdominal anatomy of a male human body louse (Pediculus humanus humanus), (From Ferris, 1951.)

Menacanthus Stramineus With Labelling

Lice (Pbthimptem)

FIGURE 4.7 Sucking lice (Anoplura) of medical and veterinary importance, showing dorsal morphology (left) and ventral morphology (right) in each case. Not drawn to scale. (A) Human body louse (Pediculus humanus immanus), female; (B) Human crab louse (Phthirus pubis), female; (C) Flying squirrel louse (Neohaematopmus sciuropteri), male; (D) Spined rat louse (Polyplax spinu-losa), male; (E) Hog iouse (Haematopinus mis), female; (F) Little blue cattle louse (Solenaptes capillatus), male; (G) Dog sucking louse (Linognartms setosus), male; (H) Longnosed cattle louse (L. muii), female. (From Ferris, 1923-1935.)

labrum esophagus pharynx-/

/// salivary 9lanti 77 "salivary '/ duct ■stylet sac -stylets haustellar teeth

■maxillae —food canal Vhypopharynx J-salivary canal <— labium cibarial pump

7/ J jiy^ V^host skin surface - haustellum (everted) / "'haustellar (prestomal) teeth 0T—stylet bundle host blood capillary

FIGURE 4.8 Head region of a sucking louse (Anoplura) feeding on a host, showing components of mouthparts and associated internal structures. (Original by Margo Duncan.)

FIGURE 4,9 Cross-section through the mouthparts of a sucking louse (Anoplura). (Original by Margo Duncan.)

canis, and Heterodoxus spiniger. These lice also parasitize foxes, wolves, coyotes, and occasionally other carnivores, Similarly, the horse sucking louse (Haematopi-nus asini}, parasitizes horses, donkeys, asses, mules, and zebras, whereas L. africanus parasitizes both sheep and goats. At least six species of chewing lice are found on domestic fowl, all of them parasitizing chickens, but some also feeding on turkeys, guinea fowl, pea fowl, or pheasants (Table I). Lice found on atypical hosts are termed stragglers.

Some sucking lice, such as the three taxa that parasitize humans, the sheep foot louse, and the sheep face louse, are not only host specific, but also infest specific body areas, from which they can spread in severe infestations. Many chewing lice, particularly species that parasitize birds, also exhibit both host specificity and site specificity; examples include several species that are found on domestic fowl, and species confined to turkeys, geese, and ducks (Table I). Lice inhabiting different body regions on the same host typically have evolved morphological adaptations in response to specific attributes of the host site. These include characteristics such as morphological differences of the pelage, thickness of the skin, availability of blood vessels, and grooming or preening activities of the host. Site specificity in chewing iice is most prevalent in the more sedentary, specialized Ischnocera than in the mostly mobile, morphologically unspecial-ized Amblycera. For example, on many bird hosts, round-bodied ischnocerans with large heads and mandibles are predominately found on the head and neck. Elongate forms with narrow heads and small mandibles tend to inhabit the wing feathers, whereas morphologically intermediate forms occur on the back and other parts of the body.

Some chewing lice inhabit highly specialized host sites. These include members of the amblyceran genus Piagetiella, which are found inside the oral pouches of pelicans, and members of several amblyceran genera, including Actornithophilus and Colpocephalum, which live inside feather quills. Several bird species are parasitized by 5 or more different species of site-specific chewing lice, and up to 12 species may be found on the neotropical bird Crypturellus soui (a tinamou).

Site specificity is less well documented for sucking lice. However, domestic cattle may be parasitized by as many as five anopluran species, each predominating on particular parts of the body, Similarly, some Old World squirrels and rats can support up to six species of sucking lice.

Because of the importance of maintaining a permanent or close association with the host, lice have evolved specialized host-attachment mechanisms to resist grooming activities of the host, The robust tibio-tarsal claws of sucking lice (Fig. 4.10) are very important in securing them to their hosts. Various arrangements of hooks and spines, especially on the heads of lice that parasitize arboreal or flying hosts, such as squirrels and birds, also aid in host attachment. Mandibles are important attachment appendages in ischnoceran and rhyncophthirinan chewing lice. In some species of Bovicola, a notch in the first anten-nal segment encircles a host hair to facilitate attachment.

FIGURE 4.10 Tibio-tarsal claws and antenna of Linognathus africanis (Anoplura): scanning electron micrograph. (From Price and Graham, 1997.)

A few lice even possess ctenidia ("combs") that are con-vergently similar in morphology to those characteristic of many fleas. They occur most notably among lice that parasitize coarse-furred, arboreal, or flying hosts. Additionally, chewing lice that parasitize arboreal or flying hosts often have larger, more robust claws than do their counterparts that parasitize terrestrial hosts.

