Fossil Insects

insect fragments viewed down a microscopePlate 1 View ‘down the microscope’ of extracted beetle fragments and fly puparia

Insects are invertebrates with exoskeletons of chitin, a substance not dissimilar to cellulose. Chitin is quite resistant to decay but can be attacked by fungi. As a general guide, if conditions are suitable for the preservation of waterlogged plant remains then insect remains will also survive. These conditions are most likely to occur in natural situations like lakebeds, palaeochannels of rivers, bogs and wetlands. In archaeological features they will be present in contexts that extend below or cut into the water table e.g. pits, well bottoms and ditches (English Heritage 2005; IAI 2007).
Beetles are the most commonly found and studied. When a beetle dies and is incorporated into a deposit, it gradually disintegrates into the component parts of its exoskeleton i.e. the head, thorax, elytra (wing cases), legs etc.

Of these various bits, it is the first three, and in particularly the elytra, which are robust enough to be preserved and variable enough to permit identification (Plate 2Carpelimus bilineatus)

Prostomis mandibularisPlate 2 Carpelimus bilineatus, found in damp decaying vegetation, commonly recovered from medieval pit fills and house floors – here heads, pronota and elytra represented.

In general, these fragments are not recognizable for what they are to the naked eye during excavation and must be extracted from deposits using specialist techniques (Kenward 1980).
The importance of insect remains, particularly beetles, to the archaeologist/palaeoecologist lies in the ability to extrapolate from the presence and associations of particular species in each context important habitat data, which can sheds light on environmental change (Robinson 2001). Some orders of insects, especially beetles and midges, have proved to be sensitive climatic indicators and are widely used in palaeo-climatic reconstruction.
Insect remains have been examined from an increasing number of palaeoecological and archaeological sites in Ireland (Whitehouse 2007). At sites such as Derryville Bog, Co. Tipperary, beetles have been used to show the character of local woodland, changes in water quality through time and the occurrence of animals in bog marginal woodland from the late Neolithic period to the Early Medieval period (Reilly 2005).

Prostomis mandibularisPlate 3 Prostomis mandibularis, wood-boring beetle, now regionally extinct from Ireland and Britain, found in Bronze Age trackway wood, Derryville Bog, Co. Tipperary.

Xyleborus dispar Plate 4 Xyleborus dispar – bark beetle, locally extinct from Ireland, found in hazel wood from Neolithic trackway, Edercloon Bog, Co. Longford.

The finding of rare or extinct beetles in archaeological contexts can give important insights into broader biogeographical issues such as where Ireland’s native insect fauna came from and how it got here from the early postglacial onward (Reilly 2008; Whitehouse 2006) (e.g. Plate 3 Prostomis mandibularis and Plate 4Xyleborus dispar).

Suggested Further Reading:
Institute of Archaeologists of Ireland (2007) Guidelines for Environmental Sampling. IAI Publications.
English Heritage (2005) Guidelines to sampling for bioarchaeological remains. English Heritage Publications.
Kenward, H.K. (1980) A tested set of techniques for the extraction of plant and animal macrofossils from waterlogged archaeological deposits. Science and Archaeology, 22, 3-15.
Reilly, E. (2005) Coleoptera, pp. 187-208. In M. Gowen, M. Philips & J. Ó Néill The Lisheen Mine Archaeological Project 1996-8. Wordwell, Bray.
Reilly, E. (2008) An ever-closing gap? Modern ecological and palaeoecological contributions towards understanding the Irish post-glacial insect fauna. Irish Naturalist’s Journal Special Supplement, 63-71.
Robinson, M.A. (2001) Insects as palaeoenvironmental indicators, pp.121-135. In D.R. Brothwell & A.M. Pollard Handbook of Archaeological Sciences. Wiley & Sons Ltd., Chichester.
Whitehouse, N.J. (2006) The Holocene British and Irish ancient forest fossil beetle fauna: implications for forest history, biodiversity and faunal colonisation. Quaternary Science Reviews 25, 1755-1789.
Whitehouse, N.J. (2007) The study of fossil insect remains in environmental and archaeological investigations: An Irish perspective, 136-164. In E.M. Murphy & N.J. Whitehouse Environmental Archeology in Ireland. Oxbow Books, Oxford.