December 1, 1891.] 



KNOWLEDGE. 



227 



the same as those of other animals, so human lice are 

 distinct from those which infest the lower mammalia, and 

 indeed each species of mammal may be expected to have 

 its own distinct parasite. Man, then, is not exceptional in 

 xittf?n')iii from these parasites, but rather in having to 

 some extent, as has taken place amongst civilized nations, 

 shaken himself free from them. 



Of these disgusting insects three species are known to 

 infest human beings, the head-louse (Fedieulus cii/titis), 

 the body-louse (/'. I'estms/jfi), and the crab-louse (P/ii/«'ni(s 

 im/umalis). The first is the kind that occurs most 

 commonly, and the last is the rarest. The two Pediculi 

 are very much alike, the body-louse being best distinguished 

 by the locality in which it is found, and by its larger size ; 

 the Phthiiius is very different from both. Taking as our 

 type tlie commonest species (Fig. 1), we may first note its 



structural peculiarities. 

 It is a flatfish, semi- 

 transparent insect, of a 

 pale ashy grey colour, 

 with a comparatively 

 small head and a very 

 large body. The head, 

 which is narrowed in 

 front and behind, 

 carries a pair of short 

 five-jointed antennre, a 

 pair of simple, rounded, 

 unfacetted eyes, and 

 the mouth organs, of 

 which more presently. 

 Behmd it merges into 

 the thorax, which 

 again is not definitely 

 marked ofi' from the 

 abdomen, but the 

 three pairs of legs 

 show how far its 

 three segments extend. 

 The legs succeed one 

 another without in- 

 terval, and the first 

 pair are placed im- 

 mediately behind the head. No wings of any kind 

 are ever developed, nor is any trace of such organs 

 perceptible ; hence some naturalists have questioned 

 whether the lice should be included amongst insects 

 at all. Not only is the thorax considerably broader 

 than the head, but this increasing breadth is continued 

 into the abdomen, so that the widest part of the 

 insect is about half-way down the body. The margins of 

 the abdomen show a scalloped edge, there being a series of 

 indentations where the segments adjoin. On each of these 

 rounded projections is placed a small circular spiracle, or 

 l)r(<athiiig hole, the terminal orifice of a short tracheal 

 branch. Six spiracles are thus arranged down each side 

 of the body, and all the short branches from them join a 

 great tracheal trunk which runs down each side parallel to 

 the margin. The whole body is covered with minute 

 scattered hairs, which are sharp-pomted and perfectly 

 straight. 



The legs are composed of the usual parts, but all the 

 joints arc short and stout, giving an appearance of clumsi- 

 ness, and the feet are extremely peculiar, their remarkable 

 structure formhig one of the distinctive characteristics of 

 a louse. The tarsus, or foot proper, consists of two small 

 joints, the division between which is not very easy to see, 

 and these are succeeded by a tcrnunal appendage in the 

 form of a single, curved, movable claw of large si/.e, which 



Fig. 1.— Heiid-Loust {Pediculus cap- 

 itis). Female, vieM-ed from beneath. 

 Magnified 22 diameters. 



is usually carried bent more or less inwards, and is 

 capable of being completely folded back upon the foot. 

 At the end of the tibia, or shank, there is a movable 

 pointed prominence, and by means of this, which acts as a 

 sort of thumb, and the great claw, the insect is enabled 

 to exercise that strong grasping and clinging power for 

 which it is noted, and which is of great importance in 

 its econoiily, facilitating its movements amongst the 

 hairs in the midst of which its life is spent. In the figiu-e 

 one of the claws is shown bent back upon the " thumb," 

 as in the act of grasping. The claws are very similar in 

 shape to those of fleas, but differ in being single on each 

 foot, instead of double. 



In the structure of the mouth organs again, lice are 

 exceptional. When the dead or inactive insect is 

 examined, no mouth organs can be seen, for, when not 

 in actual use, they are retracted within the head. The 

 mouth is of the suctorial type, the insect feeding on the 

 blood of its victims, to obtain which, an incision must of 

 course be made through the skin. But, in consequence of its 

 retractile character, there has been a great deal of difficulty 

 in determining the real structure of the sucking apparatus, 

 and it is necessary to carry out careful observations on the 

 living or recently killed insect, before the details can be 

 made out. The old Dutch naturalist Swamm eidamtook 

 great pains in investigating the matter, and showed clearly 

 that there was a suctorial proboscis, which could be thrust 

 out from the head and entirely retracted again. But, as 

 he himself says, " this proboscis is, on account of its 

 diminutive size, not to be demonstrated except with great 

 painstaking, and it is perhaps nothing but a piece of good 

 luck if one succeeds in seeing it." This being the case, it 

 is perhaps not surprising that since Swammerdam's time 

 some authorities have denied the exclusively suctorial 

 character of the apparatus, and have maintained that true 

 biting organs are present, whence they attributed the 

 irritation produced by the insects on their hosts to the 

 effects of a real pinching bite. This, however, was a 

 mistake arising from the fact that only dead specimens 

 were examined, and those too imder pressure, so that the 

 apparatus could only be seen through the skin as it lay 

 contracted inside the head, in consequence of which it was 

 misinterpreted. About t\Yenty-tive years ago Professor 

 Schiodte, a Danish naturalist, by careful observations on 

 the living insect (in this case I', irstimcnti), confirmed 

 Swammerdam's statements, and determined with greater 

 accuracy the true nature of the proboscis. He obtained 

 an abimdant supply of material from a workhouse (Danish), 

 and having enclosed some specimens in a glass tube for 

 two or three days without food, so that they might the 

 more readily fall to when released from confinement, he 

 transferred one of them to the back of his hand and pre- 

 pared to watch its movements with a lens. He thus 

 describes what followed : — " Scarcely does the abominable 

 little monster feel the heat of the skin before it lays 

 aside its former disheartened attitude, and begins to 

 feel at ease, its antennre oscillate for joy, and it 

 stretches all six legs complacently out from the body. 

 But though the pleasure and surprise at the sudden 

 transportation into congenial surroimdiugs for the 

 first moment eclipse everything else, hunger soon asserts 

 its claim, sharpened as it is by the long fast, which has 

 rendered its stomach and intestines quite transparent. 

 The animal raises itself on its legs, walks on a few steps, 

 seeking and feeling its way with its antennie, while we 

 follow it with the magnifier. Presently it stops, draws in 

 its legs a little, arches its back, bends the head down 

 towards the skin at an oblique angle, while ii pushes a 

 small dark and narrow organ repeatedly forward, and 



