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TICKS

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Originally appearing in Volume V26, Page 937 of the 1911 Encyclopedia Britannica.
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TICKS , the See also:

common name for See also:Arachnida (q.v.) belonging to the See also:order Acari, of which they constitute the two families, Ixodidae and Argasidae. Collectively the Ixodidae and Argasidae may be distinguished from other Acari by the presence of a median probe, armed with recurved See also:teeth, which project forwards beneath the mouth and between the palpi, and of a conspicuous spiracular See also:area above and usually behind the See also:base of the See also:fourth See also:leg on each See also:side. As compared with the See also:majority of Acari, ticks are of large See also:size, distended See also:female specimens of some of the See also:species measuring See also:half an See also:inch or more in length, while even the newly hatched See also:young can hardly be regarded as microscopical. The integument is tough, leathery or horny. The mouth parts consist of two small retractile mandibles, of a pair of See also:short palpi and of the toothed probe above mentioned. The palpi and probe or hypostome are attached to a movable sclerite or horny See also:plate called the capitulum. The capitulum, with its associated structures, is sometimes called the rostrum, whereas sometimes the See also:term rostrum is restricted to the hypostome alone. It is by means of the hypostome that ticks See also:pierce the integument and firmly adhere to the See also:host whose See also:blood they suck for See also:food. The two families Argasidae and Ixodidae. may be distinguished as follows. In the Argasidae the anterior portion of the dorsal See also:surface of the See also:body is extended forwards above the capitulum, so that this structure is concealed from above; the integument is fairly uniformly granular or coriaceous above and below; the palpi are See also:simple and unmodified; there is no sucker beneath the claws in the adult, and there is only a slight structural difference between the sexes. In the Ixodidae the capitulum is not overlapped by a forward See also:extension of the dorsal area, which is smooth and firmly chitinized either in front or all over; the palpi are usually modified, that is to say, their second and third segments are usually excavated internally to See also:form a sheath for the hypostome; there is a distinct sucker beneath the claws and the difference between the sexes is well marked, the See also:males having the dorsal integument thickly and continuously chitinized, whereas in the See also:females only its anterior i~Oeo°o portion bears a chitinous•plate, the See also:rest of the integument being soft to admit of its distension by the blood which is imbibed in quantity by members of this See also:sex. For a longer or shorter See also: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 See also:long been held by the natives of the countries they infest and has been recorded with corroborative See also:evidence by See also:European authors in books of travel, it is only of See also:recent years that accurate See also:information has been acquired of the See also:part played by these Arachnids in transmitting from one host to another protozoal blood-parasites which cause serious or fatal diseases to See also:man and other animals.

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 See also:

Uganda, monbata in See also:Angola, and See also:tam See also:pan on the See also:Zambezi, is widely distributed in tropical See also:Africa from Uganda in the See also:north to the See also:Transvaal in the See also:south. It was first recorded as poisonous by See also:Livingstone and is now known to be the See also:carrier of the Spirochaete of relapsing See also:fever in man, known as tick fever. Although Europeans suffer from this disease far more severely than negroes, See also:death seldom follows. The tick especially infests old huts and camping grounds and is nocturnal in See also:habit, spending the See also:day hidden in crevices of the walls or See also:floor and coming out at See also:night to feed upon the sleeping inmates. An allied species, O. turicala, occurs in See also:Mexico and See also:Texas, where it causes considerable destruction amongst poultry and is a pest to mankind as well. A similar See also:bad repute attaches to other species in different parts of South See also:America; while :Argas miniatus has been proved to be the carrier of the Spirochaete causing spirillosis in fowls in Rio Janeiro, and also in New South See also:Wale's whither it has been introduced with imported poultry. See also:Argos persicus has been introduced in the same way into South Africa from See also:Europe. As its name indicates it was first discovered in See also:Persia, where the belief in the venomous nature of its bite to human beings is both widespread and See also: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, See also:Asia and America, infects See also:dogs in Europe with the Haematozoon causing the disease known as binary 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 See also:bent or variegated tick of the Cape Colonists, infects See also:sheep with the Sporozoon causing " See also:heart-See also:water " sickness, and in Europe sheep are inoculated with the same disease by another tick, Rhipicephalus bursa.

The so-called " See also:

coast fever " in See also: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 " See also:brown tick " and the "See also:black-pitted tick." Finally Margaropus annulatus, of which there are several See also:geographical races, is the carrier of the germ causing the destructive cattle-disease variously known as " Texas " or " red water " fever in America, South Africa and See also:Australia. In the See also:United States alone the See also:annual pecuniary loss in cattle stock occasioned by the ravages of this tick disease was computed in t907 at one See also:hundred million dollars. With one or two possible exceptions, like Argas vespertilonis, which has only been obtained froth European bats, no species of tick is known to be confined to a particular host. The common sheep-tick (Ixodes vkinus) of See also:England, for example, infects cattle and clogs as well as sheep; and the pathogenetic Ixodidae above mentioned occur parasitically upon other mammals than those to which they convey the diseases specified. See also:Reptiles are infested as well as mammals, aril it is no uncommon thin; to find specimens of Ixodidae of various kinds adherent to tortoises, See also:snakes and lizards. Ticks belonging to the Ixodidae differ to a certain extent in their See also:life-histories. 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 See also: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 See also:weeks, the female See also: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 See also:

air, climb up the stalks of See also: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 See also:place become engorged with "blood. ' , After about a See also:week's feeding they drop to the ground, See also:lie dormant for a See also:month, during which See also:time they acquire their Mardi pair of legsand spiracles, and. moulting, emerge from their old skin as See also:nymphs. Nymphs repeat the behaviour of the larvae, and finally See also:moult into the adult, showing the generative orifice, which is the See also: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 See also: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, See also: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 See also:camel-tick of See also:Egypt and See also:Arabia, is alleged to be parasitic only in its mature See also:stage. Again, in Ornithodorus monbata, which is parasitic apparently only at night, the young does not See also:hatch from the See also: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 See also: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 See also:

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' See also:starvation. (R. I.

End of Article: TICKS

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