55 Figure 55. Nymph emerging from larval skin. ORNITHODOROS MOUBATA LARVAL NYMPHAL MOLT After Jobling (1925)7 PLATE XVIII nymph; surface grooves disappear and the contour becomes more con vex. The internal nymphal outline and limbs are now visible. The two fore pairs of legs move to cause pressure on the larval skin resulting in a transverse rupture from which the anterior part of the body and the anterior legs emerge (Figure 55). After all legs are free, the larval skin is abandoned. Jobling The nymphal stage, in contrast to the quiescent larval stage, is very active. Cunliffe observed four to eight nymphal instars before adulthood. He noted that most males appear at the fifth molt, most females at the fifth or sixth molt. Jobling (1925) and Pierquin and Niemegeers, however, observed no more than five molts and found the majority of male emergence at the fourth molt and the majority of female emergence at the fifth molt. Dr. G. E. Davis' unpublished records show that in his laboratory most females reach the adult stage at the fourth molt.* Discrepancies in findings among various careful observers of this subject suggest an interesting field for research. See also the section on symbiotes and growth-promoting substances, page 177.7 The interval between successive nymphal molts depends on the time of the preceding blood meal not on the time of the last molt. This is agreed by all workers. First instar nymphs require a longer period before they are capable of feeding (three to twelve *It should be noted that while argasids have several nymphal in stars, ixodids molt directly to adults from the nymphal stage. Ixodids remain on the host for several days in each stage and accommodate the huge volume of ingested blood by slow cuticular growth (whether this is true for all ixodids, as for instance males of several Madagascan haemaphysalids, should be investigated; cf. Hoogstraal 1953E). Argasids, on the other hand, feed much more rapidly and accommodate the volume of fluid ingested by stretching the skin. Lees (1952) believes that this feature necessitates the several nymphal instars of argasids. The rapid feeding of argasids on animals that are usually resting reduces the danger of their transportation under inclement conditions and to unfavorable environments; they normally remain in handy feeding range in the host's house, burrow, den, or lair. Lees cites the several blood meals that a female argasid may take to nourish several egg batches as an adaptation to maintain her fecundity. Female ixodids, which take only a single, extended meal as adults, oviposit only once over a period of several days. It is well known that ixodids de posit huge numbers of eggs but argasid eggs are relatively few in number. This discrepancy, however, is overcome by the more favorable environment for obtaining a host in which argasid larvae and nymphs usually find themselves. Survival of argasid populations does not depend on large numbers of eggs but it does in ixodids. days, mean five days; at 30°C.) than do later instars that feed on an average of two days (minimum one day, maximum five days) after molting (Jobling). Dr. G. E. Davis reports (conversation) that nymphs kept at normal room temperature require eight days before molting to the second nymphal instar and longer for successive instars. Jobling noted that first instar nymphs feed on an average of 25 minutes (minimum thirteen and maximum 87 minutes). Second and third instars average about four minutes less (minimum eleven and maximum 54 minutes), while fourth instar feeding is the longest (average 26, minimum 17, maximum 53 minutes). Jobling believes that the longer final nymphal feeding may possibly be necessary due to the requirements for metamorphosis to the sexually mature adult stage, which demands more nourishment than simple nymphal instar_to_instar development. These figures are in essential agreement with those of other students of the life cycle, mentioned in preceding paragraphs. Shortly before feeding is completed, a clear fluid begins to emerge from the coxal organs of all nymphal stages (as it also does from both adult sexes during feedings) and continues to issue until after feeding is completed and the tick has left its nost. (See REMARKS below). Nymphs are more resistant to adverse temperature and humidity factors than egg and larval stages, during which there is a much higher mortality than among nymphs. This is also agreed by all workers. The ratio of males to females is practically equal (Jobling). Males emerge from the last nymphal molt with a strong sexual urge and may fertilize several females before feeding. The average male feeding time is sixteen minutes (maximum 42, minimum nine). After feeding they are less active and less eager for females and bury themselves in soil. Three or four days later they again become active and seek females. Jobling_7 Females can be fertilized immediately after molting and several males may engage a single female before she seeks a blood meal. A female feeds for an average of 35 minutes (minimum 21, maximum 92). This feeding period is longer than those of nymphs and twice as long as that of the male. Females commence feeding about two days after molting. Jobling Frequently repeated remarks by workers of the 1905 to 1907 period that O. moubata may molt after reaching adulthood un questionably were based on erroneous identification of advanced nymphal stages as adults. The minimum time necessary for 0. moubata to complete its life cycle is 62 days for males and 73 days for females, but in practice in the laboratory there seems to be some advantage to lengthening the periods of rest after molting and before feeding (Pierquin and Niemegeers). The life cycle can be enormously lengthened by delaying feeding and mating; and, for laboratory rearing, nymphs can be produced to meet any desired schedule, within certain limits, by selective timing. The longevity of 0. moubata has excited much interest since it may be an important factor in allowing new populations to develop from a few imported specimens in areas where hosts are scarce. Hirst (1917) maintained unfed specimens alive for fourteen months and Mayer (1918) kept others alive as long as five years. Cunliffe (1921) recorded female longevity averaging 715 days under ideal conditions of temperature and humidity with food available, and 441 days when food was unavailable. Nymphs (reported as larvae) have been kept alive without food for over 710 days in the Nairobi medical laboratories ("Kenya 1928). These figures are representative of numerous other records. The prac tical importance of the long life of this species needs to be determined inasmuch as the fertility of long unfed females is much less than that of individuals that are permitted to feed at will. It has also been shown that female fertility decreases sharply five or six months following the nymphal-adult molt. No reports have been encountered that indicate a difference be tween male and female life expectancy. Parthenogenesis of O. moubata may have been observed by Cun liffe, although he hesitated to be assured that the female had not been fertilized when unobserved. Parthenogenesis definitely has been established by Davis (1951), who reared 38 of 48 indi viduals hatched from unfertilized females. Oviposition in unfertilized females was much delayed and the interval between hatching and molting of their progeny much prolonged. All progeny were females, but when these were mated with normal males both sexes were represented in the subsequent generation. According to Cunliffe, O. moubata and O. savignyi may copu late but the resulting eggs are unfertile. This is contradicted by recent, unpublished findings of Dr. G. E. Davis who writes (correspondence) as follows: I have found that the interbreeding of these two species not only results in progeny but in fertile progeny when the products of the first interbreeding are allowed to interbreed among themselves". The foregoing is a reasonably complete though brief summary of what is known about the life cycle of 0. moubata. Before leav ing this subject, attention should be called to the additional temperature and humidity studies discussed under Environmental adaptability below for these factors exert considerable influence on the life cycle. Ecology Environment and Domestic Habitats The ecology and distribution of O. moubata, as summarized in the paragraphs below, has always been considered in the light of domestic populations. The significance of the increasingly more numerous reports of the eyeless tampan in large animal burrows from the Sudan to South Africa awaits to be determined. Should it eventually be found that these two populations are a single biological entity that has happened by chance to occupy one or the other habitat, the conclusions of early workers, who believed that man has been wholly responsible for carrying this tick outward from its primitive range in the East African lowlands, will have to be modified. The arid environment preferred by domestic populations of 0. moubata restricts their presence to dry, permanent huts and structures where people gather. In its probable original area, the Somali Arid District and possibly the East African Lowland |