G. H. F. Nuttall 
♦259 
The mechanism whereby Ixodes may occasionally penetrate beneath 
the skin appears to me easy of explanation. It is an unusual and ab¬ 
normal process and I believe, in the absence of any other explanation, 
that it depends upon the amount of oedematous swelling of the host’s 
skin about the site of attachment of the tick. When the mouthparts 
of the latter become implanted in the firm subcutaneous connective 
tissue and the reaction about the puncture leads to oedema, it follows 
that the oedematous tissue must rise and gradually envelop the tick 
to a greater or less extent. In some large ticks, like Amblijomma 
liehraeum and A. variegatum, the ticks are frequently found partly 
embedded with their capitulum and part of their body inserted into 
the crater of what may be described as a miniature volcano or cone of 
oedematous skin. In Amblyomma the large size of the tick appears to 
preclude their complete disappearance beneath the skin, whereas in Ixodes 
the ticks’ bodies, being smaller, penetration will be more readily effected. 
To repeat, the penetration of Ixodes beneath the skin is not due to any 
mechanical activity on the part of the tick ; it is due to the increasing 
oedema and inflammatory swelling of the host’s skin whose surface rises 
above the subjacent tissues in which the tick’s hypostome is anchored— 
whilst the tick is sucking the host’s blood the host’s skin revenges itself 
by swallowing the tick. After the tick has penetrated beneath the skin, 
the wound it produced may heal and be obliterated. The long hypo¬ 
stome in Ixodes appears to be an essential factor in the process, for we 
have no records of Ixodidae with short hypostomes penetrating beneath 
the skin. However long the ticks may live in this situation, the firm 
cyst-like mass of tissue which forms about them necessarily renders their 
subsequent escape impossible and they must die in situ. 
