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.
Updated 6 February, 1998
Rick Speare
