278 
Annual Report fdr 1906 of the Zoologist. 
species, however, it has been found that the changes from 
larva to adult may be passed through without leaving the skin 
of the animal to which the tick first gains access. 
Ten genera of the Ixodoidea or ticks are now recognised, 
but before alluding to them in detail it is necessary to indicate 
certain points of tick structure upon differences in which the 
classification is founded. 
The body of a tick usually has a dorsal shield or scutum , 
covering nearly the whole back of the male, but comparatively 
very small in the gorged female. The scutum may or may 
not bear a pair of eyes. There is a kind of false head or 
rostrum — the part which is commonly left behind when the 
creature is too violently removed. On examination it is seen 
to consist of a base, 
a pair of palps , and 
a piercing organ con- 
sisting of mandibles 
and hypostome. The 
mandibles have 
toothed projections 
or digits at their 
extremity, and the 
hypostome is beset 
on its under surface 
with numerous re- 
curved teeth by 
which the parasite 
anchors itself (see 
Fig. 10). 
The under surface 
of the body shows 
the genital orifice 
somewhat in front, the anus towards the posterior end, and 
often (in the male especially) certain plates or shields of 
characteristic disposition. There are always, in the adult, a 
pair of stigmata or breathing apertures, not far from the origin 
of the last pair of legs. The legs themselves show important 
characters, their first joints or coxce often bearing peculiar 
teeth or spines, and their last joints or tarsi usually being 
furnished with a suctorial organ or pad for holding on to 
smooth surfaces, and a pair of claws. All these structures may 
present generic or specific differences, and there are others 
which demand attention. For example, there are various 
grooves and punctations on the body which are constant 
for the same species of tick, and the hind margin is usually 
looped or festooned more or less distinctly in a tick which is 
not replete. 
Fig. 8. — Adult female of Ixodes ricinus. 
