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Developmental Cycle

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Life Cycles Argasidae

In the Argasidae, development is gradual, with multiple nymphal stages before reaching the adult form (multi-host life cycle). This pattern is similar to that of most other Acari, to which the normal sequence of developmental stages includes a larva, 4 nymphal stages and the adult. In argasids, mated females oviposit after each blood meal, i.e., there are multiple gonotrophic cycles.

Argasid ticks exhibit remarkable diversity in their patterns of development and feeding behaviour, in contrast to the much more uniform pattern found in the Ixodidae.

Figure 1: Diagram illustrating the typical argasid multi-host life cycle with multiple parasitic phases and repeated gonotrophic cycles. (For further information see text below.)

The typical argasid life cycle is a 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. 1 in the chapter life cycles of Ixodidae. 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 stages 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 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.

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.

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.

Diapause is a major factor regulating the time of development of many of the argasid species, which must survive in empty burrows or nests for periods of many months until their hosts return or new hosts arrive.

   

Further information

  • Hoogstraal H: Argasids and nuttalliellid ticks as parasites and vectors. Adv Parasitol. 1985, 24, 135-238
  • Sonenshine DE: Biology of Ticks. Part 1, 1991, Oxford University Press, New York

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