INTRODUCTION. 15 
Fleas propagate in the dust of dwellings, in the litter of the resorts 
of the animals they infest, or attached to the hair of animals. 
Sheep ticks and bat ticks have acquired a remarkably specialized 
mode of reproduction as a consequence of their parasitic habits, which 
must be dealt with in detail in discussing that particular group. 
All of the different kinds of suctorial lice pass through the succes- 
sive stages of growth upon the particular animals they infest. The 
eggs, commonly called “nits,” are attached to the hair of the host and 
hatch into young lice which resemble closely the adults, differing only 
in size and in the hardness of certain parts. The rate of propagation 
and the time involved in passing through the different stages have 
been determined for but one species, and that under conditions which 
make it uncertain as a basis for calculating possible rate of increase 
even for that species. Much less can it be taken as a basis of measure 
‘for the development of other species. 
We know, however, that the rate of increase is at times very great. 
Frequently an animal when first observed is so thickly covered with 
vermin that the owner thinks they must have come by some other than 
the ordinary means of reproduction. The lice are frequently found in 
greatest numbers in the spring of the year, but it is quite doubtful 
whether this is due to increased rapidity of reproduction in the winter 
season. It seems most likely that the reproduction is more rapid dur- 
ing summer, but the animals are then in a thriving condition, con- 
stantly in the open air, and by their own efforts keep the lice from 
making such lieadway as to produce noticeable results. During winter, 
however, one or two generations even, from the numerous flock present 
when the animals are housed, furnishes such numbers that their pres- 
ence tells upon the economy of the animal and it is no longer able to 
contend with its foes. The resultis loss of hair, poor condition, reduced 
vitality, and, finally, unless active measures are adopted for its relief, a 
fatal result. 
The life history of the bird lice is very similar to that of the sucto- 
rial forms. Eggs are attached to hairs or feathers and the young, con- 
stantly resident upon the host, grow by easy stages to the adult form. 
Observations on the period required for the incubation of the egg have 
shown that for one species, Nitzeschia pulicaria, the time from deposition 
to hatching is from fifteen to twenty days. The rate of increase is 
apparently here less rapid than for the suctorial species, but still rapid 
enough in some species, as, for instance, the cattle and horse lice, to 
prove very troublesome to their hosts. 
The mites, ticks, mange insects, ete., travel similar cycles of life 
wherever they are confined to the host throughout life; but for the 
harvest mites and ticks, which develop for a time on vegetation, the 
eggs are usually deposited loosely on the animal or upon vegetation and 
admit of the young obtaining a portion of their growth before attach- 
ing themselves to an animal host. The young resemble the adults in 
