Ixodes scapularis, commonly known as the deer tick or blacklegged tick (although some people reserve the latter term for Ixodes pacificus, which is found on the West Coast of the USA), and in some parts of the USA as the bear tick, is a hard-bodied tick (family Ixodidae) of the eastern and northern Midwestern United States. It is a vector for several diseases of animals and humans (e.g., Lyme disease, babesiosis, ehrlichiosis, etc). They are known as the deer tick due to their ability to latch onto the White-tailed Deer.
The images shown to the left and to the right -- and in fact, most images of Ixodes scapularis that are commonly available -- show an adult that is unengorged, that is, an adult that has not had a blood meal. This is natural, since the ticks are generally removed immediately upon discovery to minimize the chance of disease. However, the "sack" that holds blood is so much larger when engorged and looks so different from the rest of the tick that it would be easy to assume that an engorged specimen of Ixodes scapularis is an entirely different tick (see photo on lower left). The "sack" is of a light grayish-blue color, whereas the tick itself is chiefly black. In identifying an engorged tick, it is helpful to concentrate on the legs and upper part of the body.
Deer tick females latch onto a host and drink its blood for four to five days. After it is engorged, the tick drops off and overwinters in the leaf litter of the forest floor. The following spring, the female lays several hundred to a few thousand eggs in clusters.