The population from Liberia reported as Aphasmatylenchus nigeriensis by Vovlas et al. (1991) is considered to be a new species and is described and illustrated here as A. liberiensis n. sp. Its primary distinguishing characteristics are the unsectored first lip annulus, the absence of sperm in the female genital tract, and the absence of males. Light and scanning electron microscopy observations were conducted on West African Aphasmatylenchus populations and the head patterns for all members of the genus are illustrated. Variability of external morphological characteristics was very low and the genus appears homogeneous in general morphology with the exception of the configuration of the first lip annulus. Fasciculi were observed for the first time in A. nigeriensis from south-western Ivory Coast, and filiform appendices similar to the appendices previously identified in the genus Radopholus were detected on the edge of the anterior cloacal lip of A. straturatus and A. variabilis; the presence of nerves in these structures cannot be ascertained with SEM. (C) Elsevier - ORSTOM.

The body wall ultrastructure of H. oryzae and H. spinicaudata is described. The cuticle consists of three major zones: a cortical zone with two layers (an outer trilaminate layer and an inner granular layer), a median zone represented by a vacuolar layer, and a basal zone consisting of a striated layer. Beneath the lateral fields, the fine structure of the cortical zone is unchanged, whereas the median zone is composed of four layers (two vacuolar layers separated by two granular layers) instead of a single vacuolar layer, and the basal zone is made of two thick fibrillar layers instead of a single striated layer. Examination of the muscle fields shows that the so-called ''Thornean cells'' of Sher (1968) correspond to an inflated sarcoplasmic portion of muscle cells with large amounts of glycogen, as revealed by cytochemical studies. This unusual accumulation of glycogen is discussed.

Observations are reported on the fine structure of the cuticle of Paratrichodorus minor, P. nanus, P. rhodesiensis and Trichodorus eburneus. The ultrastructure of the cuticle was similar in Paratrichodorus and Trichodorus species; six layers were observed, the innermost layer consisting of three multilaminate units. The anatomy of the cuticle in Trichodorids was quite different from previous observations in other orders of Adenophorea. Two sets of intracuticular canals were observed as well as a peculiar type of junction between cuticle and somatic muscles. These results confirm previous studies on the anatomy of trichodorids and support the withdrawal of the family Trichodoridae from Dorylaimida. Siddiqi's (1983) classification of the trichodorids under the order Triplonchida should be considered with attention.

The type-species of the genus Triversus, T. annulatus, originating from West Africa was studied by scanning electron microscopy. The head morphology and the coarse body annulation constitute two morphological features which appear characteristic of this genus among the Telotylenchinae. The relationships with the other species of this genus is discussed, Tylenchorhynchus triglyphus is transferred to the genus Triversus, the definition of which is amended to include conical or cylindroid female tails and non-trilobed bursae.

Aorolaimus macbethi was found in low numbers in the soils of the semi-arid tropics of West Africa up to 80 cm deep. Soil temperature, soil moisture and host plants have a significant effect on its multiplication rate. The nematode is able to enter anhydrobiosis during the dry season and is characterized by a low multiplication rate. It was slightly pathogenic to millet at low inoculum levels.

Nematode population densities increased during the rainy season, root population densities being always below 25 % of the total population, and decreased during the dry season during which nematodes enter anhydrobiosis; they varied according to the geographical location of sampling, rainfall and soil moisture (these three parameters being Linked), and to host plant. As the year proceeds, nematodes are mainly located in the upper layers of the soil (0-20 cm deep). Multiplication rates were low, generally less than 25, survival rates being higher, from 10 to 65 %; these rates varied with the year of observation and with the host plant. The record of significant and positive relationships between initial population densities of the nematode and peanut yield did not confirm previous data on the pathogenicity of the nematode to peanut.

sis were srudied in the laboratory for S. sorghi and ROlylenchulusren~formis. The results showed that the highest multiplication rates of both species were recorded at relatively low soil temperarure and high soil moisrure. Both species were able to enter anhydrobiosis during the dry season, with survival rates of 20-40 %. S. sorghi parasitized only wild and cropped cereals. During the dry season, it was under hydrobiotic conditions at depth in cropped soils and under anhydrobiosis in the upper layers of the soils under fal1ow. The restricted distribution of R. reniformis in the vegetable crops under irrigation might be explained by its narrow host range; ail other ecological characteristics were the same as for S. sorghi.

Soil temperature, soil moisture and host plant significantly affected the multiplication rate of Aphasmatylenchus straturatus and A. valiabilis. These two nematodes appeared unable to enter anhydrobiosis and to survive in dry soil during the dry season. A. straturatus is pathogenic to cowpea and peanut at inoculum levels above 1000 and 2000 nematodes per plant respectively.; A. variabilis is non pathogenic to miller at the inoculum levels tested. The distribution of the three known species of the genus Aphasmatylenchus in West Africa is discussed.

