Ischyrus quadripunctatus, the four-spotted pleasing fungus beetle (colloquially known in North America as four-spotted fungus beetle), is a species of pleasing fungus beetle (family Erotylidae) native to the Americas. Within its family, this beetle is placed in subfamily Tritominae, or – in taxonomic arrangements that prefer a more comprehensive subfamily Erotylinae – in tribe Tritomini of the Erotylinae.
It is found all across the American mainland from southern Canada to Paraguay and northern Argentina, as well as on a few islands – in the Caribbean there are historical records from St. Vincent and late 20th-century records from nearby St. Lucia, both in the Windward Islands.Skelley, Paul E. (2009): Pleasing fungus beetles of the West Indies (Coleoptera: Erotylidae: Erotylinae). Insect.Mund. 82: 1-94. PDF fulltext
Description Adult Ischyrus quadripunctatus is smallish by standards of its family, i.e. a small beetle of about 0.2-0.35 in (5-9 mm) in body length and 0.08-0.15 in (2-4 mm) body width as adult. Seen from above, the body is an elongated egg-shape, barely blunter at the head than at the wingtips, widest at the well-rounded "shoulders", and with almost parallel sides. The carapace is covered in a somewhat indistinct microscopic reticulated relief that produces a dull, then silky, and finally glossy surface as it is worn down during the beetle's final stage of life.Boyle, Wilford Wayne (1956): A revision of the Erotylidae of America north of Mexico (Coleoptera). Bull. AMNH 110: 61-172. PDF fulltext
The adults' dark-and-light color pattern is composed of patches of black and pale orange. The underside and head is all black, except for spots on each side of abdomen and prothorax, and sometimes centrally on the snout; the palps are brownish, the antennae brown with the "club" at the tip black, not pale as in some relatives. The most obvious distinguishing mark which separates I. quadripunctatus from similar species is the dark pattern on the light pronotum – four spots forming a transverse band that is a bit concave towards the head, usually unconnected from each other and the pronotal edge. A black stripe may run along the forward edge of the pronotum between the eyes, and/or at the pronotal base (where it extends across the central half or a bit wider, and may connect to the central two of the four spots). The elytra have a light-colored epipleural fold and main surface, with usually three irregular zig-zagging dark bands – through the scutellum (often broken up into a large squareish central and two smaller oval "shoulder" spots), behind half-length (almost one-fourth the elytral length except in the more extensively black center which extends as a narrow stripe all the way to the wingtip, and notched at the forward and aft margins), and at the very tip (narrow in the center, more widely extending forwards on the sides for 20-25% of the elytral length, and connected to the central band with a very narrow black stripe along the epipleura). The legs are black except for the brown tarsi, and in some populations also have light bands. The black markings, as well as the darkness of the orange color, also vary geographically, as well as between individuals.
Head The head has long antennae (by Erotylidae standards), and prominent but not outsized compound eyes with large ommatidia; the gap between the compound eyes measures 2-2.5 times the compound eye diameter across; it is somewhat narrowed at the rear and a bit wider to the front. The punctures on top of the head are as wide as one ommatidium, and each puncture is separated by 1-3 times their diameter from the next. On the epistome, the punctures are smaller, half to three-quarters the size of those on the top of the head, and separated by the same distance or slightly less. The ocular grooves barely reach the rear rim of the antennae bases, the epistome is thus unmargined; the "cheek" lobes below the compound eyes are small, round, and sharp-edged. Males have file-like structures that, when they wiggles their head, rub against the chitin and produce a squeaking sound; however, these files are short and do not seem to be very sophisticated compared to the corresponding structure in some other pleasing fungus beetles. The clypeus has no margin along the smooth front edge, and the mouth-cavity sides are flattened lobes.Deelder, C.L. (1942): Revision of the Erotylidae (Coleoptera) of the Leiden Museum. Zool. Meded. 24(3): 49-115. Fulltext
The antennae are about half to two-thirds as long as the pronotum is wide at the base; their attachments do not fully reach to the base of the pronotum, and their segments are characteristically shaped as follows: Segment 2 is rounded and usually a bit longer than wide, while segment 3 is three times as long as wide and in length almost equals the next three segments taken together. Segments 4 to 8 are longer than wide, with the eighth segment being angled at the tipward end and barely wider than the seventh; the clubbed portion of the antenna tip is covered in fine hairs and only starts at segment 9, which is a bit wider than it is long. Segments 10-11 are asymmetrical, the tenth angled at the base, the eleventh transverse; this trait distinguishes the four-spotted fungus beetle from many similar species. Altogether, the antennal club is a bit less than half as wide as it is long.
As for the mouthparts, the mentum has a broad triangular plate, notably shorter than it is wide at the base. The galea and lacinia of the maxilla resemble those of relatives Pseudischyrus and Tritoma; those of Triplax are also similar but have two small curved teeth at the lacinia tip. The end segment of the maxillary palps forms a wide asymmetric triangle, which is unusually narrow for the genus – slightly longer than it is wide. Its size is relatively modest, barely larger than the end segment of the labial palps. The latter is an asymmetric triangle, expanded medially, and slightly longer than it is wide; it is three-quarters the width of the maxillary palp end segment. The tip is cut-off and carries a tiny brush.
