REEERENCES
CHAPTER
1
THE ANOPLURA OR SUCKING LICE
The word "louse" has been applied to a great variety of
insects, and indeed to other small animals, not closely related to one another
but similar in being small and wingless. Even now, though the word is used in
a much restricted sense, it is applied to two different groups of insects. The
first of these are called the Biting Lice or Feather Lice (Mallophaga): these
insects live on the skin of birds or mammals and have mouth parts fitted for
biting solid substances. They feed on pieces of feather and fragments of
scurf, and some of them also nibble the skin and take blood which exudes. We
are not further concerned with them in this book. It is the so-called Sucking
Lice (Anoplura) which are our subject. The Anoplura are a comprehensive group,
in zoological language an Order. In technical terms the Anoplura might be
defined as follows:
Small, wingless insects flattened dorsiventrally. Antennae
- short, 3-5 jointed. Eyes reduced or absent, and ocelli absent. Mouth parts
highly modified for piercing and sucking blood of host; retracted within head
when not in use. Palps absent. Thoracic segments fused together, without the
least rudiment of wing. Legs short, tarsi with one joint and one claw: leg
adapted for clinging to hair of host. Abdomen without cerci.
Without metamorphosis, occurring on surface of host throughout
life. Parasites of mammals exclusively.
The sucking lice or Anoplura stand rather by themselves, not
closely related to other orders of insects, except perhaps to the Mallophaga
(the biting lice, or feather lice), already mentioned. The view was once held
that the sucking lice were related to the bugs (Rhynchota), but this is known
to be erroneous.
The orders are in turn divided into "families", of which four
are generally recognised in the Anoplura. Only one of these families, the
Pediculidae, is of interest to ourselves, for it includes the lice which occur
on man and monkeys: all members of this family possess eyes, a character which
distinguishes them from all other Anoplura.
To carry the matter further, the lice which occur on human
beings, with which alone this work is concerned, are classified into two
genera (Pediculus and Phthirus); in each genus there is only one
species attacking man, viz. Pediculus humanus and Phthirus pubis.
The first of these exists in two forms, the body louse and head louse; these
are best regarded as biological races rather than species, because the
anatomical distinctions between them are somewhat indefinite, though there are
more pronounced differences in behaviour and biology (page 11). Phthirus
pubis, or the crab louse, is generally confined to the inguinal region
(page 138). These lice (both Pediculus humanus and Phthirus pubis)
occur only on man and not on other hosts.
It is held that the genera Pediculus and Phthirus
are closely related: anatomical points which they have in common are an
antenna consisting of five joints, the presence of pigmented eyes, and the
fact that the tibia is provided with a process resembling a thumb, between
which and the one-jointed tarsus the hair of the host is grasped.
The points of distinction between the two genera of lice
occurring on man are briefly as follows:
Pediculus. All legs about equally strong (but
anterior legs of male stouter than those of female). Abdomen about twice as
long as it is broad. (See figure 1; pages 5 to 23.)
Phthirus. Foreleg slender with long fine claw. Middle
and hind legs strong with thick claws. Abdomen broader than long, and
compressed so that spiracles of 3rd, 4th and 5th segments lie almost in one
transverse line. Abdominal segments with protuberances at the side. (See
figure 44; pages 136 to 141).
All sucking lice (Anoplura) are obligate parasites, spending
their whole life on the skin of a mammal and living exclusively on blood. Some
200 to 220 species of sucking lice are known. Though all the hosts are
Mammalia there are certain important groups which have no parasites of this
type; for instance, the Carnivora (exclusive of the dog family), and the
Marsupials. A general account of the order is given by Ferris (I934).
The Anoplura have, so far as is known, no insects which are
parasitic upon them and probably very few enemies, except their hosts. They
harbour certain parasitic micro-organisms, some at least of which (Spirochaeta
and Rickettsia) are pathogenic to the mammalian hosts. For this reason
human lice (Pediculus) have great importance as vectors of relapsing
fever (page 90) and typhus (page 76).
The relation of sucking louse to host is often very close, one
species of louse living on one host, or on a few which are closely related to
one another. In general it seems probable that this specificity is maintained
by the parasite's reaction, which prevents it biting hosts other than the
normal.
Those who have studied the Anoplura, or lice, have found that
those mammals which are closely related to one another tend to have closely
related or identical lice. It seems that in the course of evolution the
mammals have often come to differ from one another more than the lice, so that
occasionally the insect parasite points to relationships between species of
mammal which have become rather dissimilar. To take a very simple case, the
Ground Squirrels (Citellus) of North America and Siberia are related
but different, though the lice on them appear to be identical. A more
complicated case is presented by the lice found on man and other Primates. The
lice on these hosts all belong to one of the families (Pediculidae) into which
the Anoplura are divided, and no member of that family occurs on any other
host. On a conservative view, the family may be said to consist of three
genera, Pedicinus, Pediculus and Phthirus. Of these, the
first is found only on the monkeys of the Old World (Cynomorpha). Both
Pediculus and Phthirus occur on man and higher apes, but not on
other monkeys; Pediculus has species on two of the gibbons and on the
chimpanzee (Fahrenholz); there is, however, some doubt as to whether the
record from the gibbon is correct (Ewing, 1938). Phthirus includes a
most imperfectly known species from the gorilla and has recently been recorded
from the chimpanzee (Bedford, 1936): the records from gorilla and chimpanzee
may be derived from menagerie material, and it is not conclusively known that
these animals are natural hosts of the crab louse. The orang appears not to be
infested with lice. Presumably, therefore, these two genera (Pediculus
and Phthirus) have been parasites on the human stock, and its ancestors
and close relatives, since very remote times. But in addition there are
several species of Pediculus (sometimes separated and treated as an
independent genus, Parapediculus) occurring only on monkeys in tropical
America, particularly on Spider Monkeys (Ateles). The occurrence of
these parasites (so close to man's louse) is puzzling, because on general
anatomical grounds the American monkeys are far removed from man and his
ancestry. It is possible that the lice found on spider monkeys may have been
transferred to them from human beings, but if that occurred it was in the
remote past, for considerable differences have been evolved between the
parasite of man and spider monkey (Ewing, 1938).
The examples quoted above illustrate the general truth that
related hosts carry related parasites. There are, however, a few cases in
which one must suppose that a species of louse has been transferred
successfully and permanently to a completely new mammahan host.
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[Dept of Public Health and Tropical Medicine ]
www.jcu.edu.au/school/phtm/PHTM/staff/rsbuxt1.htm
Updated 6 February, 1998
Rick Speare
