Freshwater Life - A field guide to the plants and animals of southern Africa

17	Text by Ansie Dippenaar-Schoeman and Jenny Day

Arachnids

Class Arachnida

The arachnids are a diverse group of ancient, eight-legged, mostly predatory invertebrates that include scorpions, spiders, mites and ticks. Most are terrestrial, but a few spiders and mites occur in fresh waters. Mouthparts take the form of a pair of chelicerae, ending in sharp fangs in the spiders and movable teeth in the mites.

Spiders

Order Araneae

A very successful group comprising over 43,000 air-breathing predatory species. Characterised by fused head and thorax connected to unsegmented abdomen by a narrow ‘waist’ or pedicel and by ability to produce silk.

Fish-eating spiders

Family Pisauridae

Large spiders, female body 11–27mm in length, male slightly smaller. Eyes arranged in two rows. Colour cryptic, carapace decorated with narrow white longitudinal stripes; abdomen with slightly broader white longitudinal stripes or spots. Three teeth on lower margin of chelicerae. Legs relatively long, angled slightly sideways, with numerous spines. The commonest spiders associated with fresh waters. These are wanderers, able to run rapidly over the water surface and overpower smaller insects. To catch larger prey, like small fish and tadpoles, they anchor their posterior legs to water plants, their front legs fanned out with tips resting lightly on water surface. They stay motionless, waiting for prey to pass within reach below, and then dive down to overpower it. Probably use vibrations of passing prey to locate their position. When resting, hide beneath vegetation around water ponds. Female carries egg sac below body.

1 Common fish-eating spider

Nilus margaritatus

Carapace and abdomen rich brown, with white longitudinal stripes; abdomen variable, sometimes with white spots or transverse bands; legs same colour as body. Size: Female’s body 11–22mm in length, male’s body 12–16mm. Biology: Found at freshwater pools and slow-moving streams with dense marginal vegetation. Overwinter (May–September) under rocks or logs near water. Adults are found November–March and juveniles April–May. RELATED SPECIES: Five other species of Nilus known from southern Africa. Has fairly wide distribution in eastern parts of region. Distinguished from other Nilus species by colour pattern and shape of genitalia.

Water wolf spiders

Family Lycosidae

Medium-sized, body usually 5–10mm in length. Cryptic, dull grey to almost black with white bands bordering cephalothorax; abdomen decorated with rows of small white spots. Eyes in three rows (with arrangement 4:2:2), all dark in colour, of unequal size: anterior row with four small eyes, second row with two large eyes and third row with two eyes of intermediate size, situated on front sides of carapace. Abdomen oval, covered with dense setae. Legs usually strong, of moderate length, with three claws. Semiaquatic, frequently found on banks or stony beds of rivers and ponds. Able to run with great agility on surface of water and can dive below water surface and stay underwater for up to an hour. Make flimsy silk retreats beneath leaves. Nine species known from the Afrotropical region, but only one from southern Africa.

2 South African water wolf spider

Pirata africana

Carapace slightly flattened in head region, pale brown to mottled dark grey, with narrow white band around edge; abdomen dark brown with two distinct rows of white spots; legs long, same colour as body, strongly mottled. Size: Female’s body 6–7mm in length, male’s body 5–6mm. Biology: Associated with water. Able to dive below water surface with air bubble (2) that forms around body. Females attach egg sacs to spinnerets (2a).

Long-jawed water spiders

Family Tetragnathidae

Medium-sized spiders recognised by elongated bodies and slender legs; most species straw-coloured or green. Chelicerae and fangs very long and strong (3), projecting forward in adults, more distinct in males. Along inside of long hooked chelicerae are two rows of large teeth; legs long and slender, without spines. Build orb-webs in tallish vegetation, usually near water, sometimes horizontally above water surface. Prey on flying insects, such as mosquitoes. Thirteen species found in southern Africa.

