A new species of Longitarsus Latreille, 1829 (Coleoptera, Chrysomelidae, Galerucinae) pupating inside stem aerenchyma of the hydrophyte host from the Oriental Region

Abstract A new species of subaquatic Longitarsus pupating inside the stem aerenchyma of its hydrophyte host plant is described. Eggs are laid on tender leaves and buds and the larvae are open feeders. This is the first report of an Oriental flea beetle pupating inside the stem of its hydrophyte host. A key to the species of southern Indian Longitarsus is provided.


Introduction
Larvae of fl ea beetles, in general, are subterranean root feeders. A few of them mine the leaves or feed exposed on it, while fruit borers and stem borers are extremely rare (Jolivet andHawkeswood 1995, Konstantinov andVandenberg 1996). As a rule, fl ea beetle larvae associated with terrestrial plants, including leaf feeders, pupate in soil or litter. However, root and soil under submergence are inaccessible to all life stages of fl ea beetles due to lack of adaptation for a true aquatic life. Still a few of them have adopted strategies to circumvent this limitation to harness aquatic and subaquatic plants. Jolivet (2003) reviewed the biology of subaquatic Chrysomelidae, including subaquatic fl ea beetles. In many such fl ea beetles, larvae feed on leaves and pupation occurs in the soil on shore or facultatively on the plant itself when soil is not available. In the case of beetles living on rooted emergents and fl oating plants, inaccessibility to soil force all life stages, including pupae, to adapt to the portions of the plant above water level. For example, Pseudolampsis guttata (LeConte) that feeds on the fl oating water fern Azolla caroliniana Willd. pupates on leaf surface (Buckingham and Buckingham 1981). Species of Agasicles Jacoby, 1904 are unique among the subaquatic fl ea beetles as they pupate only inside the stem internode cavity of their amaranthaceous hosts (Vogt et al. 1979). A pair of modifi ed T-shaped urogomphi secures the pupa from falling into the wedged position inside the internode cavity, from which emergence might be impossible (Vogt et al. 1979;Cox 1998). A new species of Longitarsus Latreille, 1829 from the Oriental Region, similarly adapted for pupation in stem aerenchyma is described here. Aerenchyma is a large intercellular space that acts as a mediator of internal gas exchange and maintains strength with the least tissue (Jung et al. 2008).
Th e cosmopolitan Longitarsus is the most speciose genus of fl ea beetles with about 700 species. Th ough about 100 species are known from the Oriental Region, only eight named species of Longitarsus occur in south India (Maulik 1926;Scherer 1969;Gruev and Askevold 1988;Prathapan et al. 2005) and several of them still await naming and description. Known host plants of the genus belong to at least a dozen families with marked preference for members of Boraginaceae, Lamiaceae, Asteraceae, and Verbenaceae (Jolivet and Hawkeswood 1995). Species of Longitarsus are small to medium sized fl ea beetles easily identifi ed by a very long fi rst metatarsomere which is at least half as long as the metatibia. Other salient features of the genus are the lack of transverse or longitudinal impressions on the pronotum, open procoxal cavities and the confused elytral punctures rarely forming regular striae. Members of Longitarsus are generally terrestrial and their larvae feed on the roots (Furth 1980, Ireson et al. 1991, Schwarzländer 2000, Baars 2001and Simelane 2010. Exceptions include the European Longitarsus nigerrimus (Gryllenhal) that lives in peat bogs and swamps. Booth (2000) reviewed its records from the British Isles and Jolivet (2003) summarized information on its biology.

Methods
Natural populations of the beetle under fi eld conditions were observed during 2009-2011 at Vembayam, Trivandrum, Kerala, India during several visits. Th e host plant, Limnophila aquatica (Roxb.) Alston was grown in a concrete tank partially fi lled with soil and water at Vellayani and live beetles were released on to these plants to confi rm the biology observed in the fi eld.