Laboratory studies showed that trichodorids from the semi-arid region of West Africa need relatively low soil temperature levels for reproduction. The nematodes are not able to enter anhydrobiosis. Their localization at depth is probably a response to desiccation of the soil during the dry season and to high soil temperatures in the upper layers of the soil during the rainy season. Laboratory experiments showed that P. minor is pathogenic to sorghum, cowpea and eggplant, P. nanus to millet and sorghum, and P. rhodesiensis to peanut and sorghum. In each case the weight of shoots and roots was reduced by the nematodes but otherwise there were no obvious disease symptoms.

Summary- The geographical distribution and field host plants, population dynamics and vertical distribution were studied for the nematode Helicoly/enchus dihysr.era. The factors influencing the multiplication rate and the effects of anhydrobiosis were studied for H. dihysr.era and H. mullicinclus in the laboratory and showed that absence of H. mullicinClus from semi-arid tropics of West Africa might be explained by the effects ofhigh soil temperature on multiplication rate and low survival rate after soil desiccation during the dry season. The field and laboratory observations showed that anhydrobiosis might induce a strong effeet on the physiology of H. dihyslera, nematode numbers being higher after soil desiccation during the dry season. H. dihyslera appeared pathogenic to peanut and millet.

The distribution of Scutellonema clathricaudatium in West Africa, the biotic and abiotic factors affecting its multiplication rate and its pathogenicity to peanut and millet are studied. This species appeared to be adapted to the climatic conditions of both semi-arid and rainy tropics of West Africa. No pathogenic effects of the nematode were recorded on peanut and millet. Since males have occurred during the laboratory experiments and ''vaginal glands'' (in fact the constrictor muscle of the vagina) are present in this species, the taxonomic relationships between three closely related species, Scutellonema bradys, S. cavenessi and S. clathricaudatum, need to be re-examined.

The biology of S. cavenessi was affected by soil temperature, soil moisture, host plant, present and previous host plants, length of the active contact between nematode and host plant, age of the nematode, and the previous physiological state of the nematode (hydrobiotic vs anhydrobiotic) before inoculation. This species appeared to be well adapted to the ecological conditions of the semi-arid tropics of West Africa : it is polyphagous, reproducing at relatively high soil temperature (32-34 degrees C) and low soil moisture (> 5%), with a life cycle of 100 days corresponding to the duration of the rainy season. Anhydrobiosis affected the physiology of the nematode in its reproductive activity. These biological characteristics might explain the geographical distribution according to the soil temperature and the variations of population densities according to the rainfall in the semi-arid tropics of West Africa. The study of the population dynamics showed that two periods occurred, the first corresponding to the reproductive activity of nematodes after anhydrobiosis at the end of the dry season, the second to that of nematodes produced from this first period. No pathogenic effects of the nematode to pluvial crops were recorded; this confirms previous field observations, showing that pathogenicity of S. cavenessi appeared doubtful.

The geographical distribution, population dynamics and vertical distribution in the soil were studied for the nematode Hoplolaimus pararobustus. The factors influencing the multiplication and survival rates of H. pararobustus and H. seinhorsti were studied in the laboratory. Both species appeared adapted to the ecological conditions of the semi-arid tropics of West Africa. The absence of H. seinhorsti from these ecological zones remains unexplained; the possibility of the loss of reproduction capacity after anhydrobiosis is suggested A the inoculum levels tested, these two species did not have any pathogenic effects on the plants with the exception of H. pararobustus which induced a reduction of root fresh weight for cowpea.

Two species of the genus Trichotylenchus from the semi-arid region of West Africa, T. falciformis, type-species, and T. palustris, previously belonging to the genus Uliginotylenchus, were studied under the scanning electron microscope. Eight characters are common to both species: (i) head continuous with body contour; (ii) four to six cephalic annules present; (iii) first cephalic annule ovoid, laterally elongate and fused with the second cephalic annule at the level of the amphid; (iv) lateral cephalic sectors absent; (v) submedian cephalic sectors dorsally and ventrally fused; (vi) amphidial apertures elongate, ovoid and with longer axis obliquely dorso-ventrally directed; (vii) lateral fields areolated with three incisures; and (viii) spicules with an indentation or a small trapezoidal process on the dorsal side and a short rounded process on the lateral side, both being close to the distal extremity. The synonymy of Uliginotylenchus and Trichotylenchus is confirmed. The definition of Trichotylenchus is amended.

