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 hosts 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 fleas 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 flea has been expelled and there are circular skin residues of the infection that remain. There are only lingering symptoms at this time, described in the next section. Morphology
In a study of 1000 freshly ejected T. penetrans eggs, it was found that females are generally smaller than males for all criteria. In some cases, though, females had a bigger epipharynx and maxillar palpus. Due to its burrowing activity, the chigoe flea has developed a well-developed lacinia and epipharynx that is used to penetrate the skin. Overall, the fleas’ head is relatively flattened, which again aids in burrowing through the epidermal and dermal layers.Investigators have also found that adult T. penetrans have different morphologies with respect to the shape of their head. Some have a rounded head, others have head shapes that resemble ski ramps more than anything else; still, others demonstrate head shapes that are very linear with a slight bulge at the nose. These morphologies were seen to be host-specific, as only fleas of some head-types were found in specific hosts.