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heavily infected than expected on a random basis, and some (or most) are

little infected or not infected at all. Very often, aggregation can best be

described by a negative binomial distribution. For example, MacKenzie

and Liversidge (1975) found that the distribution of metacercariae

ofStephanostomum baccatumfrom a particular locality could almost

perfectly be described by a negative binomial distribution. Studies of

latitudinal gradients in aggregation have not been made.

Restriction to microhabitats within or on host individuals

All parasites prefer certain microhabitats to others, although the degree of

microhabitat restriction varies. Thus, larvae of the nematodeTrichinella

spiralisinfect the striated muscles throughout the body of many mammal

species, whereas adults of the same species are restricted to the small

intestine of the same hosts. Different species of Mallophaga and ticks live

on different parts of the body of particular bird species (e.g. Dubinin,

1948, 1951; review in Dogiel, 1962; Fig. 9.1). Reasons for the differential

distribution of ticks, according to Dubinin (1951), are the spatial position

and function of various groups of feathers, the macro- and microscopic

structure of the feathers of each group, and the marked differences in the

microstructure of feathers of different groups responsible for differences

in aerodynamic properties. Marshall (1981) has given many other

examples of site specificity in ectoparasitic insects. Thus, Mallophaga

and Anoplura of mammals show considerable site specificity, although

apparently less so than bird lice. The most important factors controlling

distribution appear to be hair type and temperature. In cool weather

the cattle anopluranHaematopinus eurysternusoccurs largely on the

neck, but in hot weather it is found around the ears, horns and tails.

The crab louse of humans,Phthirus pubis, is found largely in the pubic

and perianal regions of the body, but it has also been recovered from

other areas of coarse hair, e.g. the armpits, beard and eyelashes. Certain

trematode species live in the stomach, others in certain sections of the

small intestine and others again in the rectum of teleost fishes and other

hosts. The aspidogastreanRugogaster hydrolagiinfects the rectal glands

and the aspidogastrean Multicalyx elegansthe gall-bladder and bile-

ducts of chimaerid fishes, whereas the nematodesDirofilaria immitisand

Dirofilaria repensinfect the heart and associated blood-vessels, or the

skin of some mammals, respectively. Site restriction has been particularly

well investigated in monogeneans of fishes. For example, five species

of monogeneans are found on the gills of the mackerel Scomber

australasicus. One species is found only on the pseudobranchs, a second

species at the base of the filaments of the main gills, a third species in the

middle of the filaments, a fourth only on the most posterior (and probably

most anterior) filaments and a fifth species is scattered over all gill

filaments along their whole length (Fig. 9.2). This site restriction is

observed even in the absence of other species, i.e. the parasites are

Niche Restriction and Mate Finding in Vertebrate Hosts 173