The evolution of the life cycles of the members of the family Dipetalonematiidae Wehr, 1935 (Filarioidea) is considered in the light of existing knowledge of spirurid nematodes. The hypothesis that the life cycles of the dipetalonematids originated from life cycles similar to those of Draschia megastoma, Habronema muscae and H. microstoma is considered to be incorrect. Alternatively, it is pointed out that in the primitive subfamily Thelaziinae Baylis and Daubney, 1926 there are forms with typical spiruroid life cycles (Rhabdochona ovifilamenta), forms with life cycles approaching those of the dipetalonematids (Thelazia spp.), and forms with life cycles intermediate between these two (Oxyspirura spp.). It is suggested that intestinal species similar to Rhabdochona gave rise to the more specialized spiruroids and forms that left the gut (Oxyspirura, Thelazia) gave rise to the dipetalonematids.
The dipetalonematids are believed to have originated from nematodes resembling the species of Thelazia and having life cycles like those of T. rhodesii, T. skrjabini and T. gulosa. Some of these worms established themselves in subcutaneous tissues. Like Parafilaria multipapillosa, they released their eggs through a break in the skin of the definitive host, thus causing a skin lesion that attracted various haematophagous arthropods which finally became involved as intermediate hosts in the life cycle. Certain species like the members of Parafilaria and Stephanofilaria (?) came to rely upon intermediate hosts that were unable to break the skin of the definitive host (Musca) and cutaneous lesions became permanent features of their life cycles. Other species became dependent upon intermediate hosts that could puncture the skin (mosquitoes, simuliids etc.) and skin lesions became unnecessary to the life cycle. The larvae of these worms then began to spread into the tissues of the skin, as found in Stephanofilaria, Onchocerca, and some species of Dipetalonema, and the infective larvae developed the ability to penetrate into the wound made by the intermediate host and perhaps, in some cases, the intact skin. Ultimately the larvae of some species habitually entered, or were deposited into, the blood stream and the adult worms were then free to colonize the vertebrate body as their larvae would then be available to the intermediate host no matter where the latter fed on the body of the definitive host; this group of worms gave rise to the many members of the family Dipetalonematidae.
The family Filariidae Claus, 1883 is briefly reviewed in the light of the above hypothesis. It is pointed out that many species, e.g. Diplotriaeninae Skrjabin, 1916, live in the air sacs of reptiles and birds and probably have life cycles similar to that of Diplotriaenoides translucidus, i.e. the eggs pass through the lungs, up the trachea and out in the faeces. It is thought that these forms may represent a separate line of evolution from that which gave rise to the Dipetalonematidae. Certain genera (Lissonema, Aprocta), occurring in the orbits of birds, probably have life cycles like Thelazia or Oxyspirura. Many other genera occurring in superficial muscles and subcutaneous tissues (Squamofilaria, Ularofilaria, Tetracheilonema, Pelecitus, Monopetalonema) may release their eggs through some sort of skin lesion. Studies on these forms are urgently needed as the details of their life cycles may shed fresh light on the origins of the more specialized filarioids.
