Experts at Bristol Zoo say a caterpillar brought in by a worried gardener could be a species never previously recorded in the region.
The saddleback has tiny spines which can sting
A woman initially took the hairy green specimen to Bristol Museum's enquiry service because she was worried it had bitten her.
She found the creature - thought to be a saddleback caterpillar (Sibine stimulea) - at a garden centre in Somerset.
The service was able to reassure her that caterpillars do not have strong enough mouthparts to bite but some species - including this one - have mildly poisonous irritant hairs.
Warren Spencer of Bristol Zoo identified the caterpillar and if correct, it is likely to be the first record of this species in Somerset - and possibly the UK.
Members of Bristol & District Moth Group and Somerset Moth Group said they had not seen this species in Somerset before.
Specialists at the Natural History Museum also can not confirm any previous records of Sibine stimulea.
The saddleback caterpillar is a recognised "pest" species which is widespread throughout the eastern and southern US.
Most of the live caterpillars have now been given to Bristol Zoo to rear into adult moths, when a final identification will be made.