During a blood meal, an infected fly (genus "Chrysops", day-biting flies) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound. The larvae develop into adults that commonly reside in subcutaneous tissue. The female worms measure 40 to 70 mm in length and 0.5 mm in diameter, while the males measure 30 to 34 mm in length and 0.35 to 0.43 mm in diameter. Adults produce microfilariae measuring 250 to 300 μm by 6 to 8 μm, which are sheathed and have diurnal periodicity. Microfilariae have been recovered from spinal fluids, urine and sputum. During the day, they are found in peripheral blood, but during the noncirculation phase, they are found in the lungs. The fly ingests microfilariae during a blood meal. After ingestion, the microfilariae lose their sheaths and migrate from the fly's midgut through the hemocoel to the thoracic muscles of the arthropod. There the microfilariae develop into first-stage larvae and subsequently into third-stage infective larvae. The third-stage infective larvae migrate to the fly's proboscis and can infect another human when the fly takes a blood meal. Microscopic examination of microfilariae is a practical diagnostic procedure to find "Loa loa". It is important to time the blood collection with the known periodicity of the microfilariae (between 10:00 a.m. and 2:00 p.m.). The blood sample can be a thick smear, stained with Giemsa or haematoxylin and eosin (see staining). For increased sensitivity, concentration techniques can be used. These include centrifugation of the blood sample lyzed in 2% formalin (Knott's technique), or filtration through a nucleopore membrane. Antigen detection using an immunoassay for circulating filarial antigens constitutes a useful diagnostic approach, because microfilaremia can be low and variable. Though the Institute for Tropical Medicine reports that no serologic diagnostics are available, tests that are highly specific to "Loa loa" have been developed in recent years. This is despite the fact that many recently developed methods of antibody detection are of limited value because substantial antigenic cross-reactivity exists between filaria and other parasitic worms (helminths), and that a positive serologic test does not necessarily distinguish among infections.