Longitarsus limnophilae
Etymology. Th is unique species is named after its host plant. Th e name is a noun in the genitive case.
Vertex shiny, impunctate, minutely wrinkled. Ommatidia fully developed. Postcallinal sulcus weak but distinct. Frontal ridge unusual, broad and not sharply raised, anteriorly widening towards frontoclypeal suture, anteriorly forming illdefi ned denticle in middle of fl at, poorly developed anterofrontal ridge. Maxillary palpus with last palpomere longest. Antenna extends well beyond apex of elytra over pronotum. Second antennomere longer than half of third; second and third together longer than fi rst, subequl to fourth; fi fth longer than fourth; fi fth to seventh subequal, eighth to tenth progressively shorter than previous antennomere. Pronotum anteriorly wider than posteriorly; 1.27 -1.34 times wider than long; anterolateral callosity posteriorly lower than anteriorly, not forming denticle at pore; posterolateral callosity protrudes beyond lateral margin; lateral margin weakly curved, Figure 1. Longitarsus limnophilae sp. n., dorsal habitus anteriorly broader than posteriorly; disc shiny with minute punctures more evident posteriorly. Elytra with well developed humeral calli, punctures distinct, width of interstices smaller than diameter of one puncture in middle of elytron. Elytral apex convex, with one long seta. Hind wings well developed. Scutellum triangular. First male protarsomere 1.60 -1.67 times longer than wide; fi rst female protarsomere 2.00 -2.43 times longer than wide. Metatibia strongly curved in dorsal view, slightly curved in lateral view. Number of spinules on dorsolateral margin of metatibia, proximal to row of stiff bristles, vary from seven to ten. In lateral view, fi rst metatarsomere 0.55 -0.57 times as long as metatibia. Proximal end of fi rst metatarsomere ventrally with thick characteristic patch of short pointed and capitate setae in both sexes. Last male ventrite internally with longitudinal ridge along middle (Fig. 5); posterior margin bisinuate.
Aedeagus in lateral view gently curved, apex acutely pointed and slightly recurved (Fig. 4); ventral side depressed with transparent window, lateral edges raised (Fig. 2); dorsal opening covered with lamina not extending to apex (Fig. 3). Arms of tegmen subequal to or slightly shorter than stem.
Remarks. Longitarsus limnophilae can easily be separated from all other south Indian species of Longitarsus by the anteriorly widening, fl at frontal ridge (in the other species, the frontal ridge is more or less narrowly raised). Longitarsus belgaumensis Jacoby closely resembles Longitarsus limnophilae in having narrowly piceous elytral suture and dark distal antennomeres. But Longitarsus limnophilae can be separated from Longitarsus belgaumensis based on the antenna extending slightly beyond the apex of elytra over pronotum (in Longitarsus belgaumensis, antenna does not extend beyond the apex of elytra over pronotum), pronotum anteriorly wider than posteriorly (in Longitarsus belgaumensis, pronotum is an-teriorly as wide as posteriorly with the maximum width in the middle), structure of frontal ridge (frontal ridge sharply raised along middle in Longitarsus belgaumensis) and genitalia.
Biology. Eggs (Fig. 13) are laid on tender leaves and buds and the larvae are open feeders (Fig. 14). Th e closely oriented tender leaves provide suffi cient cover for the larva. Mature larva enters the stem aerenchyma in the internode by boring a tiny hole and pupation occurs in it, a little above the entry hole (Figs 15, 16). Adult emerges through an exit hole nearly circular in shape with irregular margin. Adult feeds on both adaxial and abaxial surface of, mostly, tender leaves by scraping, often resulting in holes on the lamina (Fig. 12). Adults when thrown in water fl oated initially and then swam with raised antennae held back over the sides of the pronotum. After swimming for a while, some performed a short jump on to the shore.
Discussion. Jolivet and Hawkeswood (1995) have listed Bacopa Aublet, 1775 (Scrophulariaceae), a genus of hydrophytes, among host plants of Longitarsus. But no further information is available on its biology on Bacopa. Longitarsus limnophilae and species of Agasicles represent two independently evolved lineages in Alticini adapted for larval leaf feeding and pupation inside the stem. Agasicles and Longitarsus limnophilae are the only fl ea beetles known to pupate inside the stem of their aquatic host plant above the water level. Wheel shaped stem aerenchyma of Limnophila aquatica serve as a safe abode for the pupa off ering protection against natural enemies. Species of Limnophila being widely distributed aquatic weeds, Longitarsus limnophilae could be a potential biocontrol agent for them.
Th is is the fi rst report of an Oriental fl ea beetle pupating inside the stem of its hydrophyte host.