FNZ 16 - Nepticulidae (Insecta: Lepidoptera) - Identification
Donner, H; Wilkinson, C 1989. Nepticulidae (Insecta: Lepidoptera). Fauna of New Zealand 16, 92 pages.
(
ISSN 0111-5383 (print),
;
no.
16.
ISBN 0-477-02538-2 (print),
).
Published 28 Apr 1989
ZooBank: http://zoobank.org/References/9BE5D9B7-27E2-46F0-8A35-4D4912BA0D99
Identification
Non-specialists tend to come across adult microlepidoptera mainly when specimens are caught in light traps and sweep-nets. Because their wings and scales are so fragile the moths are usually by then worn and unidentifiable, at least on external features.
The most accurate way to identify adults, especially males, is by examination of the genitalia, which in the majority of species provide diagnostic characters. Preparation and interpretation of the genitalia are skilled techniques which, when mastered, can lead to successful identification using our key, based on the reproductive structures.
Collecting mined leaves with living larvae and rearing the caterpillars through to the adult stage is the simplest and surest route to identification, with the help of our guide to food plants, mine characters, and external features.
For identification to species the identity of the host plant is paramount; and the season of occurrence may be valuable supporting evidence.
Mines. It is not possible to give foolproof characters separating nepticulid mines from those of other groups. Usually they are serpertine, and sometimes they terminate in a blotch. Others may comprise only a blotch, either because the caterpillar eats round in a circle from the very beginning or because its subsequent feeding eliminates the serpentine track of earlier instars. There is usually a granular frass line which makes characteristic patterns along the mine. These patterns may vary according to species and instar. They can also be changed by weathering, particularly by rain. Some species do not defecate in the early stages, and others only intermittently. In such instances it is sometimes difficult to discriminate them from Agromyzidae (Diptera), but in the majority of nepticulid mines the frass line is diagnostic.
Other characters, or combinations of them, which may prove helpful include the length of the mine and other dimensions; the number of mines per leaf; and their position at different stages in all three planes of the leaf, since larvae tend not to mine indiscriminately throughout the thickness of the lamina.
In Britain, where species produce mines of two different lengths, CW (unpublished data) has been able to demonstrate that the shorter examples are collected in the open, e.g., on the edges of woodland and hedgerows, whereas the longer mines occur in more enclosed habitats, e.g., within woodland. The phenomenon can be further related to the surface area and thickness of the leaf. In woodland, leaves often have an increased surface area to maximise the surface presented to the sun during a limited period. As a result the leaf is usually thinner and induces the longer mine. Outside, where competition for light may be less and the leaf remains exposed to the sun for longer, the resulting mine is shorter. This in turn relates to the nutrient available within the leaf per unit length, i.e., more nutrient is available in the leaf with longer exposure to the sun, and hence the mine is shorter.
Eggs are minute, oval, and cemented to the surface of the leaf, often alongside a vein or at the leaf margin. At least in many European species the oviposition site is deliberately selected, and in some species is characteristic. Egg maps are therefore often very useful diagnostically. Recent tests suggest that some females are aware of differences in texture between the two epidermes, but in other species females can be misled into laying on the wrong side of the leaf simply by turning it over (CW). Stereoscan studies have shown the presence of a scale covering the egg which differs in texture and pattern between genera.
Larvae. On hatching, the larva eats directly into the leaf, without appearing on the outside. Not surprisingly, its legs are vestigial. The mouthparts are prognathous and mandibulate, although evidence suggests that some species subsist on the fluid content of the leaf rather than by ingesting solid particles. Larvae are usually green or yellow, but some may be pink or other shades according to which pigments are developed in the leaf. The body segments have a characteristic chaetotaxy. Orientation in the mine may be venter up or venter down, but is consistent within each species.
Cocoons are spun from dense, brown or yellow silk, the colour depending on the xanthophylls, carotenoids, and other pigments in the leaf. This can be confirmed by radioactive labelling applied to the host plant. They are broadly oval, flattened, and slightly wider anteriorly. Cocoons are usually made among debris or soil particles, or between decaying leaves on the ground. However, sometimes the larva does not reach the ground, but pupates on stems or in bark crevices, as has been observed in S. microtheriella.
Shortly before eclosion the pupa breaks through the anterior end of the cocoon. It is compressed in appearance, and has free appendages and flexible abdominal segmentation as far as segment 7.
Adults are very active, having the use of their wings soon after emergence, but usually they do not stray far from the parent plant. At rest the wings lie in a very shallow, inverted V, and meet in the middle of the back.
The number of generations per year is variable, often depending on climatic factors. In high latitudes there is usually only one, but in warmer climates there may be two or three, and even four in some species on oak (Quercus spp.). Six weeks usually suffice to complete the life cycle in optimum conditions, but this extends to several months in species with only one generation per year.