Because of their reliance on host availability, lice are subjected to special problems with respect to their long-term survival. All sucking lice are obligate blood-feeders; even a few hours away from the host can prove fatal to some species. Some chewing lice also are hematophages and similarly cannot survive prolonged periods off the host. However, many chewing lice, particularly those that subsist on feathers, fur, or other skin products, can survive for several days away from the host. For example, the cattle biting louse can survive for up to 11 days (this species will feed on host skin scrapings), and Menacunthusspp. of poultry can survive for up to 3 days off the host. Off-host survival is generally greater at low temperatures and high humidities. At 26°C and 65% relative humidity (RH), 4% of human body lice die within 24 hr, 20% within 40 hr, and 84% within 48 hr. At 75% RH, a small proportion of sheep foot lice survives for 17 days at 2°C, whereas most die within 7 days at 22° C. Recently fed lice generally survive longer than unfed lice away from the host. Although most lice are morphologically adapted for host attachment and are disadvantaged when dislodged, the generalist nature of some amblyceran chewing lice better equips them for locating another host by crawling across the substrate. Amblycerans are more likely than other lice to be encountered away from the host, accounting for observations of these lice on bird eggs or in unoccupied nests and roosts.

Host grooming is an important cause of louse mortality. Laboratory mice infested by the mouse louse, for example, usually limit their louse populations to 10 or fewer individuals per mouse by regular grooming. Prevention of self-grooming or mutual grooming by impaired preening action of the teeth or limbs of such mice can result in heavy infestations of more than 100 lice. Similarly, impaired preening due to beak injuries in birds can result in tremendous increases of louse populations. Biting, scratching, and licking also reduce louse populations on several domestic animals.

Whereas most species of lice on small and medium-sized mammals exhibit only minor seasonal differences in population levels, some lice associated with larger animals show clear seasonal trends. Some of these population changes have been attributed to host molting, fur density and length, hormone levels in the blood meal, or climatological factors such as intense summer heat, sunlight, or desiccation. On domestic ungulates in temperate regions, louse populations typically peak during the winter or early spring and decline during the summer. An exception to this trend is the cattle tail louse, whose populations peak during the summer.

Another important aspect of louse behavior is the mode of transfer between hosts. Direct host contact appears to be the primary mechanism for louse exchange. Transfer of lice from an infested mother to her offspring during suckling (in mammals) or during nest sharing (in birds and mammals) is an important mode of transfer. Several species of lice that parasitize livestock transfer during suckling, including the sheep face louse and the sheep biting louse, both of which move from infested ewes to their lambs at this time. Lice can also transfer during other forms of physical contact between hosts, such as mating or fighting. Transfer of lice between hosts also can occur between hosts that are not in contact. The sheep foot louse, for example, can survive for several days off the host and reach a new host by crawling across pasture land. Nests of birds and mammals can act as foci for louse transfer, but these are infrequent sites of transfer.

Dispersal of some lice occurs via phoresy, in which they temporarily attach to other arthropods and are carried from one host to another (Fig. 4.11). During phoresy, most lice attach to larger, more mobile blood-feeding arthropods, usually a fly, such as a hippoboscid or muscoid. Phoresy is particularly common among ischno-ceran chewing lice. Movement of the mouthparts in a horizontal plane better facilitates their attachment to a fly than in the amblycerans, in which mouthparts move in a vertical plane. Phoresy is rare among sucldng lice. This is

FIGURE 4.11 Two ischnoceran chewing lice (Mallophaga) phoretic on a hippoboscid fly, attached by their mandibles to the posterior abdomen. (From Rothschild and Clay, 1952).

probably because attachment to the fly is achieved by the less efficient mechanism of grasping with the tarsal claws.

Mating in lice occurs on the host. It is initiated by the male pushing his body beneath that of the female and curling the tip of his abdomen upward. In the human body louse, the male and female assume a vertical orientation along a hair shaft, with the female supporting the weight of the male as he grasps her with his anterior claws. Most lice appear to exhibit similar orientation behavior during mating. Notable exceptions include the crab louse of humans, in which both sexes continue to clasp with their claws a host hair, rather than each other, during mating; and the hog louse, in which the male strokes the head of the female during copulation. Some male ischnoceran chewing lice possess modified hooklike antennal segments, with which they grasp the female during copulation.

Oviposition behavior by female lice involves crawling to the base of a host hair or feather and cementing one egg at a time close to the skin surface. Two pairs of fingerlike ¿onopods direct the egg into a precise location and orientation as a cement substance is secreted around the egg and hair base. Optimal temperature requirements for developing louse embryos inside eggs are very narrow, usually within a fraction of a degree, such as may occur on a precise area on the host body. For this reason, female lice typically oviposit preferentially on an area of the host that meets these requirements.

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