58 Observations on Parasites and Parasitic Diseases 



its own particular variety of louse ; and it is a singular fact that 

 the parasite shows little or no disposition to leave the body of its 

 host for that of any other. By long cohaljitation, however, the 

 louse of one animal Avill occasionally travel to another of a 

 totally different species, and not only live upon its new victim, but 

 speedily propagate, and thus produce an attack of phthiriasis. 

 The introduction of a lousy animal among a number of healthy 

 ones of the same species is sure to be followed by all of them 

 becoming sooner or later affected. The rapidity with which 

 this sometimes takes place is very remarkable ; but it appears 

 to be always accomplished much more slowly during the winter 

 than the summer months. This circumstance is easily accounted 

 for, as the parasites are less active in the winter, both in their 

 ordinary wanderings from animal to animal, and also in their 

 powers of reproduction. It is also not improbable that some 

 kinds of lice leave the body of one animal for that of another 

 much earlier than others, and this under precisely the same 

 circumstances. 



" No man, perhaps," — writes Denny, — " ever carried his researches on the 

 habits of the louse to a greater height tlian Lceuwenhoek, who allowed his 

 zeal for science to overcome the disgust which such creatures generally produce. 

 In order to ascertain the rapidity with which a louse can propagate its species, 

 he tried the following experiment which I shall give in his own words : — 



" ' The louse is so prolific an animal that it is a common vulgar saying 

 that it will be grandfather in the space of 24 hours. Tliis I could never believe 

 to be the fact, but rather that it would require nearly a month for the offspring 

 of a louse to be capable of producing young of its kind; and in order to make 

 proof of it by experiment, 1 at first proposed to hire -some poor child to wear a 

 clean stocking for a week, with 2 or 3 female lice in it, and well-tied or secured 

 at the garter, in order to see how many young ones woirld be produced in that 

 space of time ; but I afterwards considered that I could make the experiment 

 wdth much more certainty on my own person, at the expense only of enduring 

 in one leg what most j^oor people are obliged to suffer in their whole bodies 

 during all their lives. Hereupon I put on one leg, instead of a white stocking 

 I usually wear, a fine black stocking, choosing that colour because I considered 

 that the eggs and the young lice thence proceeding would be more easily dis- 

 tinguished upon it. Into tiiis stocking I put 2 large female lice, and cutting 

 another black stocking into long slips, I bound it over the first, above the knee 

 to prevent their escaping. After wearing this stocking six days I took it off, 

 and found one of the lice had laid 50 eggs, and the other about 40. I opened 

 the one which had laid the 50 eggs, and found in its body at least 50 more, 

 and who knows how many eggs it had laid before 1 put it into the stocking, 

 and how many more eggs it might have in its body which my sight could not 

 reach ? Havins; w^orn tlie stocking ten days longer, I found in it at least 25 

 lice of three different sizes, some of which I judged were 2 days old, others a 

 day old, and the rest newly come out of the egg.' 



" To give a clearer conception of the great increase of these animals, let us 

 suppose a person to have about his body 2 male lice and as many females, and 

 that the females in 12 days' time lay 200 eggs ; and that 6 days afterwards, out 

 of those eggs, are produced 100 males and 100 females ; and that this young 

 brood in 18 days' time are grown to a size to propagate their kind, and that 

 each of these young females in the space of 12 days more lays 100 eggs, and 



