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In addition to African genera discussed herein, several others have been described from other continents, chiefly by Schulze. Many of these genera, however, meet with little general acceptance among contemporary specialists. A useful summary, including Schulze's and others' genera, may be found in Baker and Wharton (1952). In general, however, "textbook genera", as used by Neu mann and by Nuttall and Warburton, usually suffice and are of considerable practical and scientific value. They should be little tampered with if at all. Special groups of ticks may be readily categorized at the subgeneric level to provide a useful sounding board for their acceptance by specialists.
Ixodid ticks occur throughout the world wherever terrestrial vertebrates are found. They attack most land mammals and some marine forms. Some ixodids parasitize birds and reptiles, some feed on amphibians. Adults of few if any ixodids regard man as a host of predilection but many attack him in the absence of other available hosts. Larvae and nymphs are much more common and se rious posts of man than adults.
Morphologically, these ticks differ from argasids by the presence in all stages of a dorsal scutum. Ixodid mouthparts are always anterior and visible from above; the body is oval. Larvae have six legs; nymphs have eight legs and a female type scutum but lack both porose areas on the basis capituli and the genital aperture of the female. The female scutum covers only the anterior portion of the dorsum, the male scutum extends to the posterior margin of the body. Eyes may be present or ab sent. Nymphs and adults have a spiracular plate situated lateral ly, posterior of each hind leg. Palpi, clearly segmented and movable in argasids, are limited in ixodids to segments 2 and 3 and to a more or less well developed segment l; segment 4 is inserted ventrally into a pit of segment 3.
Biologically, the majority of ixodids display comparative uniformity within rather narrow limits. Exceptions are notorious but few. The number of species is small, somewhere around five hundred. Ixodids are highly adapted to parasitic life but one is prone to wonder how they have succeeded in the battle for survival. Their aimless wandering habit and dropping from hosts and their indiscriminate oviposition proves fatal to many indi
viduals, even though they survive long periods without food. Their extended life cycle subjects them to many adversities depending on season and the peregrinations of their hosts. The requirement of two or more kinds of hosts, often with divergent habits, limits their distribution to certain faunal areas. The comparatively. large size of females makes them subject to injury and tempting food for predatory arthropods and vertebrates. Certain pathogenic organisms, fungi, and hymenopterous parasites kill them. Ticks have little protection against an enemy except their ease of con cealment. They are particularly susceptible to attack during the lethargic premolting and the weak postmolting period.
On the other hand, many biological features enable ticks to survive especially well. They lay numerous eggs and withstand a comparatively wide temperature and humidity range with greater ease than many other arthropods. They survive for months or years without food and often gain considerable protection from the con cealed places in which they feed on the host. They frequently are offered a wide choice of appropriate hosts. Should they annoy the host, the animal is usually powerless to rid itself of the para site. Ticks' slow metabolic rate has certain advantages and the leathery, usually inconspicuous, integument offers some protection from living enemies, water, and chemicals. Parthenogenesis, pos_ sibly a common occurrence though not well studied, may aid survival.
The genus Ixodes, as a biological unit, shows much divergence from the usual patterns of ixodids. The biology of this genus must be considered independently but in relation to other genera. Review summaries frequently leave the student with the impression that the unique biological characters of Ixodes ticks are also characteristic of other ixodid genera.
The degree of host specificity in ixodid ticks varies from genus to genus or within certain subgroups of various genera. Generalizations on this point should be very carefully qualified.
Eggs are laid only once; promiscuously; at one time and place; and are always numerous, sometimes numbering over 10,000. Eggs hatch in from two weeks to several months, depending mostly on climatic factors.
Larvae are active and sometimes are easily visible as hun dreds or thousands of them rest on grass awaiting a host. Larvae often parasitize small mammals, birds, and reptiles, sometimes in their nests or dens. Only a few kinds choose larger animals for hosts. Thickness of the host skin is possibly an important factor in restricting larvae to smaller animals, Larvae seldom feed within a week after hatching.
After sone days of feeding, larvae molt to nymphs on the same host (two_host type of life cycle), or drop from the host and some time later molt to nymphs (three host type of life cycle) on the ground. Exceptions are the single host boophilid ticks (and a few others) that molt and remain on a single host during their lifetime. Nymphs that have molted on the ground seek a new host, sometimes the same type they fed on as larvae but, more commonly, animals similar to hosts preferred by adults. After several days of feeding, nymphs drop to the ground; be come quiescent for a time, then molt to adults. There is only a single instar in the nymphal stage (see footnote, page 141). Some species may undergo one or another of these variations in life cycle, depending on climatic, seasonal, or nutritional factors, most of which are still poorly evaluated.
There is evidence that in certain three-host species (in which larvae characteristically feed on small mammals) larvae that feed on a somewhat larger animal, such as a hare, may remain on the host, molt to nymphs, and continue feeding. They drop from the larval nymphal host only before molting to the adult stage.
Nymphs and females become tremendously engorged and new cuticle is developed during the rather slow feeding process in order to accommodate the huge volume of blood ingested. Males become only slightly distended if at all, apparently, as in argasid ticks, by stretching the integument although they may remain in a feeding position for months, even after the slaugh tered host's skin has been removed.
As reservoirs of a great variety of pathogenic organisms, ixodids are pre-eminently important, whether they act as vectors
As vectors, they transmit a greater variety of organisms
than any other arthropod. This variety is said to be greater on continental Africa than anywhere else in the world. Other injury, apparently due to toxins, in the form of tick paralysis may be locally important. Death, lameness, or serious debilitation of the ust by exsanguination or as the result of secondary infec tions at the site of attachment is not uncommon. Economic loss due to numerous punctures of animal hides by the mouthparts of large ticks is frequently reported.
Of about a hundred Amblyomma species in the world, some twenty occur in Africa and eight in the Sudan. The specific identity of most common African species appears to be settled and only in exceptional instances are specimens likely to be confused. One of the chief remaining taxonomic problems among common African amblyom mas is the A. marmoreum group, in which the range of species varian tion needs to be determined for several somewhat differing forms. Observations from the present study indicate need for further research on the relationship between A. variegatum and A. pomposum and suggest that the latter may be no more than a subspecies of the former. Recently a few workers have designated certain African populations by subspecific ranks that challenge further investiga. tion. Several West African species are known from so few specimens that their validity is questionable.
This genus has been the subject of an extensive review by Robinson (1926) comprising volume four of Nuttall's Monograph on Ticks. The African species have been keyed by Rageau and Vervent (1953).
The immature stages of most African amblyomas remain to be described with satisfactory criteria for distinguishing them.
Economically, two African species have thus far been shown to harbor or transmit human disease organisms. A. hebraeum is considered an important boutonneuse fever (tick" typhus) vector in South Africa and A. variegatum has been found naturally in fected with Q fever in French Equatorial Africa near the Sudan border. Several species are important transmitters of veterinary diseases, cause damage to animal skins, or debilitate animals through the volume of blood withdrawn or by initiating wounds that develop into ugly secondary sores,
Biologically, many gaps exist in our knowledge of African Amblyomma distribution, host-preferences, especially of immature stages, and life history. Birds are important immature-stage