Up to 7 mm long, pale green with mid- to dark brown head capsule with much darker mentum, mandible and occipital margin; strongly sclerotised subgenal margin. Eye spot single, compact, somewhat quadrate.
Well developed, about 1/3 head capsule length, non-retractile; 5 segmented, segments 2 always and 3 sometimes annulate, poorly differentiated but with both style and peg sensillum indicating apex of 2nd segment; blade subequal to segments 2-5 combined, accessory blade shorter than blade, apex of segment 1 also with small sensillum; without Lauterborn organs.
Frontoclypeal apotome present. SI, SII and SIII elongate and enlarged, somewhat sickle-shaped in lateral view, arising from distinct pedestals; SIV A rod-shaped, arising from pedestal, SIV B rod-shaped. Few simple chaetae, seta premandibularis and labral rod present. Pecten epipharyngis of 5 scales. Premandible absent.
With a small subapical tooth, longer apical tooth and 3-4 inner teeth, the innermost of which is variably separated from the mola. Seta subdentalis arising from ventral surface of mola, simple, short; seta interna arising from dorsal surface of mandible, with up to 25 simple branches.
With three median teeth (ventromentum) and 7 pairs of lateral teeth (dorsomentum). Median teeth subequal, lateral teeth diminishing in an even slope except for very small 2nd laterals. Ventromental plate not detectable. Setae submenti close to outer posterior margin of mentum.
Palp short, with all setae and sensilla short, with long chaetulae. Few serrate galear lamellae, without pecten galearis. Chaetae numerous, long and simple, with broader weakly serrate anterior lacinial chaeta. Appendix absent.
Anterior parapods fused basally, with separate crown of serrate claws. Spiracles present dorsally on abdominal segment VIII. Procercus darker pigmented posteriorly, somewhat conical, about as high as the basal width, with or without fine lateral seta, bearing apically 4 or 6-8 short setae. Posterior parapods well developed, with dark, simple claws of two distinct sizes - smaller in subapical ring.
Archaeochlus and Austrochlus larvae can be distinguished by the presence of distinct and functional posterior spiracles, rather squat hyaline and dark procerci bearing apically no more than 8 setae, and mentum with 6-7 pairs of lateral teeth and non-recessed middle tooth. Afrochlus larvae are similar but have a unicoloured dark procercus. Austrochlus larvae are distinguished only by their restriction to Australia; Archaeochlus is South African.
Archaeochlus occurs only in southern Africa: A. bicirratus and A. drakensbergensis are known only from their type localities at Qacha's Nek, between Lesotho and South Africa, close to 30S. A. biko has been collected only in Namibia between 23S and 26 46'S. An unreared, undescribed species A. 'ameib' is known from larva only, from a seepage at Ameib, Namibia.
The ecology of A. biko immature stages in Namibia is now known from larvae collected and reared from temporary pools in the bed of the Kuiseb River which had flowed in seasonal rainfall, and pools remained in the bed of the river in a canyon. Rains in the Namib and flow of the Kuiseb River is unpredictable from year to year.
The larvae of the other species, A. drakensbergensis and A. bicirratus, are confined to the sources of small temporary streams that flow over rock faces. At these sites there is an area of fine organic soil with vegetation, usually edged with mosses. The larvae are generally in the moss but, when there is a reasonable flow of water in the streams after recent rain, some move into the flowing water. Here they crawl about, apparently scraping organic matter from the rock surface, retreating to the moss area when the stream dries. In the moss, the larvae occupy moist interstitial spaces, feeding on organic detritus: no fresh vegetative matter was found in the guts. Larvae of A. bicirratus and A. drakensbergensis occur sympatrically in the sources of streams that flow over exposed rock faces high in the Drakensberg Escarpment. Metapneustic spiracles are probably beneficial in these hygropetric (thin water film) and moss habitats, where dissolved oxygen may be scarce. Pupation occurs in wet moss although some pupae may be washed into pools. Pupae are quiescent, moving only when disturbed, and do not show the active diving response characteristic of Trichotanypus, Lasiodiamesa and tanypodine pupae. Successful emergence can take place from pools.
Where adult emergence was observed, in nature and in the laboratory, adults crawled from the exuviae without taking flight. Disturbance will cause flight, but most observations suggest that a hesitant walk, reminiscent of Telmatogeton, is the normal activity. Swarming and mating have not been observed in nature, but A. bicirratus and A. drakensbergensis mated readily in small vials a few hours after emergence. Tactile stimulation appeared to initiate mating behaviour. Both sexes of a pair introduced into a vial become agitated after contact is made; grappling is followed by side-by-side alignment and mating ensues when the male abdomen is beneath that of the female, without genital rotation. Behaviour observed in the field (brundini) and in the laboratory (bicirratus and drakensbergensis) strongly suggests that succesful mating in Archaeochlus does not require swarms. Cross-species mating between drakensbergensis and bicirratus was initiated but completion was prevented apparently by a 'lock-and-key' barrier in the male genitalia. Pairs remained in copula for up to 2 minutes. If re-introduced and forced into contact some pairs mated again. Males of both species died within 24 hours of mating even when both sexes were maintained in the same humid regime. Free water was available and only females were observed drinking. Of three mated females kept in the humid regime, one laid viable eggs 46 hours after mating and then died. The remaining two lived for 5 days but experimental assessment of sexual differential mortality had to be stopped before the true potential longevity of the female under optimal conditions could be established.
How Archaeochlus survives extended waterless periods is unknown. Resistant eggs of otherwise aquatic Chironomidae have not been observed and we have no evidence that the larvae can withstand desiccation. The mandibles of female Archaeochlus closely resemble those of many insectivorous Ceratopogonidae and may indicate that survival of fertile females may be extended through feeding on invertebrates. The laciniae, although apparently weakly sclerotised also resemble those of insectivorous Ceratopogonidae though the lack of a 'fulcrum' on the mandible may be a significant morphological difference. The atrophy of female mouthparts in all other Chironomidae implies a retention of some functional significance in Archaeochlus, but until females are observed feeding the question will remain unresolved.
For both species from the Drakensberg Escarpment, survival through an extended dry season may be unnecessary. The rainy season lasts from October until March, the period of maximum mean and daily maximum temperatures. The driest period, austral winter from June until August, is also the period of snowfall (Schulze and McGee, 1978). Although this climate clearly presents Archaeochlus with low temperatures (including some daily maxima below freezing) there may be suitable habitat available all year in this area.