Scuttle flies and their relationship with tarantulas

Guy Tansley

In Vol. 7 No. 1 of the American Tarantula Society Forum, there appeared an article on scuttle flies (Phoridae) which associate with large tarantulas from South America. This article became of interest recently, as I have since acquired a large female Goliath (Theraphosa blondi) and noticed several small flies gathered on the spiders carapace and abdomen

The flies (six in total) were reluctant to leave the spider and ran (hence the common name) rather than flew to evade capture. With the aid of a pooter, all of the flies were collected, but on closer inspection of the spider I noticed there were several empty pupal cases surrounding and attached to the foveal region of the carapace. The short, velvet-like hairs on the carapace also looked as though they had been "grazed" upon. Despite this, the spider seemed unaware of the presence of the flies and the pupae, they being attached to the only part of the spider it couldn't reach when cleaning itself.

As time passed I kept a close eye on the spider and over the following two weeks, a total of nine fly larvae were collected from it and it's container. These were quite large and easily visible on the carapace and simply removed with a pair of forceps (a very long pair!), the spider even allowing me to clear it's foveal groove of empty pupal cases and larvae.

Two adult flies were preserved in alcohol and sent to Dr. Henry Disney at Dept. Zoology, University of Cambridge, England and these he identified as one of three recently discovered and described species whose larvae live on tarantulas. This species was Megaselia dimorphica which is host-specific to Theraphosa blondi. The other two species, M. preadafura and M. tinteri being host-specific to Megaphobema robustum and Pamphobeteus vespertinus respectively.

Observation shows that the larvae and subsequent adult flies feed primarily on the decaying prey of their host. This became apparent after the spider was fed large piece of raw meat. The spider spent several hours consuming the meat and fly larvae were collected on and around the discarded remains and these larvae had taken on a distinctly red appearance caused by the blood in the meat. One larva was collected from the foveal groove in the process of pupation, having gorged itself. It can be assumed that the flies also stay on the spider when they emerge and also feed on prey remains, exuvial fluid, etc. They also mate and lay eggs on the host, repeating the process all over. It seems the fly larvae do not actually harm the spider (the worst case would be larvae in the spiders chitinous skin or stray larvae entering an egg sac during construction). The larvae have been reported to be able to fast for up to two months, which would relate to the spiders fluctuating food supply, but well-fed larvae usually pupate within a month. Larvae of related species are known to possess a pair of sucker-like structures near the mouth and several rows of hooks on the underside of the rear end. This helps the larvae stay attached to the short hairs of the spider and, if groomed off, enable them to climb back easily.

These host-specific Megaselia should not be confused with related species (many Phoridae species occur in England) which feed on decaying organic material, and the only real problem is infestation of same-species hosts within a collection. My specimen is now clear of these intruders and hopefully there will be no sign of them in the future. However, if you do acquire imported spiders, it's always a good idea to check the thoroughly either before purchase or immediately afterward. If they're present, keep the spider isolated and slowly but surely the larvae can be removed.

The fruit fly-like Phoridae sub-order contains some amazing species, all recognisable by their rapid scuttling gait and humpbacked appearance. Of the British species, some are know to be parasites on invertebrates; millipedes, beetles and other larger flies. One species, Pseudoacteon formicarum, has a truly fascinating relationship with ants - the fly hovers over the ant and even chases it, laying its eggs on the soft exposed parts of the ants body, attracted initially by the scent of the ants acid.

These flies represent the close relationship between parasite and host, and make clear what can happen if a certain species becomes threatened in its natural environment. We tend to concentrate our concerns on the large host species and fail to notice the intricate interdependence of parasites which have evolved to live on one particular host. It also highlights the fact that many more connected species of insect are lost when a host species becomes extinct. In their natural environment these flies may play an important role in the spider lifestyle, keeping the burrow free from decaying material resulting in a reduction of mould growth, etc.

Dr. Disney is eager to receive further specimens of scuttle flies, especially flies from spider species other than those mentioned above. As these are relatively new discoveries to science, there may be several more types just waiting to be described.

Adult flies, pupae and larvae should be preserved in alcohol and labelled with the relevant information (name of host, date and collection site if known) and sent to : Dr. H. Disney, Dept. of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, England.

References

Two new species of Phoridae (Diptera) whose larvae associate with large spiders (Araneae: Theraphosidae)
D. Weinmann and R. H. L. Disney. J. Zool. Lond. (1977) 243, 319 - 328.

A further new species of Phoridae (Diptera) whose larvae associate with large spiders (Araneae:
Theraphosidae) R. H. L. Disney and D. Weinmann. Ent. Scand. Vol. 29:1 (1998).

Flies take advantage of tarantulas Samuel D. Marshall and R. H. L. Disney. America Tarantula Society
Forum Vol. 7 No. 1 : 3 - 4 (1998).

This article first appeared in the Journal of the British Tarantula Society, 14 (3): 96-98 (1999).