Jigger (sand flea) infested foot. Tunga penetrans (chigoe flea or jigger) is a parasitic insect found in most tropical and sub-tropical climates. It is native to Central and South America, and has been inadvertently introduced by humans to sub-Saharan Africa. Synonyms for Tunga penetrans include Sarcopsylla penetrans, Pulex penetrates, and many others. In its parasitic phase it has significant impact on its host, which include humans and certain other mammalian species. A parasitical infestation of T. penetrans is called tungiasis.
According to data provided by the U.S. Navy Oceanography Portal (http://www.usno.navy.mil/USNO/as...), astronomical twilight lasts much longer (nearly 2 hours) during the summer compared to the winter months (only about 30-40 minutes).
Morning nautical twilight begins when the center of the sun is at -12Â° of elevation and ends when the civil twilight begins, when the sun is at -6Â°. Similarly, evening nautical twilight begins just after civil twilight, the sun is at -6 Â°, and ends when the sun is at -12 Â°.