Thorax and wings
The sides of the pronotum are smoothly curved towards the compound eyes, with the base of the pronotum more than half again as wide as the headward edge, resulting in a well-rounded trapeze-shape. Behind each compound eye, there is a small bead-like margin on the pronotal edge, which is otherwise unadorned, and entirely missing between the eyes and on the entire rear side. The pronotum is covered in punctures the same size and distance as those on top of the head, a bit more densely strewn towards the sides. At each side of the pronotal base's centerline, there is a small cluster of a few larger punctures in a shallow dimple, and in the pronotum corners there are small and insoncspicuous pores. The prosternum is twice as long as its with between the foreleg attachments, flat, with a shallowly concave base; is usually pinched and adorned with a thin bead-like margin on the forward edge, which is also strongly compressed in the center, forming a pitcher-like lip. The prosternum also has a weak lengthwise keel; strongly impressed, straight and barely diverging broken lines run rearwards from the inside of the forelegs' coxal attachment.Skelley, Paul E. (2020): A new Central American genus of pleasing fungus beetles (Coleoptera: Erotylidae) from the Ischyrus-Megischyrus complex. Insect. Mund. 804: 1–11. PDF fulltext
The mesosternum is half again as wide as it is long, slightly wavy at the posterior margin, and has a more strongly curved base with a central lobe. It has some fairly indistinct small to mid-sized punctuations, and inside of the coxal attachments run straight to slightly bent lines. The scutellum is bluntly pentagonal and half as long as it is wide. I. quadripunctatus has a fully functional set of two pairs of wings. The hardened forward ones (elytra) at their widest point are wider than the prothorax where it meets the elytra, and unmargined even at the base. They bears lines of punctures sized like those on pronotum and top of the head, which extend lengthwise for almost the entire elytra; between those punctured lines, the elytra have very fine punctures which are just one-quarter to one-fifth the diameter of the punctures forming the lines. The distance from meso- to metacoxae is almost twice the distance between the mesocoxae. The long metasternum has numerous fine punctures in the centerline, and a scarce sprinkling of larger but shallow punctures towards the sides; the coxal lines are short and reach only one-quarter to the metasternum's hindward rear angles.
Abdomen and legs
The abdomen has pronounced metacoxal lines which are often unbroken and in some individuals extent halfway to the rear edge. The abdominal segments are punctuated like the metasternum, with many fine punctures in the center and fewer coarser ones towards the sides. The intercoxal process of the abdomen is somewhat widened and slightly curved, and unlike in some other Erotylidae, males and females do not have conspicuously different external abdominal structures. The attachments of each leg pair are neither particularly close or conspicuously far away. The femora of the mid- and hindlegs are angular in cross-section, and have a conspicuous sharp-edged bead on the posterior margin. The tibiae are weakly curved and do not widen conspicuously towards the tips. The first three segments of the tarsi are increasingly widened; segment 4 is reduced, attached to the upper centerside of segment 3, and rigidly joined to segment 5.
The male genitalia in general layout resemble those of many species in the closely related genera Triplax and Tritoma. The median strut of the aedeagus is barely longer than the median lobe. The latter is notably curved, cut-off at the tip, and slightly constricted by a dimple just below the tip. The long internal sac protrudes anteriorly through the median hole with almost the same length as the median lobe, and has a small anterior muscle attachment – when seen from dorsally, this appears as two black dots – but no noticeable sclerotized structures. The ejaculatory duct is enclosed and strongly sclerotized, sticking tightly to the ventral wall of the internal sac; it reaches to the median hole, and in its forward half or even beyond is strongly flattened and widened horizontally. The flagellum is hair-like, long and narrow, with a flattened ribbon-like basal half, and the tegminal arms bend abruptly upwards where they emerge from the basal piece, and have lobes on the back. The female genitalia in overall appearance are somewhat similar to those of Cypherotylus and Megalodacne, which among the Erotylidae are not closely related to Ischyrus however. A striking difference to those two is the large seminal receptacle of Ischyrus females, similar to that of the more closely related Aporotritoma pulchra in having a swollen, U-shaped and much sclerotized extension. The outside of abdominal segment 9 is covered with a dense layer of tiny prickles, giving it a dark color. The genital tube has a conspicuously large extension; when everted, it stretches the 9th abdominal segment to twice the usual length or even more.
Larva The stubby-legged sausage-shaped larva is mostly pale greyish in color. The head has dark grey patches separated by pale bands along the forehead seam and the "cheeks". The back of the first thorax segment is dark grey with a pale side arc, creating an eyespot illusion. The rest of the body has dark grey bands across the back on each segment. The back of each body segment except the last has tubercles at the sides and in the middle, which get larger and project forked spikes toward the rear; on the abdomen they are accompanied by weakly raised spiracles. The back of the 9th abdominal segment has conspicuous stubby and well-separated urogomphi.
Distribution and ecology