47 Tiny scale insects in life position

Arthropoda, Hemiptera

Fossils preserved in life position are very uncommon. Typically, after death, the plant or animal will become detached from its position in life – leaves fall off trees, pollen is blown away, and fish sink to the bottom of the lake.

The image to the top and left shows 14 female scale insects still in life position on an angiosperm leaf. Each little waxy scale is about 2.5 mm long and displays the original nacreous colouring. The image to the top and right is a diagram with white arrows showing the location of the scale insects on the leaf. The region towards the bottom of this diagram identified by the dashed white line is expanded to show the scale insects in the bottom image. Image credit: JM Bannister and A Harris. Image re-use permission kindly provided by Otago University Press and originally published in: DE Lee, U Kaulfuss and J Conran, Fossil Treasures of Foulden Maar, Otago University Press, 2022.

In the Geology Museum collections we have several examples of creatures ‘frozen’ in life position. Some particularly spectacular examples include insects which spend part of their life cycle firmly attached to a living leaf.

One of the best examples is a single, medium-sized leaf from Foulden Maar which has 14 individual scale insects still attached to it. One of few such examples known globally, each little scale has the long axis of its body parallel to the main or secondary veins on which they were feeding. All the scales are female, and they range from nine large, mature adult females (up to 2.55 mm in diameter) to five smaller young adult females that had not long moulted into their third and final instar. The larger, mature females were occupying the basal part of the leaf, while the smaller, younger individuals were on the apical part of the leaf, including one right at the tip.

Remarkably, the original nacreous, pale whitish-green colouration of the waxy scales is preserved.

Scale insects generally occur on plants that live for more than a year, and, although we can’t identify the leaf for certain, it is from an evergreen flowering plant and is probably a species in the family Elaeocarpaceae that grew near the margin of the small volcanic lake at Foulden Maar (see: World’s oldest whitebait swam in volcanic lake).

These scales are placed in the family Diaspididae (armoured scales) and subfamily Aspidiotinae. The common name refers to the wax-like, fibrous scale that is formed by the insect and which covers it. However, although these tiny fossils do not closely resemble any living species, the lack of characters needed to place them in a genus (and species) mean they cannot be given a new scientific name.

In armoured scales such as these the adult males are fragile, short-lived (a matter of hours) and lack mouthparts. The first instar immatures (termed crawlers), occur under the female’s scale cover and disperse over the leaves and stems of the host plant to colonise new leaves or are blown by the wind to other plants. Once landing on a suitable host, the crawler inserts its piercing mouthparts onto the plant to access food and begins to produce a waxy cover. The limited dispersal abilities of these armoured scales and the disappearance of their host plant from the flora of Aotearoa New Zealand likely led to their extinction.

Over 2400 species of Diaspididae occur worldwide, with 39 species still native to New Zealand that are confined to native host plants. However, there are many exotic scale insects now that are serious plant pests and are regarded as major biosecurity risks in New Zealand.

—Written by Daphne E Lee