Although ticks are commonly thought of as insects, they are actually arachnids like scorpions, spiders and mites. All members of this group have four pairs of legs as adults and have no antennae. Adult insects have three pairs of legs and one pair of antennae. Ticks are among the most efficient carriers of disease because they attach firmly when sucking blood, feed slowly and may go unnoticed for a considerable time while feeding. Ticks take several days to complete feeding.
Source: Illinois Department of Public Health