Summary- The bibliographie analysis of descriptions of species and populations within the genus Aorolaimus permined evaluation of the intraspecific variability of 39 characters used for species diagnosis and relationships. Three characters (areolations at phasmid level in the lateral fields, position of the anterior phasmid and presence of males) are weil defined and present a low intraspecific variability and a sufficient intrageneric variability for grouping the species in six groups inside which other morphological or biometrical characters did not permit an obvious separation of aU the species. Several species in the genus appear doubtful and their study under light microscopy is a preliminary before taxonominal decision. Peltamigralus regularis Siddiqi, 1985 is transferred to the genus Aorolaimus as Aorolaimus regularis (Siddiqi, 1985) n. comb. A. brevicaudalus, A. conicori, A. lriliceus are considered as junior synonyms of A. longislylus and A. lfwmei as junior synonym of A. pcu:hyurus.

The bibliographic analysis of descriptions of species and populations within the genus Aorolaimus permitted evaluation of the intraspecific variability of 39 characters used for species diagnosis and relationships. Three characters (areolations at phasmid level in the lateral fields, position of the anterior phasmid and presence of males) are well defined and present a low intraspecific variability and a sufficient intrageneric variability for grouping the species in six groups inside which other morphological or biometrical characters did not permit an obvious separation of all the species. Several species in the genus appear doubtful and their study under light microscopy is a preliminary before taxonominal decision. Peltamigratus regularis Siddiqi, 1985 is transferred to the genus Aorolaimus as Aorolaimus regularis (Siddiqi, 1985) n. comb. A. brevicaudatus, A. conicori, A. triticeus are considered as junior synonyms of A. longistylus and A. thornei as junior synonym of A. pachyurus.

Des etudes ont ete menees sur la dissemination eolienne des nematodes dans le bassin arachidier du Senegal en piegeant le sable et les poussieres dans des pots affleurant la surface du sol. Les quantites de sable et de poussieres recuperees ne dependent pas de la localisation geographique des sites d'etude ni de la culture en place pendant la saison des pluies precedente; elles augmentent regulierement au cours de la saison seche, probablement en relation avec la destruction du couvert vegetal sec par le betail. Les populations de nematodes sont caracterisees par l'abondance des nematodes mycophages et bacteriophages. Les Dorylaimida et les nematodes phytoparasites de l'ordre des Tylenchida sont peu nombreux

The fine structure of the cuticle of Aorolaimus macbethi, Aphasmatylenchus straturatus, A. variabilis, Helicotylenchus dihystera, H. multicinctus, Hoplolaimus pararobustus, H. seinhorsti and Pararotylenchus hopperi is described. Six layers are identified in Aphasmatylenchus, Helicotylenchus and Pararotylenchus species vs seven in Hoplolaimus and Aorolaimus species. The ultrastructure of the five outer layers of the cuticle is identical in all species and consists of an external cortex, an internal cortex, a granular or fibrillar layer with electron dense ovoid structures, a striated layer and an electron dense fibrillar layer; the basal zone of the cuticle consists of a thin electron-lucent layer in Helicotylenchus and Pararotylenchus species and a thick electron-lucent layer representing half of the total thickness of the cuticle in Aphasmatylenchus species; in Hoplolaimus and Aorolaimus species, two layers are present: a thin electron-dense layer consisting of densely packed osmophilic corpuscules and a thick electron-lucent layer. Intracuticular canals previously described in other genera in the subfamily occur in all species studied and may be considered constant in Hoplolaiminae; observations on lateral fields in cross section reveal a variability of their shape and of the deepness of incisures. Three major groups in the subfamily may be distinguished based on ultrastructure and relative thickness of the layers of the cuticle: i) Hoplolaimus, Scutellonema and Aorolaimus, ii) Pararotylenchus and Helicotylenchus, iii) Aphasmatylenchus and Rotylenchus. Cuticle ultrastructure in Hoplolaiminae appears totally different from that observed in other taxonomic groups of the order Tylenchida.

Paralongidorus bullatus was found deep in sandy soils of Niger and Senegal in the Sahelian zone of West Africa. Laboratory studies showed that reproduction is favored by high soil temperatures (32-36 degrees C) and moderate soil moisture (7-11% water content). Males were found for the first time. Cowpea, miller, and peanut were good hosts, whereas sorghum was a poor host. Vermiform stages were not able to enter anhydrobiosis and the nematode probably survives the dry season in the egg stage or by migrating deep into the soil. Paralongidorus bullatus was pathogenic to the most important crops of these Sahelian countries (millet, peanut, and sorghum) and it is suspected to be a major pathogen of peanut in semi-arid regions of West Africa.