Engorgement
The following images all depict the adult female of Ixodes holocyclus. Colour and markings change markedly as engorgement progresses. It is the third tick, the moderately engorged adult female (width, at level of the spiracles, more than 4 mm) which is most commonly removed from dogs with tick poisoning. If the fully engorged tick were found on a dog it suggests that the dog has a degree of immunity.
Adult female - No engorgement
Adult female - Early engorgement
Adult female - Moderate engorgement
Adult female - Full engorgement
These pictures are not to scale with each other. Because the size of the 'shield' (scutum) does not change as the female tick engorges, you can use it to compare the relative sizes.
Read more about this topic: Ixodes Holocyclus