4 Boyd’s long-jawed water spider

Tetragnatha boydi

Elongate straw-coloured body, abdomen with distinct dark median stripe and with blunt tip above spinnerets. Legs long and slender, without spines. Size: Female’s and male’s bodies 6–7mm in length. Biology: Very common in southern Africa; occur widely from Africa to Mexico. Make orb-webs in grass near water at night and rest on vegetation during day. Egg sacs attached to vegetation. RELATED SPECIES: Differ from other species in genus by shape and number of teeth on chelicerae and shape of genitalia.

Water mites

Order Acarina ‘Hydracarina’

A few aquatic mites (hydracarines or hydrachnellae) are found in wetlands, one or two in rivers. Often brightly coloured; bodies globular and unsegmented; four pairs of legs, one pair of eyes. Most adults carnivorous, but at least one larval stage parasitic (usually on aquatic insects). Reproduction sexual. Larva resembles adult but with six legs. About 160 species known in southern Africa, 34 from Cape Floristic Region. Distribution records woefully inadequate, so degree of endemism unknown. Identification to species depends on microscopic details. Species listed merely give an impression of the group’s diversity.

1 Water mites

Family Torrenticolidae: Torrenticola species

Heavily sclerotised, often with a thickened ridge around the body. Size: Body 0.5–1mm in length. Biology: Live in springs and running waters worldwide, except in Antarctica. About 14 species known from southern Africa.

2 Water mites

Family Hydrachnidae: Hydrachna species

Pliable cuticle with papillae; often brightly coloured. Size: Relatively large, up to 3mm in length. Biology: Found mostly in ponds and other wetlands. Larvae often parasitise water boatmen (Hemiptera: Corixidae) (p.182). Four species are known from southern Africa, mostly in the northeastern parts of South Africa.

3 Water mites

Family Sperchonidae: Sperchon species

Cuticle soft, often with papillae; colour yellowish to brownish, often with darker spots. Size: 0.3–1mm in length. Biology: Genus widely distributed, mostly in ponds and other wetlands. Larvae are parasitic. Nine species known from southern Africa, mostly in the northeastern parts of South Africa and Zimbabwe.

4 Water mites

Family Arrenuridae: Arrenurus species

Fairly robust mites with a hard, sometimes patterned cuticle, protruding in places. Usually brown or orange but sometimes pale in colour. Size: Fairly large for mites, up to 2mm in length depending on species. Biology: At least some species feed mostly on ostracods. Larvae parasitise aquatic odonates or dipterans, including mosquito larvae. More than 20 species known from southern Africa.

5 Water mites

Family Hygrobatidae: Atractides amplexa

Robust, brownish, often speckled, with a hard patterned cuticle, often with a ridge circling the body from front to back and side to side. Size: Body about 0.8mm in length. Biology: Mostly riverine. One of 14 species of Atractides known from southern Africa.

6 Water mites

Family Limnesiidae: Limnesia species

Ovoid, often bright red, with smooth cuticle and large forward-pointing pedipalps. Eyes large, usually obvious. Size: Body up to 2mm in length, depending on species. Biology: Some species occur in running and others in standing waters. Eggs are laid in gelatinous masses; larvae parasitise dipterans, and young adults feed on the eggs and juveniles of the same species of fly. Nine species known from southern Africa.

7 Water mites

Family Unionicolidae: Neumania species

Body ovoid, often pale, with red fluid visible inside. Legs slender, bunched up under the body. Size: Variable, body up to about 1mm in length. Biology: Parasitic in mussels throughout their lives. About 20 species in five genera in southern Africa.

8 Beetle mites

Family Hydrozetidae: Hydrozetes species

Small, hard, amber to dark brown, teardrop-shaped. Size: Body 0.4mm or less in length. Biology: This genus belongs to a different subfamily from that housing the other water mites; not all species are aquatic and larvae are not parasitic. Eats mainly particles of detritus. Hydrozetes usually live in permanent ponds and streams. Several have been noted in acidic rivers and wetlands in South Africa, but only Hydrozetes capensis has been described.

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Arachnids

Class Arachnida