Triplax frosti

Adults of this beetle are about 3.3-5.4 mm long and 1.6-2.9 mm wide. Their elongated body has smoothly curving sides and strongly shiny elytra. Like in many of their relatives, head and pronotum are reddish-orange, while the elytra are black. The scutellum is orange-yellow, lighter than the pronotum if anything, providing an easy distinguishing trait against T.thoracica (albeit only obvious with a magnifying glass). As usual for the members of the "breast-plated" species complex, T.frosti has orange legs and antenna bases and lacks a conspicuous margin to the forehead sides, which also form a well-rounded obtuse angle towards the mouth. The "stems" of its antennae are reddish-orange up to segment 7, with segment 8 dark chestnut brown, and the black segments 9-11 forming elongated "clubs". It can be distinguished from most other Triplax – even very close relatives – by having an all-orange underside, no distinct margin at the base of the elytra, and conspicuously expanded maxillary palps; T.thoracica is identical in these regards, but apart from the dark scutellum color differs in having the spaces between the lengthwise lines of large punctures on the elytra strewn with smaller punctures all the way from the elytra base (instead of smooth and unpunctuated at the base as in T.frosti). …

T.frosti is found in the USA and Canada from Nova Scotia to Virginia, west at least to Manitoba and Nebraska, and from there more sparsely northwest to British Columbia. Unlike most of its relatives, it avoids subtropical climates, ranging south only to Virginia and perhaps Tennessee. Altogether, though it is by no means rare, its range is not well understood; it appears to occur in an arc running southeast to west along the former massive prehistoric Lake Agassiz-Ojibway.

The northernmost limits are at least Upper Canard, Nova Scotia, Kouchibouguac National Park in New Brunswick, Duparquet, Quebec, Malachi, Ontario, Teulon, Manitoba, Redfield, Saskatchewan, and Edwand and Fort McMurray in Alberta. In the west, it ranges at least to Grand Forks, North Dakota (where it occurs on the university campus) and Sioux County, Nebraska. From its southern limit, there are few records; it occurs at least to Pennington Gap, Virginia, but its limits due west are essentially conjectural. This beetle is most rarely encountered outside late May to mid-September; most often, it is seen from June to early August. …