The Melolonthinae: Chafers and June Bugs

Within the bewildering array that is beetle diversity, one of the more readily recognisable groups is the Scarabaeoidea, the assemblage that includes dung beetles (which, as it happens, are what I currently spend most of my days looking at) and related forms. Members of this group are easily distinguished from other beetles by their distinctive antennae, ending in an asymmetrical club with segments extending to one side like a set of fingers. Several families, many of them further subdivided into subfamilies, are currently recognised within the scarabaeoids. One of the most commonly encountered scarabaeoid subgroups is the subfamily Melolonthinae, commonly known as the chafers.

Green scarab beetles Diphucephala sp., a common genus of day-flying melolonthines here in Australia, copyright Boobook48.

Somewhere in the region of eleven thousand species around the world have been assigned to this grouping; as always, doubtless many more could be recognised by those who take the time. Melolonthinae is generally recognised as a subfamily of the family Scarabaeidae, sharing with other scarabaeids features such an antennal club in which the segments are relatively narrow and can be smoothly pressed against each other, and an exposed pygidium (the last dorsal plate on the abdomen, forming what you might think of as the 'butt plate'). Some authors have recognised melolonthines as a distinct family but this is the less commonly utilised option. Melolonthines belong to a group of mostly plant-feeding subfamilies in which the row of abdominal spiracles bends downwards towards the rear so at least the last pair remains visible when the elytra are closed. Within this cluster, melolonthines tend to be characterised more by lacking the features of the other subfamilies than by distinctive features of their own (more on that in a moment) but general features include mandibles that are not visible when looking down on the top of the head, fore coxae that do not protrude much ventrally, equal claws on each leg (at least on the mid and hind legs) and only one visible spiracle when the elytra are closed. The labrum (the piece at the front of the mouthparts that might be thought of as the insect's top lip) is usually hardened and may be more or less fused with the clypeus (the lower- or foremost section [depending how you look at it] of the front of the head capsule). Many melolonthines are noticeably hairy and/or dull in comparison with other scarabaeoids but others may be shiny and/or metallic in coloration.

Sugarcane white grub beetle Lepidiota stigma, copyright Bernard Dupont.

For the most part, melolonthines are plant-feeders at both larval and adult stages of the life cycle (Lawrence & Britton 1991). The greater part of the active life cycle is taken up by the larval stage which may last for many months (Britton 1957). Larvae mostly live underground, feeding on plant roots and humus. A number of species have made themselves known as significant pests in this manner because of the damage they may inflict on pastures or agricultural crops (the grass grub Costelytra zealandica comes immediately to mind as a good example of this in my native New Zealand). Pupation also occurs underground in subterranean cells and mature adults may remain dormant in these cells for some months waiting for conditions to be just right for emergence. Once they do emerge from the ground, however, the adult life span is quite brief, only lasting a few weeks or even days. Because of this brief emergence, and because their habit of waiting for specific environmental cues means that large numbers may appear seemingly all at once, many species have been awarded vernacular names that reflect their seasonality such as June bug (in the Northern Hemisphere) or Christmas beetle (in the Southern). Some species will feed on foliage as adults, some may visit flowers for pollen and nectar, other particularly short-lived species will not feed as adults at all. The majority of adult melolonthines are active at dusk or night, spending the days sheltered in secluded locations, but a number of flower-feeding species are active by day (Britton 1957).

The infamous grass grub Costelytra zealandica, illustrated by Desmond Helmore.

The classification of melolonthines can charitably be described as an absolute mess. As noted above, we can confidently say that they belong to a clade with other subfamilies of plant-feeding scarabaeids (the Cetoniinae, Rutelinae and Dynastinae) but the features setting them apart from these other subfamilies are likely to be primitive for the group. As such, it comes as little surprise that phylogenetic studies have failed to establish the Melolonthinae as monophyletic (e.g. Eberle et al. 2018; Woolley 2016). However, it seems that no-one thinks that an adequately expansive study that would allow them to be appropriately divvied up has yet been done. Matters are not helped by the absence of a well-established internal classification for melolonthines. Various distinct subgroups can be recognised and between twenty or thirty tribes have been recognised around the world. But the relationships between these tribes remain uncertain, as does the tribal position of many genera. Much of the revisionary work that has been done has been conducted at a regional level only. Thus, for instance, the tribal classification of Australian melolonthines established by Britton (1957) applies only to Australian species and the tribal distinctions Britton recognised may end up falling apart if one attempted to apply them to species from elsewhere. Not that the authors should be criticised for this situation: after all, when one is dealing with over 11,000 species, things rapidly tend to become unmanageable.

REFERENCES

Britton, E. B. 1957. A Revision of the Australian Chafers (Coleoptera: Scarabaeidae: Melolonthinae) vol. 1. British Museum (Natural History): London.

Eberle, J., G. Sabatinelli, D. Cillo, E. Bazzatto, P. Šípek, R. Sehnal, A. Bezděk, D. Král & D. Ahrens. 2018. A molecular phylogeny of chafers revisits the polyphyly of Tanyproctini (Scarabaeidae, Melolonthinae). Zoologica Scripta 48: 349–358.

Lawrence, J. F., & E. B. Britton. 1991. Coleoptera. In: CSIRO. The Insects of Australia: a textbook for students and research workers 2^nd ed. vol. 2 pp. 543–683. Melbourne University Press.

Woolley, C. 2016. The first scarabaeid beetle (Coleoptera, Scarabaeidae, Melolonthinae) described from the Mesozoic (Late-Cretaceous) of Africa. African Invertebrates 57 (1): 53–66.