CHRYSOMELINAE Latreille, 1802
This group includes some of the most popular and typical leaf beetles. Most species can be found throughout the year, often in large colonies.
This large subfamily includes more than 3000 species in about 130 genera but this situation is likely to change as several of the larger genera are divided into many subgenera and include many subspecies e.g. Chrysolina Motschulsky, 1860 includes about 450 species and about 250 subspecies in more than 60 subgenera. The group is cosmopolitan, being represented on most of the World’s islands and extending into high latitudes e.g. Phratora polaris Schneider, J.S., 1886 occurs in Iceland. The classification is not settled but the group is generally divided into 2 tribes, one of which, Timarchini Motschulsky, 1860 which is sometimes treated as a subfamily, is monogeneric; Timarcha Samouelle, 1819 includes about 130 species and 30 subspecies in 4 subgenera, it is primarily western Palaearctic with most species distributed around the Mediterranean and North Africa while another 2 are Nearctic. The remainder of the subfamily are classified within the Chrysomelini Latreille, 1802 and this is generally divided into 5 subtribes; Entomoscelina Chapuis, 1874, (Holarctic and Neotropical) Chrysolinina Chen, 1936, (Cosmopolitan but for Australia) Doryphorina Motschulsky, 1860, (Cosmopolitan) Gonioctenina Wilcox, 1972 (Holarctic) and Chrysomelina Latreille, 1802 (Cosmopolitan) although others e.g. Paropsina Motschulsky, 1860, which overlaps with Gonioctenina, are sometimes recognized. Some warmer regions are especially rich in endemic genera e.g. 23 in the Neotropical region, 15 in Tropical Africa and 20 in Australia, and some very large genera are restricted to certain regions e.g. Doryphora Illiger, 1807 includes more than 450 species from Central and South America, Paropsisterna Motschulsky, 1860 includes about 120 species from Australia and New Guinea, although some are adventive elsewhere, Peltoschema Reitter, 1880 includes about 100 species from Australia, and Chrysolina is Holarctic, African and Indian. Most species occur in temperate and drier tropical regions, the Western Palaearctic is generally rich in species; the central European fauna includes more than 100, while the Nearctic is, given the extent in terms of latitude and longitude, relatively poor with about 140 species of 16 genera. Australia is particularly diverse, although lacking in Chrysolinini, with more than 40 native genera and 750 species or about 25% of the total for the subfamily.
Timarcha tenebricosa larva
© The Trustees of the Natural History Museum, London http://data.nhm.ac.uk/dataset/collection-specimens
Members of the subfamily are mostly medium-sized and very conspicuous, typically round-oval, convex and brightly coloured and/or metallic although there is a wide morphological variety e.g. Paropsis Olivier, 1807, with about 70 described species from Australia and New Guinea, are commonly referred to as tortoise beetles, they are brightly coloured and patterned and some might easily be taken for members of the Cassidinae. Both adults and larvae feed on foliage and sometimes flowers but apparently not pollen, a very wide range of plant families are hosts but some are especially popular e.g. Rosaceae, Asteraceae, Salicaceae, Brassicaceae and Ranunculaceae, most species are monophagous or oligophagous and relatively few are polyphagous, monocotyledons are generally used only by polyphagous species when conditions dictate. Under good conditions many species may produce huge populations after only a year or two, such ‘outbreaks’ can by sudden and unexpected and so inevitably many species have become significant horticultural and agricultural pests, the most notorious of which is probably the Colorado potato beetle, Leptinotarsa (=Doryphora) decemlineata (Say, 1824). Similarly many have been trialled as biocontrol agents of invasive weeds, often introduced beetles feeding upon adventive weed species e.g. the native Palaearctic Chrysolina hyperici (Forster, 1771) introduced to the United States to help control St. John’s wort-and reduce the spread of St. John’s-wort disease. Most adults are diurnal, hence the often gaudy colouration, but some are nocturnal as well e.g. Chrysolina oricalcea (Muller, O.F., 1776), and some are strictly nocturnal e.g. C. marginata (Linnaeus, 1758). Eggs are almost always laid exposed on the host plant, generally singly or in small groups under the leaves or towards the tips of fresh stems, and one species, Doryphora paykulli (Stål, 1859) from Central America displays parental care with the female guarding them. In some Timarcha the eggs are covered with excreta or regurgitated food and deposited on or buried in the soil below the host and in some riparian species e.g. of Phaedon Latreille, 1829 or in Hydrothassa Thomson, C.G., 1859, they are inserted into depressions cut into leaves or stems. Some species are ovoviviparous, laying eggs that hatch almost immediately after laying, and some are truly viviparous, producing first-instar larvae. Ovoviviparity is generally associated with loss of the spermatheca in adults and of the egg-burster in larvae. Larvae usually feed exposed on the leaves, generally in groups, and feed from the edges towards the mid-rib and so feeding damage is usually obvious and often extensive, they develop quickly and pass through 3 or 4 instars before pupating, mostly within a cell in the soil beneath the host but also, rarely, among the foliage or on the stems. Predation of eggs and larvae tends to be high while pupae and adults are rarely attacked, adults possess glands on the thorax and abdomen which release volatile defence chemicals which may be sequestered and modified from the host or produced independently. Larvae have similar glands but seem to be much more vulnerable. Members of Timarcha, which are flightless, will readily reflex-bleed from the mouth and leg joints when alarmed. In temperate regions many species are univoltine with eggs laid in the spring, larvae develop during spring and summer and new-generation adults in mid- to late-summer, in many these new-generation adults overwinter and oviposit in the spring after a period of feeding but some oviposit in the autumn and the eggs and/or larvae will overwinter. Adults overwinter among litter and moss or in tussocks or under bark and so will be found by searching or from extractions. Most species are fully winged and disperse by flight; many may suddenly appear in the spring or disappear in the autumn as they move between summer breeding sites and more sheltered overwintering sites.
