898 
INSECTS AND THE IE RELATION 
again. Copulation is effected ; the fecundated female when 
replete falls off the animal, and shortly after commences 
ovipositing, and we have therefore a complete metamorphosis 
with larval, nymphal, and adult stage passed upon an animal 
host. 
The blue-tick of Eedwater and Anaplasmosis ( Boophilus 
annulatus) passes all three stages and effects both moults on 
the one host. The red-legged-tick of horse-biliary fever (Rliipi- 
cejphalus evertsi) passes the larval and nymphal stages on one 
host, and moults from nymph to adult on the ground. The 
brown-tick of East Coast fever, and sheep gastro-enteritis 
(Rh. ajpjpendiculatus), the bont-tick of Heartwater ( Amblyomma 
variegatum), and the dog-tick of tick fever ( Haemaphysalis 
Leachii) require three hosts, moulting from larva to nymph, 
and from nymph to adult, on the ground. 
With the exceptions of the viruses of sheep gastro-enteritis 
and of Heartwater, which belong to the ultravisible class, the 
organisms conveyed by the above-mentioned ticks belong to 
the Protozoa ; but, unlike trypanosomes, which swim about 
in the blood, these are non-motile, and are found within 
the red blood corpuscles. From the pear-shaped body of the 
first-discovered member they are termed Piroplasma. 
Although it is known that these Pirojplasma must undergo 
some developmental cycle in the ticks, research has not yet 
clearly demonstrated it ; but experimental evidence shows that 
for each disease, the cause of which is distinct, a cycle different 
or modified is undergone. 
It is by this knowledge of transmission that stock-owners 
are enabled to circumvent disease : either by destroying all 
ticks or, failing this, those forms or stages which acquire or 
which give infection. 
We have emphasised that the Ixodidae do not directly 
pass from one animal to another ; and in the case of the blue- 
tick of Eedwater it was stated that all stages, from larva to 
adult, were spent on a single host. It might therefore be asked 
how such a tick could carry disease. Experiments show that 
the young larvae, born of a parent that fed upon an ox in which 
the Eedwater parasites are present, will convey the disease 
when they first attach themselves to a host. 
