I came across this very large (c. 5 cm in length) digger wasp which was searching the leaf litter in Doi Suthep-Pui National Park, northern Thailand. It is a scoliid wasp called Megascolia azurea (Subfamily Scoliinae; Tribe Scoliini). There are five subspecies. (1) Given the location, perhaps it is ssp. siamensis Bertem). Regiscolia is an alternative name for this genus, which also includes Megascolia procer (the Giant Scoliid Wasp) which is one of the largest wasps in the world with a wingspan of 11.5 cm (2).
It looks quite similar to a species found in southern Europe, called Megascolia maculata, which is the largest European solitary wasp. M. maculata has however, yellow spots on the abdomen rather than the orange-red ones seen here. Both species have reddish-golden hairs, called ‘vestiture’, on the last three (in the female) abdominal segments (see photo in Link 2).
What was this wasp doing? Perhaps it was looking for prey? The so-called mammoth wasps, Megascolia maculata flavifrons, are external parasites of the larva of European Rhinoceros beetles (Oryctes nasicornis). They lay a single egg on the larvae of Rhinocerous beetles (as shown on the photo in Link 4), so perhaps this wasp was similarly searching for beetle larvae?
Whilst I was taking the photographs of this wasp, I did not notice the strange creatures riding on the back of the wasp (above, encircled), although they are quite prominent. I now realise that they are pseudoscorpions, which are known for their phoretic (hitch-hiking) habits. Pseudoscorpions are not scorpions, but are a type of arachnid, with 8 legs like a spider. They have been recorded hitching lifts on a variety of invertebrates, including large beetles, using them as a means of moving between trees (5). The main benefits of phoresy are to be able to exploit new habitats that would not be available to the pseudoscorpions under their on steam! (6) It has been called a symbiotic relationship but it is not clear to me what the benefits to the wasp are? Maybe the pseudoscorpions feed on parasites or clean up the wasps’ nests? One review suggests that the pseudoscorpions seem to gain most from the association. (6)
One interesting fact about this species, is that only the female has a red head, an example of sexual dimorphism. The male has a black head and is slightly smaller. These wasps also have iridescent blue wings which have a rippled surface (see below) which appears wavy in cross-section (7).
The pseudoscorpions are, it has to be said, rather unsightly, at least to my eye! And on first impressions, they look as though they are parasites. But there is a strong possibility that this is a symbiotic relationship and that the wasp benefits in some way from the association. So perhaps she does not mind having the hitch-hikers riding on her back and she is probably large enough not to be impeded by their presence. There is probably much to learn about this particular relationship although it might not be easy working with a nest-full of these large wasp!
- Poinar Jr, G. O., Curcic, B. P., & Cokendolpher, J. C. (1998). Arthropod phoresy involving pseudoscorpions in the past and present. Acta arachnologica, 47(2), 79-96.
- Sarrazin, M., Vigneron, J. P., Welch, V., & Rassart, M. (2008). Nanomorphology of the blue iridescent wings of a giant tropical wasp Megascolia procer javanensis (Hymenoptera). Physical Review E, 78(5), 051902.