Family LIMNICHIDAE

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Family LIMNICHIDAE

Compiler and date details

31 December 1999 - Andrew A. Calder, CSIRO Entomology, Canberra, Australian Capital Territory, Australia

Introduction

Limnichids are small, oval, strongly convex beetles clothed with a decumbent or erect and occasionally pruinose vestiture. Their general habitus is reminiscent of byrrhids in the way the head and appendages are retracted when resting but are much less convex and possess an elongate prosternum and a distinct clypeal suture and clypeus. Worldwide the family consists of about 271 species in 33 genera and 4 subfamilies (Cephalobyrrhinae, Limnichinae, Thaumastodinae and Hyphalinae) (Wooldridge 1986; recent literature). The Hyphalinae was proposed by Britton (1971) to accomodate an unusual intertidal beetle from the Great Barrier Reef that was showing important affinities with both the Limnichidae and Elmidae. The described Australian fauna comprises four genera (Hyphalus Britton, Byrrhinus Motschulsky, Limnichus Dejean and Paralimnichus Delève) and 9 described species in two subfamilies: Limnichinae and Hyphalinae. This is an underestimate of the richness of our fauna and revisionary work on the group is badly needed. Paralimnichus was placed by Kasap & Crowson (1975) in the Chelonariidae. Wooldridge (1983), however, believed it was a true limnichid even though the partially divided eye, reduced fourth tarsal segment and lobed third tarsal segment distinguished it from all other limnichids. Paralimnichus is placed in the Limnichidae in this catalogue following Lawrence (1987) as it has most of the defining characteristics of this family.

The family was first considered by Erichson (1847) as a tribe in the Byrrhidae that was later raised to family rank by Thomson (1860). Subsequent classifications continued to treat the limnichids as either a subfamily or tribe of Byrrhidae and this was the case when Lea (1907, 1920) revised the Australian species. However, Hinton (1939) reinstated the Limnichidae to family status based on study of the structure of the digestive, reproductive and nervous systems and considered the limnichids to be nearest to Dryopidae.

The first Australian limnichid described was Limnichus australis collected in Tasmania (Erichson 1842). Macleay (1871) added another two species from Gayndah while Blackburn (1896) described another collected during the Horn Scientific Expedition to Central Australia in 1894. Lea (1920) was the next author to describe limnichids revising all the previously described species and adding another five new species. Another fifty years elapsed before the Australian limnichids were studied again. Britton (1971) erected a new subfamily for an unusual, fairly common intertidal beetle found on Heron Island. Mazur (1984) noted that the species Macleay described as Limnichus frontalis was in actual fact an histerid of the genus Stictostix Marseul and not a limnichid.

The biology and habits of the Australian limnichids are not well known. Many species occur in riparian situations alongside creeks and rivers, the adults often strongly attracted to light in large numbers. Immature stages are unknown for the Australian species, although Paulus (1970) has described the habits and larvae of two European species. These species are terrestrial and live in tunnels in the soil under layers of moss and algae on which they feed. Lawrence & Britton (1991, 1994) noted that unknown limnichid larvae have been found in sand, mud and leaf litter apparently feeding on decaying organic matter. The adults and larvae of Hyphalus insularis Britton appear to feed on the thin film of algae (Cyanophyta) that covers the surface of beach rock in the intertidal zone (Britton 1971). Britton also observed the larvae protruding tufts of gill-filaments from a caudal chamber that was obviously an adaptation for living underwater.

Diagnosis

Limnichid adults are very small to minute, rounded, convex, black beetles often covered with a greyish or yellowish pubescence or occasionally glabrous. Adults range in length from 0.9 to 4.5 mm. The head is strongly deflexed and fits into the prothorax. A large, well-defined frontoclypeal suture is present, delimiting a distinct clypeus. The antennae are slender, 11-segmented and either filiform or with a 3 or 4-segmented club. The pronotum has well defined, sharp lateral edges and the base is sinuate. The procoxae are transverse with exposed trochantins and the procoxal cavities are open behind both internally and externally. The prosternum is produced in front of the procoxae. The mesocoxae are widely separated and are partly closed by the mesepisternum. The hindwing has an open radial cell and no anal (wedge) cell. The abdomen has five visible sternites usually with the basal three connate or all five connate as in Paralimnichus. Abdominal segment 8 has functional spiracles. The legs of limnichids can be folded into cavities on their undersides, similar to byrrhids. The tarsi are usually simple and 5-segmented but are 4-segmented in Hyphalus and lobed in Paralimnichus, similar to some byrrhids (Lawrence 1982; Lawrence & Britton 1994).

Limnichid larvae are elongate, cylindrical, slightly curved and are distinguished by possessing a consolidated maxillolabial complex such that the cardines are contiguous, the maxillary articulating areas are absent and the postmentum is mostly joined to the stipites. The body is clothed with erect setae. The head is prognathous, partially retracted into the prothorax and has five or six stemmata each side. The mandible lacks both a mola process and prostheca. The antennae are short and 3-segmented. The legs are 5-segmented. Segment 9 lacks urogomphi and the spiracles are biforous (Lawrence 1991; Lawrence & Britton 1994).