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Encyclopedia Britannica - Main :: THE-TOO |
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TICKS , the common name for Arachnida
body
i~Oeoo portion bears a chitinousplate, the rest of the integument being soft to admit of its distension by the blood which is imbibed in quantity by members of this sex. For a longer or shorter period of their lives ticks are parasitic upon vertebrate animals of various kinds; but although the belief that the bite of certain tropical species is poisonous has long been held by the natives of the countries they infest and has been recorded with corroborative evidence by European authors in books of travel, it is only of recent
Both the Argasidae and Ixodidae contain pathogenic species, of which the best known are the following: Ornithodoros monbata, belonging to the Argasidae, and called bibo in Uganda, monbata in Angola, and tam pan on the Zambezi, is widely distributed in tropical Africa from Uganda in the north to the Transvaal in the south. It was first recorded as poisonous by Livingstone and is now known to be the carrier of the Spirochaete of relapsing fever in man, known as tick fever. Although Europeans suffer from this disease far more severely than negroes, death seldom follows. The tick especially infests old huts and camping grounds and is nocturnal in habit, spending the day hidden in crevices of the walls or floor and coming out at night to feed upon the sleeping inmates. An allied species, O. turicala, occurs in Mexico
Texas
carrier of the Spirochaete causing spirillosis in fowls in Rio Janeiro, and also in New South Wale's whither it has been introduced with imported poultry. Argos persicus has been introduced in the same way into South Africa from Europe. As its name indicates it was first discovered in Persia, where the belief in the venomous nature of its bite to human beings is both widespread and historical. It is singular that the Argasidae; which are for the most part parasitic upon birds, contain the only species of ticks, especially O. monbata, wnicn are known to be seriously harmful to mankind; whereas amongst the Ixodidae no human pathogenic species has been ascertained to exist, although several forms have been proved to be highly destructive to domestic mammals of different species. The most important of these are the. following: Dermacentor reticulatus, a species widely distributed in ,Europe, Asia and America, infects dogs in Europe with the Haematozoon causing the disease known asbinary fever," and has been asserted to be answerable for the so-called spotted or tick fever in man in the Rocky Mountains. The same canine disease results in South Africa from the bite of Haemaphysalis leachi. Amblyomma hebraeum, the bent or variegated tick of the Cape Colonists, infects sheep with the Sporozoon causing " heart-water " sickness, and in Europe sheep are inoculated with the same disease by another tick, Rhipicephalus bursa. The so-called " coast fever " in cattle in South Africa is conveyed by two distinct species of the genus Rhipicephalus, namely by R. appendiculatus and R. simus, which are locally known respectively as the " brown tick " and the "black-pitted tick." Finally Margaropus annulatus, of which there are several geographical races, is the carrier of the germ causing the destructive cattle-disease variously known as " Texas
snakes
Mature males and females are found together upon the same host. Fertilization is effected by the male transferring spermatophores into the genital orifice of the female by means of his proboscis. The gorged and fertilized female quits her hold of the host, and falling to the ground, proceeds after a short delay tolayher eggs in some sheltered spot. The number of eggs laid is enormous, one computation putting it at twenty thousand. After oviposition, which may extend over several weeks, the female dies. The newly-hatched young has only three pairs of legs and is without sppiracu((ar and genital orifices. These young, or larvae as they are called, after the integument has hardened by exposure to the air, climb up the stalks of grain or herbage and cling with outstretched legs waiting for passing animals. They seize hold of the first that brushes by and crawling to a suitable place become engorged with "blood. ' , After about a week 's feeding they drop to the ground, lie dormant for a month, during which time they acquire their Mardi pair of legsand spiracles, and. moulting, emerge from their old skin as nymphs. Nymphs repeat the behaviour of the larvae, and finally moult into the adult, showing the generative orifice, which is the mark of maturity. The adult securesa host in the same way as the young. Both sexes feed upon blood; whereas the male alters but little in ap~.p~earance, the female becomes enormously distended..Cron the foregoing epitome which applies to many species, Rhipicephalus appendiculatus for example, it is evident that every individual tick has to find a host on three occasions, namely, as larva, nymph and adult. In R. bursa, however, the moult that a Rostrum or hypostome; b, b, Palpi; c, Capitulum; f, Abdomen, ments of leg; transforms the larva into the nymph takes place on the host, and in Margaropus annulatus the transformation of larva into nymph ands nymph into adult is effected without the temporary sojourn on the ground. Another species, Hyalomma aegyptium, the so-called camel-tick of Egypt and Arabia, is alleged to be parasitic only in its mature stage. Again, in Ornithodorus monbata, which is parasitic apparently only at night, the young does not hatch from the egg until it has attained the nymphal stage. It is an interesting and important fact that the newly hatched young of certain species, Margaropus annulatus for instance, before it has fed, if produced by a female carrying the germs of spirillosis, can infect healthy organisms with the disease. From this it is evident that the Spirochaetes pass directly from the mother tick to her offspring. Duration of life in ticks depends upon the conditions of their existence. Under favourable conditions, when food is obtainable, growth is rapid, the time from the hatching of the young until it reaches maturity and dies after oviposition being, for example, about eleven weeks in R. appendiculatus and only abont three weeks in M. annulatus. On the other hand, when food is not obtainable', life may be indefinitely prolonged if the tick be guarded from enemies and from atmospheric conditions inimical to existence. Examples of Ixodes vicinis have been kept for two years and three months with-out feeding, and specimens of Argas persicus were still alive after four years' starvation. (R. I. P.) End of Article: TICKS If you wish, you can link directly to this article.
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