Diagram illustrating the typical argasid multi-host life cycle with multiple parasitic phases and repeated gonotrophic cycles (Although there are some noteworthy exceptions, e.g., Ornithodoros lahorensis, which are 2-host ticks or Otobius megnini and O. lagophilus, which are 1-host ticks). Symbols (letters) same as in Fig. 3.1. Following embryogenesis (1) and hatching (2) the hungry larvae attack vertebrate, hosts inhabiting the nest, burrow, or other niche environment. Following host contact (3), larvae attack (4) and feed (5). Larvae feed rapidly (RF). (In some argasid species, larvae do not feed but molt directly to the N₁ stage. In others, larvae feed slowly and molt twice (to N₂) and the N₁ is a non-feeding stage.) Following engorgement (6), fed larvae detach (7), drop off and ecdyse (8), molting into the first nymphal stage (N₁). Hungry N₁ nymphs again attack hosts that enter the niche, repeating the cycle (9-12). This cycle of host contact, rapid feeding, engorgement, detachment and ecdysis in the niche occurs several times (13-21), as indicated by the letter n after these symbols and the arrows indicating the repeat of these processes. The precise number of nymphal stages is indeterminate and not necessarily only as shown in the figure. As many as 8 nymphal instars have been recorded in some species. Following emergence, adults may first mate (22) in the niche or attack hosts. Following host contact (23), adults feed rapidly (24), engorge (25) and drop off (26). Mated females oviposit (27) small egg clutches, whereupon they return to attack hosts and feed again. Adults seek hosts, feed and engorge several times and fed mated females oviposit after each blood meal (28-36). The number of gonotrophic cycles is indeterminate (N₀).

Argasid ticks feed rapidly (except in larvae of certain Argas and Ornithodoros species), and the females feed and oviposit frequently (i.e., multiple gonotrophic cycles). The mated females deposit small egg masses (< 500 eggs/cycle).

There are often many nymphal molts in the life cycle. Almost all argasid ticks have a multi-host feeding pattern (Hoogstraal and Aeschlimann, 1982). In the majority of species, i.e., those that feed on mammals and birds (but excluding bats), larvae seek hosts, feed rapidly (15-30 minutes) and drop to molt in the sand, duff or cracks and crevices of the natural habitat. Fed larvae molt to first stage nymphs (N₁) resembling miniature adults in body characteristics, especially the leathery, mammillated body cuticle, but lacking the genital pore and any evidence of sexual dimorphism. These N1 nymphs also attack hosts, feeding rapidly as did the larvae, and retreating after their meals to molt in some sheltered locality.

An important factor enabling the nymphs to feed rapidly is their ability to eliminate excess blood meal water in the form of coxal fluid, a clear, colorless liquid excreted from the coxal glands during or shortly after. The fed nymphs molt again to yet another nymphal stage, N₂ and cycle of host seeking, feeding and molting is repeated. In some species, 5, 6 or even 7 nymphal molts occur before the ticks mature to adults.

The highest recorded number of nymphal stages is 8 (Hoogstraal, 1985). The number of nymphal states is not consistent, even within the same species. Nutritional factors, especially blood volume taken in previous stages, is believed to be an important indicator of the number of nymphal stages. Moreover, males usually emerge sooner than females, i.e., males require 1 or 2 fewer nymphal stages than do females. In Ornithodoros parkeri, the lightest N₃'s produced a mixture of males, females, and N₄ nymphs. In general, nymphs in the lowest weight classes gave rise to more males than females, while the heaviest nymphs produced more females (Pound et al., 1986).

This is similar to the situation seen in ixodid ticks, where there is a linear relationship between unfed and fed weight of the immatures and the sex of the adult; the lightest immatures developed into males, the heaviest into females (Arthur and Snow, 1966).

In the Argasidae, the passage of so many nymphal stages contributes to a much longer life cycle than in the Ixodidae. In addition, many argasid ticks can resist long periods of starvation during their development, so that the life cycle can be extended for many years. An example of a typical argasid life cycle is that of the soft tick, Ornithodoros turicata americanus.

In the Argasidae, adults become sexually active after emergence from the last nymphal molt, and they do not require a blood meal to initiate gametogenesis. Mating occurs before as well as after blood feeding, but rarely if ever on the host itself. However, the gonotrophic cycle is completely dependent upon the blood meal (except in those rare instances in which mated females oviposit autogenously, i.e., without feeding).

As in the genus Ixodes, unfed females do not normally oviposit even if they are mated. Following feeding, mated females commence oviposition, depositing small egg clutches containing as many as several hundred eggs per batch. When oviposition is completed, the ticks remain vigorous; the hungry females then seek hosts, feed and oviposit again. The number of gonotrophic cycles varies considerably between individuals within the species, as well as between species, although it rarely exceeds 6. This pattern of repeated gonotrophic cycles, often following long periods of waiting between blood meals, enables argasid ticks to disperse their progeny gradually over time, often across a span of many years. The argasid strategy is very different from that of the ixodids, where all of the progeny of a fecund female are produced in one massive surge of ovipositional activity.

Several important deviations from the typical argasid life cycle are known. In many of the Ornithodoros species parasitizing bats (e.g., O. kelleyi, Sonenshine and Anastos, 1960), larvae remain attached for many days, feeding in a manner similar to that of ixodid ticks. This pattern is also common in argasid ticks parasitizing birds. Ornithodoros amblus, a parasite of pelicans, cormorants and other shore birds, exemplifies an argasid life cycle typical of those species with a slow feeding larval stage (Khalil and Hoogstraal, 1981). The lengthy attachment period provides an opportunity for dispersal to other localities and may have evolved as an adaptation for this purpose. Alternatively, this king feeding period may be necessitated by the fact that these larvae lack coxal glands, and time is needed to eliminate excess blood meal water.

Regardless of the reason, the greatly engorged larvae retain sufficient nutrition from the single blood meal for 2 successive molts. The first molt produces the N₁, which does not feed but molts again to the N₂.

Quite the opposite occurs in species of the subgenus Ornithodoros, e.g., O. Moubata and O. savignyi. In these species, the larvae do not feed, but molt directly to N₁ nymphs. Nonfeeding males also occur in the life cycles of some species, similar to the pattern observed in some species of Ixodes.

An unusual variation occurs in the life cycle of the Spinose ear tick, Otobius megnini. In this species, only two nymphal stages occur. Moreover, these highly specialized argasids are 1-host ticks, with a high degree of host and body site predilection regulating their feeding and development. O. megnini is an obligate 1-host tick. In the central Asian species, O. tartakovskyi, nymphs and adults remain on the host for extended periods during winter, feeding and developing there in the same manner as 1-host ticks, but dropping and molting off the host during warmer periods of the year. In contrast to O. megnini, O. Tartakovskyi is a facultative 1-host or multi-host parasite.

In the genus Argas, most of which parasitize bats or birds, the larvae of almost all species feed slowly, similar to the O. amblus pattern noted above. An interesting exception occurs in the larva of Arges cucumerinus, a parasite of marine birds, which feeds in 7-25 minutes (Hoogstraal, 1985).

Diapause is a major factor regulating the time of development of many of these species, which must survive in empty burrows or nests for periods of many months until their hosts return or new hosts arrive. Khalil and Hoogstraal (1981) noted that O. amblus did not oviposit during winter when held at 27-28 °C and normal daylight, although they did oviposit when held under a long day (16 hours) photophase