FNZ 49 - Lithinini (Insecta: Lepidoptera: Geometridae: Ennominae) - Introduction
Weintraub, JD; Scoble, MJ 2004. Lithinini (Insecta: Lepidoptera: Geometridae: Ennominae). Fauna of New Zealand 49, 48 pages.
( ISSN 0111-5383 (print), ; no. 49. ISBN 0-478-09357-8 (print) ). Published 29 Apr 2004
The Lithinini are a diverse cosmopolitan tribe belonging to the geometrid subfamily Ennominae, and comprise 36 genera and 100-150 species. One of their most unusual characteristics is their predominant host association: the vast majority of species with described life histories have pteridophagous larvae. This fern-feeding habit has been documented for all genera indigenous to New Zealand. Specialist fern-feeders are quite rare among phytophagous insects, probably because few herbivores have succeeded in circumventing the array of phytochemical defences present in pteridophytes (Cooper-Driver 1985; Soeder 1985). Lithinini is the only major lineage within the Geometridae that is characterised by pteridophagy, and the phylogenetic diversity of their fern hosts (10 genera in 6 families for the New Zealand fauna alone) is indicative of either long association or extreme plasticity in coping with pteridophyte defensive chemistry.
The tribe as a whole is difficult to characterise on the basis of universally shared derived characters, but a number of important diagnostic features have been discovered. The male genitalia (Fig. 27-34) have a characteristic shape when mounted conventionally (venter uppermost) on a microscope slide. The valves usually curve upwards close to the uncus rather than splay out laterally; the transtilla is characteristically triangular, and many taxa have a pair of setose processes arising from the anellus; and the furca usually has an apical spine or cluster of setae. The slender-bodied adults usually have a discal spot and, on the ventral surface of the hindwing, a postmedial band or row of spots on the veins.
The larvae of taxa with described life histories (9 of 36 genera) all have supernumerary ( > 6) SV setae on the 6th abdominal segment, as opposed to the 3 or 4 SV setae characteristic of most ennomine Geometridae. Larvae have a striking and very characteristic defensive response when disturbed: they drop or 'jump' from the host, spasmodically twisting and curling, and this generally results in a rapid descent through vegetation and dead fern fronds which protects them from predators. This defensive behaviour is particularly effective in species associated with ferns such as Pteridium, as dense mats of dead fronds accumulate below the ferns, and their growth habit (generally multiple fronds and often vegetative propagation) makes it easy for larvae that have 'dropped' to climb back on to host plants. Larvae feed openly on the fronds of their host plants, and adults are often disturbed from resting places on or near host plants.
The three genera endemic to New Zealand have a larval character that may prove to be widespread in Lithinini, and provides a functional/morphological connection to the aforementioned defensive behaviour: seta L1 on one or more of segments A6-8 is extremely slender and sensitive to touch/vibration, apparently acting as a 'trigger' for the jumping and coiling/twisting response (J.S. Dugdale, in litt. ).
Pupae of Lithinini have 3 pairs of modified setae anterior to the terminal pair of cremastral setae. The homologies of these 3 pairs of setae were interpreted by Dugdale (1961, fig. 7a) as SD1, L1, and D1, with the terminal pair as D2. This character was first proposed as diagnostic for the North American members of the tribe by Forbes (1948), and holds for all taxa with pupae described to date. It may, however, represent a shared primitive character within the Geometridae.
The Lithinini of the temperate New World were reviewed by Rindge (1986), Pitkin (2002), and those from Chile and Argentina are being studied by Parra ( in prep ). Holloway (1987) discussed fern-feeding in the Afro-Indo-Australian lithinine genus Idiodes and provided an overview of potentially related fern-feeding ennomine genera. The tribe has been treated within the context of several faunistic works, e.g., Forbes 1948 (N.America) and McGuffin 1987 (Canada). The New Zealand fauna has been treated within major works that cover or include the family Geometridae (Meyrick 1884a, b, 1917; Hudson 1928, 1939).