Hobson’s Spatial Calibration Algorithm For Dragonflies For Migrations Research
Insects
Although often less recognised, large biomasses of insects migrate long distances each year including moths, butterflies, locusts and dragonflies. The use of common tracking methods such as exogenous markers are impractical for tracking insect migration because of its small size and the high improbability of recapture, however the use of naturally occurring intrinsic markers such as stable isotopes can be a very useful option.
Some dragonflies show migration patterns quite similar to those of songbirds, however a major difference is that insect migrations often involve more than one generation throughout the annual cycle. “Individual insects that carry out a complete return migration have not yet been found”. Dragonfly larvae develop in water bodies which undoubtedly contributes to the strong relationship between its wing chitin – which is derived from their aquatic environment at formation – and long-term precipitation O and H isotopes. Findings from the above study were subsequently applied to 49 Pantala flavescens from November to December through the Maldives. Every October-December large numbers of the Globe Skimmer dragonfly appear in the Maldives. It is suspected that they are wavering of the migratory flight from India to East Africa which in addition may only be one leg in a multi-generational journey (which begins following the Inter-tropical Convergence Zone – This dragonfly species has been described as an obligate Inter-tropical Convergence Zone migrant as it follows the falling rains by taking advantage of these converging air masses).
The use if isoscapes and analysis of stable hydrogen isotopes in the wings of Monarch butterflies enabled researchers to determine that butterflies breeding in North America were the same ones who were overwintering in Mexico.
Aims
The objectives of Hobson (2012) was to determine the relationship between precipitation in North America and the chitin in dragonfly’s wings. They devised an algorithim to create a spatially explicit “wing-chitin isoscape to be applied to the study of migrant dragonflies and potentially other aquatic emergent migratory insectsStrontium isotopes can be used for a range of forensic applications including animal migration research and animal tracking and are sometimes used in insect studies, however a critical assumption in its use – that there is no fractionation from bedrock sources has never actually been experimentally tested. Flockhart, et al (2015) assessed this assumption using adult monarch butterflies fed on obligate larval host milkweed plants which were grown on seven different soils collected from across Canada. ”.
Methods
Hobson (2012) et al used the composition of H isotopes in a population of Common Green Darners through Texas in conjunction with a statistical model used to depict the probable natal origins of these dragonflies which would be a potential mix of local and migrating dragonflies.
Results
Thankfully “the measurement of intrinsic isotopic markers in metabolically inert tissue like wing chitin now allows the tracking of migratory insect populations through the use of continental patterns in stable isotope values of inorganic and organic substances”.
Results obtained by Flockhart et al (2015) suggest that strontium isoscapes in bedrock and soil may serve as a reliable biological marker across large geographic areas, adding another element to our bank of tools in animal migration research.
Using Hobson’s spatial calibration algorithm for dragonflies, results strongly suggest that the migrations of this species is much longer than previously thought, possibly involving a trans-Himalayan high-altitude transverse with probabilistic origins primarily in northern India. Hobson (2012) found a strong relationship between hydrologic geospatial patterns and between North American dragonfly wings of three species. These findings enabled the team to create a dragonfly wing isoscape that could be used to infer natal origins of migratory individuals.