The majority of species are between 3 and 20mm in length; the smallest are probably species of Gibbiomela Daccordi, 2003 at 1.5mm and the largest are species of the Australian genus Promechus Boisduval, 1835 at 30mm. Most are convex, elongate-oval and continuously curved with the head produced forward and, apart from a few dorsal trichobothria, glabrous. Many are brightly coloured, patterned and/or metallic and some tropical species need to be seen to appreciate the striking variety of vivid colouration, the most spectacular occur in warmer regions e.g. see Paropsis Olivier, 1807, Paropsisterna Motschulsky, 1860, Calomera Hope, 1840, Rhaebosterna Weise, 1917 or Alfius Reid, 2006 from Australia or the Neotropical Platyphora Gistel, 1857 but temperate species can also be impressive e.g. Chrysolina, Chrysomela Linnaeus, 1758 or the Nearctic Zygogramma Chevrolat in Dejean, 1836 (the list is extensive). The head is prognathous and usually retracted, often deeply so, into the thorax, temples variously developed or absent, vertex and frons smoothly convex to flat and often with a median longitudinal impression, supraorbital grooves usually absent. Eyes very variable in size, flat to very convex, circular to transverse and long and without or with only a small emargination, glabrous. Antennae 11-segmented with the basal segment large and then filiform to weakly broadened or asymmetric towards the apex, insertions visible from above and widely separated, placed laterally in front of the eyes. Frontoclypeal suture present or absent, clypeus truncate or emarginate, labrum free and truncate to deeply inwardly curved anteriorly. Mandibles short, broad and often strongly curved before the apex, and bi- or tridentate. Maxillary and labial palpi prominent and with a cylindrical to conical or fusiform terminal segment. Pronotum transverse, often widely so, and generally widest near the base and narrowed anteriorly or parallel-sided, the base often as wide as the elytra across the shoulders, and the lateral margins are usually visible from above. Disc smoothly convex, often with basal depressions or, rarely, with tubercles inside the angles; anterior angles rounded or projecting and posterior angles sharp or nearly so. Sub-lateral grooves variously developed, often missing. Prosternum flat to weakly convex in front of the coxae, often with ridges which accommodate the antennae, process very variable; reaching the middle of the coxae in Gastrophysa Dejean, 1836 to long and expanded behind the coxae or transverse; flat to strongly elevated. Procoxal cavities oval to transverse, narrowly to widely separated and open or closed externally, the coxae not, or only very weakly, projecting below the level of the prosternum. Scutellum usually obvious, triangular to rounded, and sometimes abruptly elevated. Mesoventrite relatively long and often excised or curved anteriorly for the reception of the prosternal process, rarely raised to meet the pro-coxae e.g. in Plagiodera Dejean, 1836, or produced into a process extending forward the anterior prosternal margin, the posterior margin without tubercles and sometimes hidden by a raised anterior part of the metaventrite. Mesocoxae flat and round to transverse-oval; narrowly to widely separated. Metacoxae horizontally transverse but not reaching the elytral epipleura, narrowly to widely separated. Hind wings usually well-developed and often brightly coloured, sometimes reduced or absent e.g. in Timarcha. The ventral surface is at most only weakly convex and usually almost flat, the pronotum and elytra are laterally curved and the appendages fold into grooves so that they can become very compact when disturbed and many display thanatosis. Elytra variously convex, often strongly so, form round to elongate; at most 2x the width and 2-4x the pronotal length, completely covering the abdomen and continuously rounded or angled apically. Punctation very fine to strong, often with a mixture of both, and often with distinct punctured striae, sometimes e.g. in some Chrysolina, running in pairs longitudinally. Lateral margin often reflexed ventrally, in some with a group of wing-folding spicules towards the base and above the epipleura, and in some with a distinct sub-apical group of setae e.g. Chrysolina. Microsculpture varies from very fine, almost non-existent, to dense and strong so that the elytra appear dull, in many stronger in the female. In the apterous Timarcha the elytra are fused. Abdomen with 5 free ventrites or, rarely, with the first and second connate e.g. in Plagiodera Dejean, 1836. Legs well-developed and more-or-less equal in size. The femoral-trocantal joint is weakly to very strongly oblique and so the femora sometimes almost touch the coxae. Femora weakly broadened medially, in a few species the male pro-femora are toothed, tibiae generally rather flattened, parallel or expanded towards the apex, terminal spurs absent but the outer apical angle may be produced, sometimes strongly so e.g. in Gonioctina Dejean, 1836, outer margin sometimes flattened or depressed to accommodate the tarsi, and there are often various combs of thick, short setae towards the apex. Tarsi 5-segmented, tarsomere 3 expanded and often hiding the tiny fourth segment, terminal segment elongate and usually extending well beyond the lobes of the third segment. In some e.g. Timarcha the basal 3 segments are lobed, often more strongly so on the pro-tarsi and more broadly in the male. Species rarely exhibit morphological sexual dimorphism but in some the legs or mandibles are more developed in the male and in general females are larger and broader than males. Males are generally easy to determine by the dense pads of flat and truncate adhesive setae beneath the tarsal segments, in females these are generally simple and laterally directed.