868 SOME POINTS OF GENERAL INTEREST 
Cockroaches may harbour plague bacilli and carry them 
for a considerable distance in their intestines. Dr. Hunter 
was able to recover cultures from faeces of these insects. 
Fleas, however, are the most important of them all, and 
on them we shall dwell at somewhat greater length. 
Life History of Fleas . — These undergo a complete meta- 
morphosis. The adult female lays eggs, about the size of a 
small pin’s head, on the ground. These are not attached to 
the fur of the host. One to five eggs are laid at a time, and 
they hatch in about two days in such a climate as that of 
Bombay, and it would probably be about the same time 
here. A larva issues from the egg, and in this stage the insect 
is very vulnerable. It has chewing or biting mouth-parts, 
and lives on any kind of animal or vegetable refuse. These 
larvae are remarkably long in the case of Pulex cheojpis, the 
common rat flea, and comparatively short in P. irritans, the 
human flea. After about a week, the larva becomes sluggish, 
ceases to eat, and spins a cocoon of white silk-like fibres, and, 
enclosing itself in this, turns into a pupa. The surface of the 
cocoon is frequently covered over with dust or other small 
particles of rubbish which adhere to the fibres. In from seven 
to fourteen days the pupa turns into the perfect flea — the imago, 
and escapes from the cocoon. The mouth-parts of the flea 
are so constructed that it can only live on liquid food. Young 
fleas, which have just escaped from the cocoon, can live with- 
out food for one to two weeks, whereas, after having taken 
their first meal, older fleas die within a week if deprived of 
food : at least, this was the case in the Bombay experiments, 
and I will just pause here to warn you that we must be cautious 
in our generalisations. Things which occur in one country, 
and under a certain set of conditions, will not necessarily 
be the same in another land where the conditions are 
different. 
The time taken from the egg to the imago or perfect flea 
is about twenty-one to twenty-two days, under favourable 
circumstances ; but under unfavourable ones this period may 
be greatly prolonged. Climatic conditions affect the develop- 
ment of some species of fleas. Dampness in the surroundings 
of a breeding-place hinders the development, and adult fleas 
