This is from the wikipedia article on Paederus Beetles:
Once pederin is on the skin from the initial beetle contact, it may also be spread elsewhere on the skin. "Kissing" or "mirror-image" lesions where two skin areas come in contact (for example, the elbow flexure) are often seen. Washing the hands and skin with soap and water is strongly recommended, if contact with a rove beetle has occurred.
Initial skin contact with pederin shows no immediate result. Within 12–36 hours, however, a reddish rash (erythema) appears, which develops into blisters. Irritation, including crusting and scaling, may last from two to three weeks.
It's also been suggested that the plague of boils that God inflicted on the Egyptians was actually an outbreak of Paederus Beetles:
Blister beetles and the ten plagues
Several interpretations of the biblical ten plagues (Exodus 7:14—12:30) postulate that the insects of the third and fourth plagues gave rise to the boils of the sixth plague.1 The notion that these were arthropod-borne epidemics, such as bubonic plague, trypanosomiasis, or leishmaniasis, is unlikely because the Israelites probably would not have escaped such widely transmitted diseases. Reports of invasions of blister-inducing rove beetles in southwest Asia2 suggest a novel explanation.
Many rove beetles (Paederus spp, Staphylinidae) have a toxic haemolymph called paederin that causes painful necrotic blisters when a beetle is crushed on the skin.3 Rove beetle populations are generally small but under the right environmental conditions, they can reach spectacular size. At night, beetles are attracted to lights and commonly descend on inhabited areas, blackening walls.4 The ensuing vesication can injure thousands of people and has forced the evacuation of entire communities.3—5
The first two plagues may have produced ideal conditions for massive breeding of Paederus. In the first plague, “the water of the Nile turned to blood”, probably because of a bloom of toxic phytoplankton (a red tide).1 The anoxic conditions in the river killed the fish and forced “frogs onto the land of Egypt”, causing the second plague.1 Rove beetles, which breed in the marshy banks of the Nile and scavenge tadpoles and carrion,3 were provided heaps of decaying frogs on which their numbers could flourish. Thus arose the third and fourth plagues, in which “grievous swarms of insects invaded Pharaoh's palace and the houses of Egypt”. Rove beetle swarms are normally focal, which may explain why the insects plagued the Egyptian community but spared the neighbouring Israelites.
Soon thereafter, “boils ([sh'chin]: boils or eruption) breaking forth with blains ( [avahbu'ot]: blisters or boils) on man and beast” formed the sixth plague. Paederin-induced blisters erupt 1—4 days after exposure; thus, victims frequently do not associate the beetles and skin lesions causally and think of them as separate events.
We propose that the third and fourth plagues were an invasion of Paederus, probably P alfierii, a blister-causing rove beetle that lives in the Nile delta,3 whose population exploded under the conditions of the first two plagues. The swarm was localised to the humid Nile delta area, affecting only the Egyptian community, and the subsequent outbreak of blisters, occurring several days later, was perceived as a separate event—the sixth plague.
