Page images

nonmotiIe Zft

The egg of Q. moubata is among the largest kown from ticks.


A newly laid egg (Figure 44) is slightly ovoid, glistening golden yellow, and measues approximately 0.9 x 0.8 mm. Later it becomes reddish brown. Eggs from older females are light to dark brown in color. An irregular, faint, whitish, polygonal reticulation and interrupted radiating streaks may be seen through the cuticle. The internal larva becomes discernable four days after the egg is deposited and occupies the whole egg by the sixth or seventh day (Figures 45 and 46). Zrln alkaline haematin product originating from haemoglobin in the maternal blood meal has been demonstrated in eggs (Wigglesworth l943).;7

Eight days after the egg has been laid (temperature 30°C.), the larva emerges by alternate contractions of the anterior and posterior ends of the body (Figures 47 and 48) that ruptue the shell (Figure 49) and expose the larval dorsal surface. The shell may be completely detached in this manner, but usually remains on the ventral surface covering the mouthparts and legs (Figues 50

to 54). ZrUobling;7

When movements necessary for emergence are completed, the

larva becomes escent till the n hal molt. That larvae are

er Hatching-E53 dz-not ee has been conclusively established for over a century, though several recent textbooks on medical entomology report differently. All observers have noted the quiescent stage between hatching and molting, and have differed only in the time required for a larva to molt to a nymph. Davis (1947) found that this molt occurred only a few hours after emer. gence from the egg. Robinson (1942) and Jobling stated that larvae molt four days after emerging from the egg (minimum, three days; maximum, five days). The various early observers reported periods of from three to 23 days from hatching till the nymphal molt.

The sacculated gut of a newly hatched larva is filled with a red ish brown fluid Qdigglesworth 1943). The inference is that this is an alkaline haematin resulting from the ingestion of haennglobin by the mother tick._7

Before molting, the larva pales in color; its legs and mouth. parts shrink. Its skin becomes detached from that of the internal

[graphic][merged small][merged small][merged small][graphic]

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. Z'Jobling;7

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 nnlt, 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 sug. gest an interesting field for research. Z-See also the section on symbiotes and growth.promoting substances, page l77g;7

The interval between successive nymphal molts depeds 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 acconmndate 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 1953B). Argasids, on the other hand, feed much more rapidly and accommodate the volue 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 envi. ronments; 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 favor. able environment for obtaining a host in which argasid larvae and nmphs 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 suc. cessive instars.

Jobling noted that first instar nymphs feed on an average of 25 minutes (minimum thirteen ad 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 requiremnts for metamorphosis to the sexually mature adult stage, which demands more nourishment than simple nymphal development. These figures are in essential agreement with those of other stuents 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 host. (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

6 urge and may fertilize several females before feeding. The aver.

age male feeding time is sixteen minutes (maximum 42, minimum nine). After feeding they are less active and less eager for females an bury themselves in soil. Three or fou dayslater they again be. come active and seek females. Z-Job1ing;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. Z-Jobling_7

Frequently repeated remarks by workers of the 1905 to 1907 period that 0. moubata may molt after reaching adulthood um» questionably_were Based on erroneous identification of advanced nymphal stages as adults.

The minimum time necessary for O. moubata to complete its life cycle is 62 days for males and 79 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 four. teen 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 practical 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 between male and female life expectancy.

Parthenogenesis of Q. 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 1.8 indi

« PreviousContinue »