Triplax

Triplax
Adult of type species Triplax russica
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Pancrustacea
Class:	Insecta
Order:	Coleoptera
Suborder:	Polyphaga
Infraorder:	Cucujiformia
Family:	Erotylidae
Subfamily:	Erotylinae
Genus:	Triplax Herbst, 1793
Type species
Silpha russica Linnaeus, 1758
Synonyms
Ogcotriplax Heller, 1920 Paratritoma Gorham, 1888 Platichna C.G. Thomson, 1863 Platychna Kuhnt, 1909 Pseudotriplax Heller, 1920 Tritomapara Alvarenga, 1970 "Tritomarapa" (lapsus)

Triplax is a large genus of small beetles in the pleasing fungus beetle family Erotylidae. They are found almost anywhere where fungi grow, but seem to be less common (though still widespread) in the tropics and absent from Australasia and the Pacific region. They are placed in subfamily Tritominae, or – in taxonomic arrangements that prefer a more comprehensive subfamily Erotylinae – in tribe Tritomini of the Erotylinae.¹²

Adult Triplax beetles are smallish by pleasing fungus beetle standards, a quarter-inch (6-7 mm) long at most, with the smallest individuals of the smallest species less than one-tenth of an inch in length (2.3 mm). They are usually around half as wide as they are long, and have a rather flat back. Usually pill-shaped when seen from above, with head and wingtips evenly rounded and the sides almost parallel, some species have a more tapering shape, with slightly pointed wingtips and a blunter head. From the closely related genus Tritoma, Triplax can be distinguished by a flatter and more elongated body, with narrower elytra, and shortened lines on the prosternum which do not extend forward of the attachment points of the forelegs.³⁴

They have the typical black-and-reddish colors found in many Erotylidae. Specifically, most Triplax have black elytra and a contrasting lighter or darker reddish-blown pronotum and head. In some species, the head is also black, in others the entire pattern is reversed; some have reddish patches or bands across the elytra.¹⁴

Head
The head has small compound eyes with tiny ommatidia, indicating a diurnal or crepuscular lifestyle, but with overall rather little reliance on eyesight. Male Triplax adults possess file-like structures at the sides of the neck attachment, which, when the beetle wiggles its head, rub against the chitin and produce a squeaking sound; these structures are developed similarly to those of, e.g., Dacne pubescens, and in Triplax species with basally margined elytra they are also present in females. The epistome has a narrow broken margin around the tip. The "cheek" lobes below the compound eyes are well-developed, and very large in some species. The antenna length is usually more than half of the pronotal base width, with antenna segment 3 always shorter than the segments 4 and 5 together, and a "clubbed" antenna tip which is at least twice as long as it is wide.⁴

As for the mouthparts, the galea and lacinia of the maxilla resemble those of the related genus Tritoma, the galea being almost triangular, with a tip carrying sparse bristles at the outer side and a dense pack of bristles on the inner side. The lacinia is also covered in dense bristles, and – unlike in Tritoma – at the tip has two small black incurved teeth, set so closely together that they may appear as a single tooth. The maxillary palps are more or less strongly curved inwards and have a first segment that is at least as long as segments 2 and 3 together. Their end segment is somewhat triangular, abruptly cut-off, and at least about twice as wide as it is long; in some species its length is only 20% of its width. Its tip carries a small brush, which usually is well visible, and consists of a single row of microscopic fleshy finger-like cylinders, which on their sides have many small bristles. In some species, this brush is reduced in size, usually hidden within a slit at the palp tip, and visible only at the sides. The labium has a mentum that is fairly narrow and strongly varies from being very much reduced and triangular to being well-developed and pentagonal between species; however, even between the individuals of a Triplax species, size and shape of the mentum are somewhat variable. The labial palps end in a more or less widened hatchet-shaped segment, which is sparsely covered in short bristles, with a longer strong bristle at the outer side. At its tip, it has a tiny brush, much smaller than that at the end of the maxillary palps, and composed of a single row of short smooth nubs.⁴

Body, wings and legs
The prothorax is almost as wide at the base as the wings at their widest point. The pronotum is fairly wide and usually trapezoid-shaped, with the rear edge around half to three-quarters again as long as the forward edge. A thin margin runs around the sides and rear of the pronotum, but is absent from the front between the compound eyes. In each corner of the pronotum, there is a sizeable navel-shaped pore, which may attain a conspicuously large size in some species. The prosternum may have lengthwise lines at the center margin of the leg attachments extending to the rear edge of the prosternum, but at most a tiny bit forward; more usually though, these lines are very weakly impressed or entirely missing. On the metasternum, the lines between the leg attachments are likewise absent or vestigial. On the base of the abdomen, lines between the hindleg attachments are usually present, with the tip of the abdomen protruding as a narrow bent process between the hindleg attachments.⁴

The scutellum is bluntly pentagonal, in some species almost heart-shaped. The elytra are ornamented with regular lines of small punctuations; they have a distinct margin at the base, except in those species whose females lack stridulation organs on the head. The attachments of each leg pair are set narrowly together. The tibiae are more or less widened towards the tip depending on species (often, but not reliably, slightly wider in males), and have a bluntly rounded upper/outer edge. Segment 4 of the tarsi is reduced, attached to the upper centerside of segment 3, and firmly joined to the base segment 5.⁴

Triplax beetles are almost found worldwide, except in the Australian region, but are most common in the temperate regions of the Northern Hemisphere. Almost half of the known diversity is found in Europe and west-central Asia, whicle just a dozen species is found in East and Southeast Asia. Roughly one-fourth of the described species occurs in Africa, and about one-fifth in the Americas (with 75% of these restricted to North America). Thus, the genus probably originated in the general Pontocaspian region.⁴

They are woodland species, as usual for their family, and live near the shelf and tree-growing gilled fungi on which they feed. Their larval development is little-studied in detail, even though their larvae do not seem to be hard to observe, being regularly noted in passing in the literature. Active whenever the ground is not frozen, a particular preference for Pleurotus (Agaricales: Pleurotaceae) gilled and Inonotus (Hymenochaetales: Polyporaceae) shelf fungi was noted; at least in North America, most species of Triplax feed exclusively or nearly on one or the other of these fungus genera. While most pleasing fungus beetle species are not very social and tend to keep to their own kind, Triplax are noted for their gregarious habits – not rarely, dozens of adults can be found eating together on a single small mushroom. They also often occur in mixed-species groups; some, e.g. T. thoracica, habitually associate with several closely related species, suggesting some form of niche partitioning.¹³⁴

The following list includes species which some recent authors separate in genera Paratritoma and Tritomapara, elevating Triplax to one of the larger erotylid genera, with about 100 species overall: ²⁵⁶

Triplax macra and Triplax thoracica seem to form cryptic species complexes with their respective closest relatives.

In addition to the living species, a number of 30-35 million year old fossils have been assigned to Triplax; most were at first placed elsewhere:⁵

• †Triplax contienensis Alekseev, 2014 (Rupelian)
• †Triplax diluviana (Wickham, 1914) (Priabonian)
• †Triplax materna (Wickham, 1912) (Priabonian)
• †Triplax petrefacta (Wickham, 1916) (Priabonian)
• †Triplax submersa (Wickham, 1912) (Priabonian)