The alimentary canal and nervous system resemble those of Mallophaga more than they do those of Hemiptera. The oesophagus leads into a large stomach bilobed in front; at the posterior extremity of this there open the four Malpighian tubes, and behind these there is a well-marked small intestine. The nervous system consists of a cephalic ganglion and of three other closely approximated ganglia, the posterior one the larger. It remains doubtful whether or not the first of these three ganglia is the infra-oesophageal one.[[544]]
Fig. 293.—Mouth-organs of louse. (After Graber.) b, c, Chitinous envelope into which the beak can be withdrawn; d, head of the beak, with crown of spines; g, labrum; h, delicate tube protruded (very rarely seen in this state); m′, unpaired muscle.
The species of lice, so far as known, are not numerous, some six genera and about forty species being all that are recorded; they occur on various kinds of mammals, including some that live in water. Seals have a genus, Echinophthirius, peculiar to them. Monkeys are specially liable to be affected by lice; the genus that chiefly occurs on them is Pedicinus, a very distinct one, in which there are only three instead of five joints to the antennae. Perhaps the most remarkable louse is Haematomyzus elephantis, that of the elephant; it has a long proboscis in front of the head. As a rule each species of louse is confined to one species of Mammalia, or to very closely allied forms. Man is said to be infested by three species, Pediculus capitis, P. vestimenti and Phthirius inguinalis; Meinert is of opinion that P. capitis and P. vestimenti are only one species, and Schiödte appears also to have thought this probable. Andrew Murray was of opinion that the heads of different varieties of men are infested by distinct varieties of P. capitis. His conclusion was chiefly based on examination of specimens preserved by Charles Darwin; it requires confirmation. Very little is known as to the life-history of the louse. Leeuwenhoek made himself the corpus vile for an experiment, from which he concluded that the Pediculus vestimenti is very prolific. That scientific men did not know whether the louse bites or sucks was formerly made the ground for a taunt. Schiödte has given an almost pleasing account of the way in which he settled this,[[545]] showing that the sucking action is beyond all doubt. Accounts of disease called Phthiriasis, attributed to lice, are to be found in many old books, but the evidence does not warrant us in believing anything more than that persons suffering from some disease, and in a neglected and filthy condition, were horribly infested with these disgusting Insects.
It is usual to say that Pediculidae are Hemiptera degraded by a long exclusive persistence in parasitic habits. At present, however, this must be looked on as a pious opinion, rather than as an induction from our knowledge of their morphology and embryology; for this is at present too imperfect to warrant any final conclusion.
NOTES AND CORRIGENDA
VOL. VI.
Note to P. [4]: Classification of Hymenoptera. Mr. W. Ashmead has published[[546]] a sketch of a new classification of Hymenoptera, in which the points we have suggested are given effect to; the first division of the Petiolata being carried out with reference to the position of the ovipositor, while part of the Proctotrypidae is brought into the Aculeate division. We cannot, however, commend this arrangement as final, for several points have not received sufficient consideration.
Note to P. [172], line 22. For the words "We shall subsequently see," substitute "We have previously said" (p. [161]).
Note to P. [350], line 10 from bottom: instead of "only one genus," read "only one Old World genus."