host's skin, expels eggs, and dies. Due to the nature of the discussion, overlap with other sections, particularly the one on symptoms, is unavoidable. Stage 1 is characterized by the penetration of the skin by the female chigoe flea. Running along the body, the female uses its posterior legs to push its body upward by an angle of 45–90 degrees. Penetration then starts, beginning with the proboscis going through the epidermis. By stage 2 (day 1–2), penetration is complete and the flea has burrowed most of its body into the skin. Only the anus, the copulatory organs, and four rear air holes in fleas called stigmata remain on the outside of the epidermis. The anus will excrete feces that is thought to attract male fleas for mating, described in a later section. The hypertrophic zone between tergites 2 and 3 in the abdominal region begins to expand a day or two after penetration and takes the appearance of a life belt. During this time, the flea begins to feed on the host's blood. Stage 3 is divided into two substages, 3a and 3b. Stage 3a is 2–3 days after penetration is complete. In substage 3a, the flea's midsection swells, balloon-like, to the size of a pea. This expansion of the flea causes its integument to be stretched thin, a process called physogastrism. The swelling ends in the condition of physogastry, and results in the appearance of a white halo around a black dot at the center of the lesion. That dot is the rear end of the flea. In substage 3b, the chitin exoskeleton of tergites 2 and 3 increase in thickness, which gives the structure the look of a miniature caldera. Egg release is common in substage 3b, as are fecal coils. The eggs tend to stick to the skin. At about the 3rd week after penetration, stage 4 begins, which is also divided into two substages. In 4a, the flea loses its signs of vitality and appears near death. As a result, the lesion shrinks in size, turns brown, and appears wrinkled. The death of the flea marks the beginning of substage 4b (around day 25 post-penetration) as the body begins to eliminate the parasite through skin repair mechanisms (e.g. shedding and subsequent skin repair). At this phase, the lesion is seen as brown or black. By the 5th stage of tungiasis, the carcass of the T. penetrans